CHEMISTRY

CHEMISTRY.

Chemistry. CHEMISTRY, as a regular branch of natural science, is of comparatively recent origin, and can hardly be said to date from an earlier period than the latter third of the past century. It is true that, before that time, many men of high talent and wonderful ingenuity had devoted themselves to chemical studies; and the very name of the science, which, with the prefixed article, forms the well-known word Alchemy, is of Arabian origin. The early Greek philosophers had some vague yet profound ideas on the subject, but their acquaintance with it was limited chiefly to speculation a priori, founded on a general and often inaccurate observation of natural phenomena. Yet their acuteness was such, that some of their speculations as to the constitution of matter coincide in a most wonderful manner, as will be hereafter shown, with those which are now beginning to prevail among the most profound modern philosophers.

In these early ages, and long afterwards, those who turned their attention to the experimental part of the subject, which the Greeks hardly attempted, did so with some other object in view than the mere cultivation of chemistry as a science for its own sake. They either laboured to discover and produce new remedies, and in this manner were led to the search after the panacea or universal remedy, the elixir vitae, and similar desiderata, which indeed formed the chief occupation of alchemists, as chemists were then called, for many centuries; and these transcendental researches, while they failed in their professed objects, yet led to the discovery of many of the best known and most important compounds of chemistry, as well as of many most valuable medicines. Or else, on the other hand, led by speculations as to the nature of the material elements, which are much less absurd, or at least were then much less absurd, than we are apt to suppose, they attempted to effect the transmutation of one element into another, and especially of the baser metals into gold and silver. In this way arose the search after the philosopher's stone, supposed to be capable of effecting this transmutation, and after the universal menstruum or solvent, which was expected to have similar effects, or at least to facilitate the necessary processes. In this pursuit, as in the other, the alchemists failed; but their indefatigable industry and their wonderful ingenuity led them to numerous discoveries neglected at the time in many instances, but which have had a most marked and important influence on the development of the modern science.

It has already been said, that the alchemists did not study chemistry as a science, but only practised it as a means of attaining their objects. They proceeded on certain speculative notions, adopted a priori, and supposed to be established truths. But at last the time came, when the materials they had collected with so much industry and zeal were made the foundation, on the Baconian or inductive system, of a new science. This could not take place until after the development of physics, or mechanical philosophy, which treats of those properties which are common to all material substances, and which development immediately followed or accompanied that of astronomy.

From the very nature of chemistry, it was impossible that it should take a truly scientific form, until the balance was applied to it. Up to that time, speculations a priori, and erroneous interpretations of observed phenomena, retarded its progress, although new and important discoveries were daily made. But Lavoisier, who first employed the balance in the study of some of the most important phenomena, such as those of combustion, on a certain theory of which the then existing science was founded, effected a complete revolu-

tion in chemistry; and from that time, 1760-1770, the science of chemistry has made rapid and continuous progress.

Our limited space will not permit us to enter more minutely into the history of chemistry, and we shall therefore at once proceed to give some account of the science as it now stands.

It is not easy, and fortunately it is not necessary, to define chemistry in a few words. We may describe it as the science which treats of the properties of the different kinds of matter, or elementary bodies, which exist in our universe, the laws which regulate their mutual actions, and the proportions in which they combine together to form the compounds which, for the most part, constitute the animal, vegetable, and mineral kingdoms, as well as the properties of these compounds. But it is necessary here to explain the term elementary body or element.

ELEMENTARY BODIES.

The ancients, as is well known, admitted four elements, earth, water, air, and fire, of which all things were composed. But it is a mistake to suppose that they used the term element in the same sense as we do. That which they understood by it was rather the forms under which matter is presented to us, than the nature or essence of material substances. Thus, by earth they understood solid matter; by water, liquid matter; and by air, matter in the state of gas or vapour. Fire was their name for what we call heat or caloric, which causes solid bodies to become liquid, liquids to become gaseous. And in this sense the four elements of the ancients do indeed include all material substances.

The meaning now attached to the word element is very different from this. We observe that there are different kinds of matter, whether solid, liquid, or gaseous; and on investigation we find that some of these may be resolved into two or more different substances, some of which again may in like manner be resolved into others, but at last we come to substances in which we are unable, with all our appliances, to detect more than one kind of substance or matter. Some such substances occur in nature, although most natural substances contain more than one kind of matter. To take a few examples. Common salt can easily be shown to consist of two different kinds of matter; of a metal, sodium, and of a non-metallic body, chlorine. Water can be resolved into two gaseous non-metallic substances, oxygen and hydrogen. Cinnabar consists of mercury and sulphur; marble of three bodies, the metal calcium, the non-metallic body carbon, and oxygen. But it is out of our power, by any means yet known to us, to detect any other substance in sodium, chlorine, oxygen, hydrogen, mercury, sulphur, calcium, and carbon; sodium yields only sodium, sulphur only sulphur, and so on. And when this is the case—when a body cannot be proved to contain more than one kind of matter—it is called an element or elementary body, because such bodies are, in fact, the elements of which the material world around us is made up. Some elementary bodies are found in nature as such—for example, mercury, sulphur, carbon, gold, silver, iron, copper, and a few others; but in general two or more are found united, as in air, water, rocks, earths, plants, and animals.

It must be observed, that when we call a substance, such as carbon or chlorine, elementary or simple, we do not assert that it is absolutely and certainly simple, or may not be found hereafter to contain more than one kind of matter; we only mean that, to us, or so far as our knowledge extends, it is so. Thus, at the beginning of the present century potash and soda were considered elementary, be-

Chemistry cause no one could prove them to be compounded; yet soon afterwards, by the aid of a new power, that of galvanism, Davy succeeded in showing that they contained each a metal, potassium and sodium, united to oxygen. It may happen that some future chemist should find the means of proving that these metals themselves contain more than one kind of matter, in which case they would no longer be called elementary. Nay, it is even considered probable, that all the metals may ultimately prove to be compounds, not elements; and the same opinion is entertained regarding such elements as chlorine, bromine, and iodine. But so long as this is not demonstrated, these bodies, and all others in the same position, must be retained on the list of elements.

The researches of chemists have, up to this time, detected about 60 elementary substances; of which 12 are non-metallic, and the rest all metals. The non-metallic bodies are also called metalloids.

Of these elements, the whole of our earth, including the waters and the atmosphere belonging to it, and likewise all the living organisms, whether animal or vegetable, are made up. But it is only a small number of the elements which occur in any great abundance; for the majority, especially of the metals, are only found in a few rare and scattered minerals.

The following table exhibits the elements arranged alphabetically. The second column contains the symbols or abbreviations used to represent them in chemical notation, and the third contains the numbers which represent their respective combining proportions, as ascertained by experiment, that of hydrogen being made the standard of comparison = 1. The meaning and value of these numbers will be presently explained.

Those elements, the names of which are printed in small capitals, are the most abundant and the most important; for of them, practically, the mass of the globe and all its inhabitants, the sea and the air, are composed.

Thus, the air consists for the most part of only two elements, oxygen and nitrogen; water of two, oxygen and hydrogen; sea salt of two, sodium and chlorine; calcareous matter, such as marble, limestone, chalk, and Iceland spar, as well as the earthy part of shells, of three elements, carbon, calcium, and oxygen. Silica, quartz, or sandstone, contains silicon and oxygen; gypsum consists of sulphur, calcium, and oxygen; bone earth of phosphorus, calcium, and oxygen; woody fibre of carbon, hydrogen, and oxygen; muscular fibre of carbon, hydrogen, nitrogen, sulphur, and oxygen; pure clay of aluminum and oxygen; iron ore of iron and oxygen, or of carbon, iron, and oxygen; lead ore of lead and sulphur; copper ore of copper and sulphur. The ashes of land plants contain much carbonate of potash, that is carbon, potassium, and oxygen; those of sea plants, carbonate of soda, or carbon, sodium, and oxygen. Many, indeed most, rocks and soils are more complex, being mixtures of several of the substances just enumerated. Thus, granite, gneiss, and mica slate, all contain quartz, felspar, and mica; and the two latter minerals are compounds of silica with clay or alumina, potash, lime, &c. Clay slate, grauwacke, and other rocks, consist chiefly of felspar; and the beds of coal contain much carbon, with less hydrogen, nitrogen, and oxygen, than the vegetables from which they are derived. Soils are rocks, more or less disintegrated; but contain the same substances, namely, quartz or silica, clay or alumina, limestone, felspar, gypsum, &c., which are found in the rocks which yield them, generally mixed with decaying vegetable matter or mould.

The preceding statements will show how small is the number of the more important elements; that is, of those which constitute an important part of the earth's crust. And even among the substances named there are one or two, such as the ores of lead and copper, which occur only in veins and in comparatively small quantity. But these metals, and a good many others, such as gold, silver, mercury, zinc, antimony, chromium, arsenic, cobalt, nickel, platinum, magnesium, &c., although, in comparison with the more abundant elements their quantity is small, yet occur in sufficient abundance to be applied to innumerable useful purposes. Iron, although its ores are scarce, compared with the common rocks, is yet, in small proportion, so universally diffused that it may be reckoned among those elements which make up the chief part of the earth's crust.

It will be seen that of all the elements oxygen is the most universally present, forming a constituent part, indeed, of all rocks and soils, excepting only rock salt, which is hardly to be called a true rock; of all plants and animals, of water, and of the air. It cannot, therefore, be doubted that oxygen is of all the elements the most important.

Next to it, in the mineral kingdom, come silicon, aluminum, calcium, carbon (in limestone), sodium, chlorine

Chemistry. (these two in salt), hydrogen (in water), nitrogen (in air), magnesium, iron, potassium, and sulphur, and in smaller proportion, phosphorus, iodine, bromine, and fluorine, besides the ores of metals.

But in the animal and vegetable kingdoms, while nearly the same elements form their mass, the proportions vary much. Carbon is the predominant and characteristic element of all organized structures; after it come hydrogen, nitrogen, oxygen, phosphorus, and calcium (in bones), sulphur, potassium, chlorine, sodium, and in smaller proportion iodine and fluorine. Aluminum hardly occurs in the organized world.

It is thus obvious, that there is no essential distinction, so far as concerns the mere nature of the elements, between the mineral and the organized kingdoms of nature. The principal elements of both are the same; and the difference lies, first, in the predominance of oxygen in the former, and of carbon in the latter; and, secondly, in the more complex nature, as we shall hereafter see, of the chief compounds which enter into the formation of organized tissues.

It is hardly necessary to point out, that by the very definition we have given of the term element, it is implied that we cannot transmute one element into another. We do not mean to assert that it is absolutely or physically impossible to transmute one of our so-called elements into another, but only that we cannot do this up to the present time. And there is every reason to believe that if we should ever succeed in transmuting one of our elements into another, it would be in consequence of our discovering, which is quite conceivable, that one or both of the transmutable bodies was in reality a compound, and not a true element. There are certain groups which have so many characters in common to all the members of them, that the simplest explanation would seem to be, that all these members contain one common ingredient or element, to which the common properties are to be ascribed, and which, in each case, is combined with a different element, to which the differences are due.

Thus all the metals agree in having the metallic lustre, and in conducting heat and electricity, besides having a general resemblance in their chemical characters; and it is conjectured that all metals contain one common ingredient. But if this be so, the metals cease to be simple elementary bodies, and must be compound. Again, there is a wonderful and accurately graduated analogy between chlorine, bromine, and iodine, in all their characters; and the opinion is pretty generally held, that they also will ultimately prove to be compound bodies, and to possess a common ingredient. But this, in the meantime, is but conjecture.

When we shall have explained the views now held as to the atomic constitution of matter, we shall mention another conjecture as to the possibility of the transmutation of bodies truly elementary. For the present it may suffice to point out that if such transmutation be supposed possible, it must be under circumstances very different from those which commonly exist. For if it were possible, under existing circumstances, that one element should become converted into another, the whole foundation, not only of chemistry, but of nature, would be destroyed. If, for example, carbon, the predominating and essential element of organized beings, could, under ordinary circumstances, be converted, as has been alleged, into silicon, one of the chief elements of mineral nature, how could plants or animals, to which carbon, as such, is essential, possess any stability? As soon as the carbon of any vegetable or animal tissue became silicon, that tissue must cease to exist. Again, if such transformation were possible, under the usual conditions of experiment, chemical analysis would be utterly impracticable, since it would be impossible to determine with accuracy the amount or weight of an element, liable at any time to become a different one. It is easy to see that an indispen-

sable condition of the very existence of the material world we inhabit is the stability of the elements of which it is made up. An unstable element is almost a contradiction in terms; and, therefore, practically, the transmutation of elements, under the usual conditions of experiment, must be regarded as necessarily unattainable, although it may be conceivable under widely different circumstances. And when we speak of the indispensable stability of elementary bodies, it is altogether independently of the question whether these be really elementary, or, as is perhaps more probable in many cases, compounds which we are unable to decompose, or prove to be compounds. Thus, whether carbon be really elementary or not, it must be a stable element to us, otherwise the compounds it forms could have no stability.

Having explained what is understood by elements or elementary bodies, we shall next proceed, before describing the elements individually, to mention briefly the important laws which regulate chemical action between different elements, and especially those which have reference to the proportions, by weight as well as by volume, in which they combine together. It will also be necessary to explain the atomic hypothesis, which is adopted to furnish an explanation of the facts alluded to.

CHEMICAL COMBINATION.

Chemical combination, or chemical action, is to be distinguished from such actions of matter on matter as are physical or mechanical. Thus, all bodies are acted on by the force of gravitation, and, as we say, attracted towards the earth, and towards each other. A rod of sealing wax briskly rubbed on cloth or silk attracts light bodies; a magnet attracts iron filings. In all these cases, one mass or portion of matter acts on or attracts another, and motion is the result. These actions, or attractions, of gravitation, electricity, and magnetism are exerted between different portions of matter, but not necessarily between different kinds of matter; and they are also exerted at sensible distances, which in the case of gravity or of magnetism may be very great distances.

In all these respects chemical action is different. It is exerted invariably between different kinds of matter. It operates at insensible distances, and the result is, not sensible motion, but the formation of one or more new substances, by the combination of those which act on each other. The evidence of this is found in changes of properties, both physical and chemical. Of course the simplest case is that in which two elements act on each other, or combine to form a new body, which is said to be a compound of the two. Two portions of sulphur or two of iron cannot act chemically on each other, but can only exhibit mechanical attractions, such as those of gravitation, cohesion, and the like. But if a portion of sulphur and a portion of iron be placed in contact, at a certain temperature, chemical action ensues; the sulphur and iron combine together, and a new substance is the result. This new substance contains these two elements, and is called the sulphuret of iron. It is neither yellow and easily fusible like sulphur, nor metallic, malleable, and tenacious, like iron. It is of a brassy colour, with some degree of metallic lustre, and very brittle. Here the new body or compound formed has properties quite distinct from those of its elements. Now this is universally the case where chemical combination takes place. The change of properties is usually complete. Thus oxygen and hydrogen, two permanent gases, combine and produce water, a liquid at ordinary temperature. Iodine, which is a black solid, forms with mercury, which is white and metallic as well as liquid, a fine scarlet crystalline compound; and with lead, a compound which forms hexagonal plates of the colour and lustre of gold. Sulphur with mercury forms vermilion. Nitrogen and hydrogen, two inodorous and taste-

Chemistry. less gases, combine to form the caustic and pungent ammonia. The same law holds in regard to compound bodies, which act on each other. Thus sulphuric acid, which is highly corrosive, and caustic potash, a substance used as an escharotic, unite to form sulphate of potash, a mild neutral salt; and there are hundreds of similar cases.

In every case of a chemical compound, we can show that it contains two or more elements, and we thus distinguish compound bodies from such as are simple or elementary.

In every case of chemical action between two substances, whether simple or compound, combination takes place. But in a great many instances decomposition also occurs; that is, substances previously combined are separated. In the greater number of cases both these changes happen; some of the substances present enter into combination, while others are separated; or the same substance separates from that with which it had been united, and combines with another. Hence the almost infinite variety of the results which the chemist can produce.

When zinc is introduced into a solution of chloride of tin, a compound of tin and chlorine, the zinc combines with the chlorine, forming chloride of zinc, and the tin is separated in bright metallic crystals. Here we have both combination and decomposition. When iodide of potassium is added to chloride of mercury, what is called double decomposition, which, however, implies double combination, takes place. The potassium leaves the iodine to unite with the chlorine, while the mercury leaves the chlorine to unite with the iodine, and thus, while the two original compounds are destroyed, two new ones, the chloride of potassium and the iodide of mercury, are formed.

There are certain conditions which favour and promote, others which impede or oppose, chemical action. Thus, two substances, both solid, rarely act on each other, because the force of cohesion among the particles of each prevents them from coming into sufficiently close proximity to those of the other. In some few instances, as when iodine and phosphorus act on each other, their mutual attraction overcomes the obstacle offered by cohesion.

In general, the best plan is to liquefy one of the substances, or both, either by the aid of heat, or by the use of some solvent, such as water. If this be done, and if the attraction or tendency to combine be powerful, combination will generally follow.

The liquid form is the most favourable to chemical action, because it permits the particles to come into close proximity. Hence most substances are employed in a state of solution.

The gaseous form is unfavourable to chemical action, because the particles of elastic fluids or gases are at a great distance from each other compared to that which separates them in the liquid state. Yet, where the attraction is powerful, gases do act on each other. Thus oxygen and deutoxide of nitrogen gases instantly combine. But in general gases do not act on each other, unless under the influence of the sun's rays, or of a high temperature. Chlorine and hydrogen do not unite in the dark, but combine with explosion if placed in the sunshine, or if a flame be applied to the mixture. Oxygen and hydrogen do not combine till either a flame is applied or an electric spark passed through the mixture; in both cases they combine with explosion.

Chemical action frequently takes place between a solid and a gas, as when potassium absorbs oxygen; or between a liquid and a gas, as when water absorbs ammonia.

Heat favours chemical action, although at the same time, by increasing elasticity, it tends to separate bodies already combined. But it appears to exalt the energy of chemical attraction in a still higher degree. Hence heat is constantly employed by the chemist, both to liquefy solid bodies, and thus indirectly assist chemical action, and to increase directly the force of chemical attraction.

When several substances are present in a solution, various

circumstances contribute to determine the result. If two Chemistry. of the substances present can form a compound which is very insoluble in the menstruum employed, that is, in which the force of cohesion is very great, that substance will be formed, and this will decide the other changes. Thus, when sulphate of soda is mixed with nitrate of baryta, since sulphuric acid and baryta form a compound absolutely insoluble in water, they combine, and the sulphate of baryta being thus formed, the nitric acid must combine with the soda and form nitrate of soda.

In like manner, if any of the substances present have a great tendency to assume the form of gas, that is, if its elasticity be great, it will escape as gas, and thus determine the result. If carbonate of lime be mixed with nitric acid, the carbonic acid, which has a far greater elasticity than nitric acid, escapes as gas, and the lime of course combines with the nitric acid. It is often said that carbonic acid is a weak acid, and is expelled by a stronger, such as nitric acid; but it is the elasticity of carbonic acid which causes it to appear weak.

At ordinary temperatures, sulphuric acid expels silicic acid from its combinations. But at a red heat, silicic acid expels sulphuric acid, because at that high temperature the elasticity of sulphuric acid is very great.

When a number of different substances are present in a solution, and when these are capable of combining two and two,—as, for example, several acids with several bases, each acid being capable of combining with each base,—the result is determined by all the above circumstances; by the temperature; by the relation of the solvent to the compounds that may be formed; by the force of cohesion in these possible compounds, and by the force of elasticity in such of the bodies present as tend to assume the elastic form.

It was formerly the custom to give tables of affinity or of decomposition, showing the supposed comparative force of chemical attraction between two or more acids, for example, and any given base; or between two or more bases and a given acid. That body which expelled another and took its place was called the stronger, and was said to exert a more powerful affinity, or chemical attraction, than the substance it expelled for the third body. Thus, sulphuric acid, which expels nitric acid from nitrate of potash, was said to have a stronger affinity for potash than nitric acid. But in this case, the change, which only takes place fully with the aid of heat, is determined by the tendency of the nitric acid to assume the gaseous form, that is, by its elasticity; and it affords no proof that nitric acid is weaker than sulphuric. Nay, as we have seen, an acid, apparently very feeble at ordinary temperatures, may become, at a red heat, capable of expelling sulphuric acid, by which it is itself expelled in the cold. In consequence of these and similar considerations, tables of affinity are no longer used, since they convey no information but the bare fact, that at a given temperature, and under certain circumstances, certain changes occur, and cannot tell us the real comparative force of affinity or attraction between any two or more substances of the same class, and a third with which both can unite.

When one body leaves another with which it had been combined to unite with a third, the result used to be called an example of elective affinity; as if the body B, in the compound AB, chose or preferred the body C, and thus formed the compound BC, A being set free. But it is now considered, that when a body C is placed in contact with a compound AB, the compound BC is only formed when it has a greater cohesive force than AB; or when A has a greater elasticity or cohesion than C, or when both causes are combined. And we have no means of ascertaining whether the attraction between B and C be greater than that between A and B, considered apart from the influencing circumstances.

There is another case, namely, where two bodies refuse

Chemistry. to act on each other till a third is added, which tends to combine with the new compound that may be formed. Thus, zinc does not decompose water till sulphuric acid be added, and then hydrogen is set free, while the oxygen of the decomposed water is found to be combined with zinc and sulphuric acid, forming sulphate of oxide of zinc. This was formerly called a case of predisposing affinity, and the acid was said to promote the action in virtue of its attraction for the oxide of zinc about to be formed. It is obvious, however, that it is absurd to speak of the attraction of sulphuric acid for a body not yet in existence, and still more so to ascribe to this attraction the formation of that body. The truth is, that when zinc is in contact with water, no change occurs; the forces tending to preserve the existing state of things being superior to those which tend to disturb it. But when the acid is added, an additional disturbing force is brought into play, the existing arrangement is overturned, and the zinc, oxygen, and sulphuric acid unite to form the sulphate of oxide of zinc, while the hydrogen, from its elasticity, is disengaged as gas.

There are, besides, some remarkable instances, in which certain substances appear, by their mere presence, to promote chemical action without taking a share in the change, as the sulphuric acid does in the case last mentioned. Thus, oxygen and hydrogen, which, when mixed, may be kept for any length of time without combining, rapidly combine if allowed to come in contact with platinum, whether in a compact, dense, polished plate, in a porous spongy form, or in the form of powder. And yet the platinum does not in any way enter into the change, but remains, as before, uncombined. It must be admitted that we cannot satisfactorily explain this striking fact. The proposed explanations will be mentioned under Hydrogen.

In like manner, yeast or ferment induces the fermentation or decomposition of sugar, if placed in contact with it, yet neither gives anything material to the sugar, nor receives anything from it. It is supposed to act by the communication of a mechanical impulse or motion to the particles of the sugar, which motion or impulse destroys the existing equilibrium, and a new equilibrium is established, which is permanent under the existing circumstances.

Some have included these two last-named modes of action under one head, under the name of catalysis or catalytic action. But independently of the circumstance that this is merely giving a name to the phenomenon which we cannot explain, and not explaining it, it would seem that we can hardly conceive that the same cause which produces the combination of oxygen and hydrogen by mere contact, should also by mere contact cause the decomposition of sugar, or the separation of bodies already combined.

Such is a brief account of the circumstances which promote or oppose chemical action. Let us now consider the subject in reference to the quantity of the substances which combine, and we shall find it to be a matter of the highest importance; the study of which, in fact, has created the existing science of chemistry.

COMBINATION IN DEFINITE PROPORTIONS.

The researches of chemists have established a most important law; which is, that when two or more substances unite to form a new compound, they do so in definite, fixed, invariable proportions.

Thus, hydrogen and oxygen unite to form water. Now, when 1 grain (ounce, pound, &c.) of hydrogen is thus converted into water, the water produced weighs, invariably, exactly 9 grains (ounces, &c.) Consequently hydrogen and oxygen unite, to form water, always in the proportion of 1 part of hydrogen to 8 parts by weight of oxygen. If we analyze (after purifying it) the water of a river, of a lake, of the sea, of rain, or of snow and ice, whether newly formed, or produced ages ago, we shall always find that 9 grains of

water contain 1 grain of hydrogen and 8 grains of oxygen. Chemistry. If we mix these elements in any other proportion, such as 1 to 10, or 2 to 8, and cause combination to take place, we shall find, in the former case, 2 parts of oxygen, and in the latter 1 part of hydrogen, remaining uncombined. In short, we cannot form water which shall have a composition different from that just stated. If we should be able, and this is possible, to cause these two elements to combine in any other proportion, the resulting compound, as will be seen hereafter, would be, not water, but a totally different compound; and this compound, in its turn, would be found, whenever formed, to be as invariable in the proportion of its elements as water itself.

If we analyze the oldest marble, geologically speaking, we shall find it to consist of carbon, calcium, and oxygen, in the proportions of 6 parts, by weight, of carbon, 20 of calcium, and 24 of oxygen, which make up 50 parts of carbonate of lime. If we analyze the newest chalk, or if we prepare, artificially, carbonate of lime, and analyze it, the results will be precisely the same. These three elements, to form this compound, unite in the above proportions invariably.

This law admits of no exceptions; and it is of the very essence of any compound, and a point on which its properties as well as its existence depend, to contain always the same relative amount of its component elements.

It is easy to see that, unless this were so, chemistry as a science could have no existence, for analysis would be impossible. If the proportion of any of the elements of a compound were variable, it would be impossible to attach any value to such a compound. If, for example, iron ore, lead ore, or silver ore, being pure compounds, contained at one time 50 per cent. of the metal, at another 5 per cent., how could such ores be valued? This, in fact, constitutes the difference between a true compound and a mixture, in which the proportions are never twice the same. But all true compounds, if pure and free from admixture of foreign matter, are uniform in their composition. This is the first great law, in regard to quantities, which regulates chemical combination.

The second law is, that if two bodies are capable of combining in more proportions than one, that is, of producing more compounds than one, then a very simple ratio exists between the quantities of the same element in these compounds, when referred to the same amount of the other element. This ratio is usually that of a multiple by a small whole number.

Thus, hydrogen forms with oxygen two compounds with properties totally dissimilar; namely, water or protoxide of hydrogen, and the deutoxide, binoxide, or peroxide of hydrogen. In water, as we have seen, the proportion is 1 of hydrogen to 8 of oxygen. In the peroxide, the hydrogen being made 1 as before, the oxygen is not 8, but 16, that is, 8 multiplied by 2. Again, lead forms with oxygen two compounds, or oxides, which contain—

In some cases there are three or more compounds of the same elements, but the law holds invariably. Thus, nitrogen forms with oxygen the following five compounds:—

Here the oxygen in the first compound is multiplied successively by 2, 3, 4, and 5. Such cases, however, are rare. There are few instances in which two elements combine in more proportions than two or three.

In the cases just cited, the ratio is the simplest possible;

Chemistry. but there occur ratios somewhat different. Thus iron forms with oxygen several compounds. We find, in the

Here the ratio in the second compound, if we consider the first as in the ratio of 1 : 1, is that of 1 : 1.5, and in the third, it is 1 : 3. Compounds in which the ratio of 1 : 1.5 is observed are called sesqui-compounds. In some rare cases we find the ratio of 1 : 2.5, and of 1 : 3.5. In all other instances, and these constitute the vast majority, the quantity of the element which varies, the other being supposed stationary, is multiplied by a whole number, such as 2, 3, 4, 5, and very rarely 7.

This law is called the law of multiple proportions, and in reference to it compounds may be arranged in two categories. In the first we have the amount of one element, the other being supposed to be fixed, increasing by the simple multiples 2, 3, 4, &c, so that we have a simple arithmetical series.

In the other, we have the following series of numbers for the quantities of the variable element, or some of them, namely—1, 1.5, 2, 2.5, 3, 3.5.

The next law of combination is, that the numbers representing the weights in which bodies combine together are mutually proportional. That is to say, if a certain weight of A combine with a certain weight of B, and if the same weight of A combine with a certain weight of C, then the numbers which represent the weights of B and of C which combine with the same weight of A, will also represent the weights of B and C which will combine together, if they can combine. Or if not, then the combining weight of either B or C will be a multiple or submultiple of the number referred to.

Thus, 8 parts of oxygen unite with 1 of hydrogen to form water, and 1 part of hydrogen combines with 16 of sulphur to form hydrosulphuric acid. Now, when oxygen and sulphur combine, they do so either in the proportion of 8 parts of the former to 16 of the latter, as in hyposulphurous acid; or in those of 16 of oxygen to 16 of sulphur, as in sulphurous acid; or 24 of oxygen to 16 of sulphur, as in sulphuric acid.

Again, 8 parts of oxygen unite with 40 of potassium, and 40 of potassium unite with 16 of sulphur; the quantity which, or a multiple of it, we have just seen to combine with 8 of oxygen.

Lastly, 1 part of hydrogen, which, as we have seen, unites with 8 of oxygen and 16 of sulphur, unites also with 36 parts of chlorine. And 36 parts of chlorine unite with 8 of oxygen, and also with 40 of potassium, the quantity of that metal which combines with 8 of oxygen and 16 of sulphur. Chlorine, however, combines with oxygen in more proportions than one; and here, as with sulphur, the law of multiples holds, for 36 of chlorine combine with 8, with 24, with 32, with 40, and with 56 of oxygen.

It will now be obvious to the reader, that if we ascertain by experiment the proportions by weight in which all the other elements combine with one, such as oxygen (which can combine with all the others except only fluorine), these numbers will at once tell us in what proportions these other elements which combine with oxygen will combine among themselves. Or, if the numbers thus obtained should in any case be found not to represent the combining proportion of one element with another, it will only be because in that instance one of the elements combines according to a multiple or submultiple of the number representing the weight of it, which combines with 8 of oxygen.

It is precisely in this way that the numbers in the third column of the table, p. 438, have been obtained. They represent the proportions by weight, in which, or in some

instances in multiples or submultiples of which, they combine, not only severally with 8 parts of oxygen, but with each other, according to the third law, which declares that these numbers are mutually proportional.

It will be observed that in this table hydrogen is made the standard of combining proportions, its number being = 1. And it is for this reason that oxygen is represented by 8, being, as we have seen, the quantity which in water is combined with 1 of hydrogen. Hydrogen has been chosen in this country as the standard of comparison, because, being the lightest of all bodies, it has the smallest combining number, and if that be made = 1, the numbers of the other elements will generally be whole numbers, and thus we get rid of fractions as far as possible. If we were to make hydrogen = 10 or 100, the other numbers would have to be increased in proportion.

On the Continent, oxygen is made the standard, and is made = 100. Hydrogen then becomes 12.5; sulphur 200, chlorine about 450, and so on. The smaller numbers of the English scale are more easily retained, and, as already mentioned, there are fewer fractions. But any one who wishes to do so, can easily convert the numbers of the hydrogen scale into those of the oxygen scale. For this purpose, he has only to multiply the former by 12.5; and conversely, to reduce numbers of the oxygen scale to those of the hydrogen scale, we divide them by 12.5.

It is a very remarkable fact, and one no doubt connected with the intimate constitution of matter, that when hydrogen is made unity, nearly all the other elements are represented by whole numbers. In other words, their combining proportions are multiples of that of hydrogen by whole numbers. Dr Prout first advanced this as a law, which was much contested, and for a time it was supposed to be overthrown by the results of experiment. But as our methods of analysis have improved, it has been found that every year more elements are brought under Prout's law; and it seems probable that, as this improvement advances, that law will be ultimately found to apply universally. For the present, some important elements, such as chlorine and potassium, cannot be brought exactly under it, although we have used whole numbers in speaking of these elements, in order to avoid fractions. The true numbers of these elements, according to the best and most recent authorities, are given in the table.

It must be carefully remembered that these numbers are in no respect whatever theoretical, but represent the actual results of the best analyses. They are quite independent of all hypothesis, and it must be received as a simple observed fact, that the elements combine according to these numbers, or multiples or submultiples of them, whether we can explain it or not. It will be at once perceived that this important fact constitutes the only true foundation for chemistry as an exact science.

It cannot be doubted that this essential fact depends on some cause connected with the constitution of matter. But as we know nothing certain concerning the constitution of matter, nay, nothing whatever of matter, except its properties, that is to say, the various ways in which it affects our senses; so we are as little able to explain why the elements combine in fixed and invariable proportions, as we are to explain why or how the earth and the sun gravitate towards each other.

In all departments of natural science, we find ultimate facts, like this of definite proportions, or like gravitation, of which we know only that they exist; and those who imagine that, for example, they explain gravitation by saying that there exists an attraction between the gravitating bodies, which they call gravity, or the attraction of gravitation, delude themselves with words. To say this is merely to repeat the simple fact, that the bodies in question somehow tend towards each other, in different terms; and it furnishes not

Chemistry, even the shadow of an explanation of the phenomenon. Newton's celebrated law of gravitation was never intended, as some imagine, to explain gravitation, which no man can explain, but only tells us that all masses or portions of matter tend towards each other with a force which varies directly as the mass, and inversely as the square of the distance. This law enables us to measure and calculate the force of gravitation, but throws no gleam of light, nor does it pretend to do so, on the nature and mode of action of that force. Why do two bodies tend towards each other, and how is it effected? These are questions which Newton never attempted to answer, well knowing that they are beyond the reach of our faculties.

In like manner, the questions, Why do two elements combine chemically? How is their union effected? Why do they unite in fixed and definite proportions? have never been answered, and probably will never be answered, so long as our faculties remain the same. We can only say that they do unite, and that they unite according to certain laws which we can investigate and ascertain.

It is not wonderful that we should be utterly unable to answer such questions as we have stated, when it is considered that we cannot even define matter, or say what matter is. Of matter we know only the properties, not the essence.

But philosophers, in their eager anxiety to explain everything, have formed certain hypotheses as to the constitution of matter; and although none of these has been demonstrated to be true, and the knowledge of the true nature or essence of matter is, in all probability, beyond the reach of our faculties, yet some practical advantage may be derived from assuming as true a certain hypothesis concerning matter, from which we can deduce, in a simple and logical manner, the facts of combination in definite proportions, of combination in multiple proportions, and of proportional or equivalent numbers. This hypothesis is that which is known as the Atomic Theory. We shall now proceed to explain it in its application to chemistry; but the reader must carefully bear in mind, that although the Atomic Theory is only an hypothesis, assumed in order to furnish some explanation of the above-named facts, yet, whatever the ultimate fate of that hypothesis, and it is next to impossible that it should ever be demonstrated, the facts which it is intended to explain are observed and ascertained truths, which must remain, even were the atomic hypothesis proved to be false.

ATOMIC THEORY.

From the very earliest periods philosophers speculated on the nature and constitution of matter; and among the Greeks two opinions were held, both of which have ever since had supporters among those who studied natural science.

According to one opinion, matter is divisible ad infinitum, so that the smallest conceivable portion of matter may yet be divided into two or more smaller portions, and these again into others still smaller. Those who argue in favour of the infinite divisibility of matter appeal to experiment, which shows that matter is really divisible to an extent far beyond that of which our senses, aided by the microscope, can take cognisance. And they add, that we cannot conceive a mass of matter so small, that we are not able to imagine it to be divided into two halves, and these again each into two halves, and so on ad infinitum.

Now, all this is quite true. The actual divisibility of matter is amazingly great. One grain weight of gold can be beat out so thin as to cover with a perfect metallic surface 54 square inches; and gold is present, therefore, on every visible point of this large surface, even when examined by a high magnifying power. The one grain of gold, therefore, has been divided into at least as many minute parts as there are visible points in 54 square inches, viewed, let us

say, under a magnifying power of 1000 linear. Platinum Chemistry, can be drawn out to a wire so fine as to be with difficulty seen. And these facts are as nothing compared to the division effected by chemical means. If one grain of iron or of copper be dissolved in an acid, and diluted with a gallon of water, the presence of the metal may be detected in every drop of the liquid. Now an ordinary drop weighs one grain, and in a gallon of water there are 70,000 grains. But each drop may be divided without difficulty into 1000 parts, since we can easily weigh $\backslash frac\{1\}\{1000\}$th of a grain on our balances. And in every one of these parts we can detect the metal, even with the naked eye, by the use of proper tests. Again, under the microscope, each of these $\backslash frac\{1\}\{1000\}$th parts of a grain of the liquid may be so magnified as to appear equal to the original drop, and of course may be again subdivided into 1000th parts, in which it cannot be doubted that we should still be able to detect the metal. Now, the gallon of liquid will yield 70 millions of minute drops, each weighing $\backslash frac\{1\}\{1000\}$th of a grain; and 70,000 millions of the microscopic drops we have supposed to be derived from these under a high magnifying power. So that, if, as there is every reason to believe, the iron or copper can be shown to exist in each of these last particles, we shall have divided the one grain of metal, by chemical means, into 70,000,000,000 parts. And there is no reason to think that, even then, we should have reached the limit of actual divisibility, if there be a limit to it.

But all this does not prove that matter is infinitely divisible, for there may be a limit, though beyond the reach of our senses.

Now, the other opinion, which was held by some of the early Greek philosophers, is this: that matter is indeed divisible to a prodigious degree, but not infinitely; that there is a limit to divisibility, and that this limit depends on the constitution of matter. Matter is believed, by those who hold this opinion, to be formed of very minute particles which are called atoms, from two Greek words, signifying "that which cannot be cut or divided;" and when division reaches these, it can go no further, and must stop.

It is no doubt true, that however minute the supposed atoms may be, we can conceive of them as halved, and of the halves again as again divided. But while there is no limit to our conception of the smallness of atoms, this by no means proves that there may not be a limit to the actual divisibility of matter. To see this, let us consider what division really is; and we shall see that the knife, the hammer, the pestle, and the solvent, all agree in separating one part of a mass of matter from another, or in causing the parts to assume a new arrangement. The knife, which is perhaps the simplest agent of division, is a form of the wedge, and, being forced into the vacant space between two portions of a mass of matter, separates these. But that this may be done, there must be empty spaces between the parts of which the mass of matter is made up. And this is the case. Every mass of matter has multitudes of such empty spaces or pores, into which the dividing instrument penetrates. Matter itself is impenetrable; and when we penetrate a mass of it, we force the instrument into its pores, the matter being displaced or yielding on all sides, but not being itself penetrated. Consequently, if we suppose a mass of matter devoid of pores, it would not be possible for us to divide it, since matter is impenetrable, and there is no space for the instrument to enter. All natural matter, however, is porous, and consequently divisible; and the same is true of the smallest particles which our senses can appreciate.

But the very nature of the atoms, supposed by the second hypothesis to exist, is to be destitute of pores; to be in fact units of matter, entirely filling the space within their periphery, which the minutest fragment of ordinary matter does not. And when we have conceived a mass without pores, we have conceived an atom, that is, a particle which

Chemistry, cannot be divided, although we can conceive a particle of half its size, and so on.

According to this theory then, matter is made up of such atoms, or entirely solid, indivisible particles, which are not in absolute contact, but probably touch each other at one point only; and of the pores, or vacant spaces between them. When heat expands matter, it forces the atoms farther apart; when cold contracts it, they come nearer together. When cut or bruised, they are more or less completely separated from their original cohesion; when dissolved, the atoms are separated by the solvent. When beaten out thin, or drawn into fine wire, they are made to assume a new arrangement, either in many parallel lines or in one or a few such lines. In short, these atoms may be separated, or newly arranged, but they cannot possibly be divided; and therefore when, in the process of division, we come to the atoms, we must stop. They are, however, so very minute, that our means of division fail us before we reach them; so that the limit to divisibility in practice is short of that fixed by the nature of matter, according to this view.

For this reason we cannot demonstrate the actual existence of atoms, but the hypothesis which assumes their existence agrees perfectly with all the known phenomena exhibited by matter, whether physical or chemical, and more particularly with the laws of combination in fixed and multiple proportions; whereas the theory of the infinite divisibility of matter leaves these phenomena entirely unaccounted for.

It was Dalton who, reflecting on the facts of combination in definite proportion, first thought of applying to explain these the atomic hypothesis of the constitution of matter. In order to do this, however, we must assume, not only the atomic constitution of matter, but also three other hypotheses, namely, first, that the atoms of each element possess a weight which is invariable; secondly, that the weight of the atom of each element is different from that of all others; and, thirdly, that the elements combine atom to atom, and so forth.

If, for example, we assume that an atom of oxygen weighs 8 times as much as an atom of hydrogen; and if we further assume that 1 atom of oxygen unites with 1 of hydrogen to form water; it is easy to see that, in that case, water must contain oxygen and hydrogen in the proportions ascertained by experiment, namely, that of 8 parts by weight of oxygen to 1 part of hydrogen. And if we suppose 2 atoms of oxygen to combine with 1 atom of hydrogen, the compound thus formed must contain 16 parts, by weight, of oxygen to 1 of hydrogen, which, as we have seen, is the case in the deutoxide of hydrogen.

It must never be forgotten, that in applying the atomic theory to explain the facts of combination, we begin by a series of assumptions. We assume, first, that matter is formed of atoms; secondly, that these have fixed weights; thirdly, that these weights are different in different elements; and, fourthly, that when elements combine, they do so either atom to atom, or 1 atom to 2, 2 to 3, 1 to 3, &c. &c. All these are pure assumptions, which we cannot demonstrate; but these being made, and admitted, the whole facts of combination in fixed and multiple proportions may be naturally deduced from them, and indeed may even be predicted. This is the strongest argument in favour of the truth of these hypotheses, but yet they are not demonstrated truth; while the facts are facts, even if our hypotheses should be abandoned.

This, then, is what is called the Atomic Theory of chemistry. It supposes matter to be formed of exceedingly minute but indivisible particles or atoms, which possess weight, and which have different weights in different elements, but invariably the same weight in the same element. It is, however, impossible for us to know the absolute weight

of these atoms in any case; all that we can do is to ascertain their comparative weights, on the further supposition that elements unite atom to atom, or in some very simple ratio. Thus, if we suppose that one atom of hydrogen and one atom of oxygen unite to form one (compound) atom or molecule of water; then, since we know as a fact that the proportions, by weight, in water, are 1 part of hydrogen to 8 of oxygen, we see that, admitting the suppositions we have stated, whatever be the absolute weight of an atom of hydrogen, an atom of oxygen must weigh 8 times as much. The smallest portion of water we can weigh may possibly contain a million of atoms of each element; but the number signifies nothing, so long as they are supposed to unite one atom with one atom; the relative weights of a million of atoms of each being the same as those of one atom of each element. If, then, we make hydrogen, as being the element whose atoms are the lightest in the standard, and express the relative weight of its atom by 1, the weight of the atom of oxygen will be 8.

In this way the relative weights of all the elements are ascertained, being referred to hydrogen as a standard; and the reader will at once see that these weights—atomic weights, as they are called—coincide with the combining proportions given in the table at p. 438. These numbers, therefore, are called indifferently atomic weights, combining proportions, or equivalent numbers, and, for shortness' sake, equivalents. They are mutually equivalent, because, as has been shown, when a body B leaves another A, to unite with a third C, the weight of B, at first united with a given weight of A, combines with or is equivalent to a weight of C, which is equivalent to or combines with the same weight of A. In other words, when one element is substituted for or replaces another, it is always in the proportion indicated by these numbers, or occasionally in a multiple of these. The terms, combining proportion and equivalent number or equivalent, are preferable to that of atomic weight, inasmuch as they simply express a fact, and do not involve any hypothesis. We shall therefore use, in general, the term equivalent, contracted, when desirable, into eq., as being unobjectionable.

The facts of multiple proportion follow naturally from the atomic theory. Thus, in the compounds of nitrogen and oxygen, the equivalents of these bodies being ascertained to be 14 and 8 (hydrogen = 1), we have only to suppose that,

and the resulting numbers will be as formerly stated.

In the case of such compounds as exhibit the ratio, in their composition, of 1:1.5, or 1:2.5, we cannot, without contradiction, speak of such compounds as formed of 1 atom of one element and 1½ or 2½ atoms of another. The half of an atom, of that which is indivisible, ex hypothesi, is a contradiction in terms. To avoid this, while using the terms of the atomic theory, we double the numbers and say that these compounds are formed by the union of 2 atoms of one element with 3 or with 5 of another. The two oxides of iron consist of

Alumina or sesquioxide of aluminum, sesquioxide of manganese, and sesquioxide of chromium, all likewise consist of 2 atoms or eqs. of metal and 3 of oxygen; and this analogy in composition is attended with remarkable analogy in properties.

It only remains to mention, that the compounds formed by elementary bodies combine together among themselves

Chemistry. precisely as the elements do. It rarely, if ever, happens, at least in inorganic or mineral chemistry, that elementary bodies and compounds combine together, unless in the case of such compounds as play the part of elements, and are hence called compound radicals. Few of these, however, are known, save in combination; and in general, we find that compounds unite with compounds, elements with elements. Thus, oxygen and sulphur unite with hydrogen and metals; but sulphuric acid, a compound of sulphur and oxygen, combines, not with hydrogen or metals, but with water and the oxides of the metals.

The combining proportion, atomic weight, or equivalent of a compound, is the sum of those of its elements. Thus water, formed of 8 parts of oxygen, and 1 of hydrogen, enters into combination in a proportion expressed by 9, the sum of these. Sulphuric acid, a compound of 1 eq. of sulphur and 3 eqs. of oxygen—that is, of 16 parts of sulphur, and 24 of oxygen, by weight—has the number $40\; =\; 16\; +\; 24$ for its equivalent or combining proportion. Potash, a compound of 1 eq. potassium, and 1 eq. oxygen, or (in round numbers) 40 parts of potassium and 8 of oxygen, has the equivalent $48\; =\; 40\; +\; 8$. And when potash and sulphuric acid combine, so as mutually to neutralize each other, they do so in the proportion of 48 parts of potash to 40 of sulphuric acid, forming neutral sulphate of potash, a compound of 1 eq. of each compound. There is another sulphate of potash, which is composed of 1 eq. of potash, 2 eqs. of sulphuric acid, and 1 eq. of water, and is termed the bi-sulphate of potash. So that the law of multiple proportions holds in regard to compounds as well as to elementary bodies.

We have no means of ascertaining the absolute size or volume of the atoms of any substance, and consequently we cannot directly find the relative volumes. But there are facts which indicate that the atoms of some elements are of the same size as those of certain others. For in compounds which crystallize, some of the elements may be removed and replaced by others, without affecting the form or angles of the crystal; which could hardly happen if the atoms of the replacing element differed much in size from those of the element which it replaces. This leads us to consider the subject of crystallization, and that of isomorphism, or the substitution of one element for another, without affecting the crystalline form of the compound.

This treatise is not the place for a full discussion of the subject of crystallization, which is one of great extent and importance, both in regard to chemistry and to mineralogy. Crystallography is now, in fact, a distinct branch of science.

It is sufficient for our purpose to state, that when substances are so placed as to assume the solid form, whether from the liquid state, or that of gas or vapour; and when this takes place slowly, and so that the particles or molecules can arrange themselves according to their natural tendencies,—they assume regular geometrical forms, which are termed crystals. Each substance which is capable of crystallizing, whether simple or compound, exhibits always the same form, except in a very few cases to be presently mentioned. This is so uniformly the case, that many substances may be recognised by this crystalline form alone.

Two circumstances require notice; first, that although the same substance always takes the same form (with the exceptions above alluded to), yet, within certain limits, the outward form may vary; that is, provided all the forms which occur are geometrically derivable from one, which is the fundamental form. Thus sea-salt crystallizes in cubes, but it also appears in regular octahedrons; fluor spar appears in cubes, in regular octahedrons, and in regular tetrahedrons; alum forms both cubes and regular octahedrons. This is because the cube, the regular octahedron, and the regular tetrahedron, constitute really but one fundamental form, termed the regular system, and are all geometrically and mechanically derivable one from the other. Calcareous

spar forms rhombohedrons, but it also forms regular six-sided prisms and six-sided pyramids; also three-sided pyramids, and a prodigious number, amounting to several hundreds, of modifications of these forms; but all reducible, both theoretically and practically, to the fundamental rhombohedron of Iceland spar. In point of fact, then, calcareous spar exhibits only one form, but modified. Salt, alum, and fluor spar, also each exhibit geometrically but one form, variously modified. But although, as in calcareous spar, the extent of modification may be very great, it is never seen to crystallize in the form of the cube, regular octahedron, or regular tetrahedron, nor in any other form not geometrically derivable from its fundamental rhombohedron.

The second point is, that many different substances have the same crystalline form, as is seen in the case above quoted, of alum, salt, and fluor spar; to which may be added galena, iron pyrites, several metals, and many chlorides, bromides, iodides, fluorides, and sulphurets. This arises from the fact that the number of crystalline fundamental forms is very small compared with that of crystallizable bodies. It is chiefly in the regular system, that of the cube, that we see so many different substances assuming the same form, because the regular system is a very limited one; whereas in the other systems—in which the angles may vary, since we may have oblique rhombs or prisms of many different angles, but can have only one cube—there is much greater variety. We cannot, therefore, strictly say that each substance has a different form, but that each substance has a form to which it adheres, although other bodies may have the same.

It is evident that crystalline form must depend on the fact that the molecules of bodies are arranged in right lines, and at certain angles. When the right angle prevails, we have the cube and its derivatives, and the rectangular prism, which differs from the cube in the unequal length of its axes. When other angles occur, the result is an oblique prism, a rhombohedron, or an oblique octahedron, &c.

The great frequency of the cube and its derivative forms probably depends on the tendency of molecules of equal size to arrange themselves in lines at right angles to each other. But there are many instances of substances crystallizing in this form, which depend on isomorphism, that is, on the fact that, in certain compounds, one of the elements may be replaced by another without altering the crystalline form.

Chloride of sodium forms cubic crystals. But if the sodium be removed, and replaced by its equivalent of potassium, the new compound still crystallizes in cubes. Nay, if we remove the chlorine from the chloride of sodium, and replace it by bromine and iodine, the form is still unaltered. And if in the bromide or iodide of sodium we replace the sodium by potassium, the new salts assume the same form. Here, then, are six salts, the chlorides, bromides, and iodides of potassium and of sodium, which have the same crystalline form, the cube. These salts are said to be isomorphous.

To take another example. Alum, which is composed of sulphuric acid, alumina, potash, and water, forms regular octahedrons. But if the potash be replaced by soda or by oxide of ammonium, we obtain two salts, soda alum and ammoniacal alum, which are not to be distinguished from common alum by the form of their crystals. And, further, if the alumina, which is a sesquioxide, be replaced by the sesquioxide of iron, of manganese, or of chromium, we obtain three new alums of the same form. In each of these, as in common alum, the potash may be replaced by soda, or by oxide of ammonium, without affecting the crystalline form. So that we can have 12 alums, all differing in composition in some important point; yet all isomorphous.

Without quoting more examples, although many more might be adduced, it will be obvious from these that in each of the two groups the identity of form among the members

Chemistry. of the group depends on an analogy in composition; or, in other words, when one element can replace another without change of form, the replacing element is analogous in properties to that which it replaces; and, further, the function and position in the compound of the replacing element are the same as those of that for which it is substituted.

Thus, in chloride of sodium the chlorine is negative, the sodium positive; and while the positive sodium is replaceable by the positive potassium, a body singularly analogous to it, the negative chlorine is only replaceable by the negative iodine or bromine, the analogy of which to chlorine is very striking.

Alum consists of 1 eq. of the sulphate of a protoxide (sulphate of potash) with 1 eq. of the tersulphate of a sesquioxide (tersulphate of alumina), and 24 eqs. of water. The potash (oxide of potassium) in the sulphate, is replaceable by soda or oxide of ammonium, bodies entirely analogous; while the alumina (sesquioxide of alumina) can only be replaced by other sesquioxides, such as those of iron, manganese, and chromium, which are extremely analogous to it.

When we consider these facts, it would appear that the atoms, or molecules (groups of atoms) of the replacing body, besides their general analogy to the body replaced, are most probably of the same volume with those of the latter. This may help to explain how they can take the place of the expelled body, and yet not alter the form of the compound. For if their volume were different, it is not easy to see how the angles of the crystal should not be altered.

We can give no further explanation of isomorphism than this, that certain elements appear to be themselves isomorphous, and when this is the case they can replace each other in compounds without affecting the crystalline form.

Among the elements, various groups of such as are isomorphous, that is, with those of the same group, have been detected. These groups are given in the following table; and it will be seen that, as a general rule, the bodies in each group are not only isomorphous, but also in the highest degree analogous in properties.

The following isomorphous groups have been established, and the existence of more is highly probable:—

It is remarkable that groups of three are very frequent. Of course, where two or more elements have the same crystalline form, or are isomorphous, similar compounds of these elements must also be isomorphous. Thus, if potassium,

sodium, and lithium be isomorphous, their protoxides, their chlorides, their sulphurets, must likewise be isomorphous, each with those of the same kind, chlorides with chlorides, &c.

What has been stated regarding the fact that substances of a totally different nature may have the same form, must not be confounded with isomorphism. Both alum and common salt crystallize either in cubes or in octahedrons, but they are not isomorphous; they merely happen to agree in crystalline form. But potash alum is isomorphous with soda alum, chloride of sodium with chloride of potassium, and so forth.

When we find two compound substances, of analogous properties to be isomorphous, this fact leads us to conclude that they are analogous in constitution. Thus, alumina is isomorphous with oxide of chromium, and analogous to it. Now, alumina is a sesquioxide, and we conclude that oxide of chromium is likewise a sesquioxide; a conclusion amply confirmed by other considerations. Selenic acid is found to be isomorphous with, and highly analogous to, sulphuric acid; arsenic acid is isomorphous with, and analogous to, phosphoric acid. We conclude, that since sulphuric acid is a teroxide and phosphoric acid a pentoxide, so selenic acid will prove to be a teroxide and arsenic acid a pentoxide. And this is found to be the case.

When two salts not isomorphous are in solution together, and the solution is evaporated, the two salts will crystallize either successively if of different solubility, or at the same time if of equal solubility, but quite distinct each from the other. The molecules of the one are only attracted by those of its own kind. The two salts may thus be easily separated. But if two isomorphous salts be dissolved, no matter in what proportion, every crystal will contain both, and they cannot be separated by crystallization. This is often a source of great inconvenience. Thus, when potash alum (common alum) is contaminated or adulterated with the isomorphous iron alum, it is impossible to purify it by crystallization. The iron alum is so similar to common alum that no one would suspect it, from its taste or colour, to contain iron, although in general the salts of sesquioxide of iron have a strong inky taste and brown colour. But when used in dyeing or calico printing, the presence of a little iron alum renders the alum totally useless, nay, injurious. It is the isomorphism of the two alums which causes them to adhere so firmly together.

In a few instances, as may be seen by the tables, the same element appears in two isomorphous groups; that is, in one set of compounds it is isomorphous with one group, in another set with another.

It has been stated, that the same body always crystallizes in the same form, inasmuch that many bodies may be recognised by their crystals. But it was also mentioned that there were some exceptions to this general rule, and these are very curious.

We can conceive readily that the same composition might be found in two entirely different crystalline forms; for two compound bodies may have the same composition, and yet may differ in constitution, that is, in the arrangement of the same atoms. For example, if two compounds each contained 3 eqs. of the same metal and 4 of oxygen, they might yet be totally different; for one might be made up of two compounds, namely, one formed of 2 eqs. of metal and 2 of oxygen, and another of 1 of metal and 2 of oxygen—together, 3 of metal and 4 of oxygen; the other might be made up of two different compounds—of one containing 2 of metal and 3 of oxygen, and one containing 1 eq. of metal and 1 of oxygen—together, as before, 3 of metal and 4 of oxygen. And it would be quite natural that these compounds should have different crystalline forms.

But when we find elementary bodies crystallizing in two distinct forms not mutually derivable, it is not easy to un-

Chemistry. derstand how this should occur. Yet it does occur, and not unfrequently.

Sulphur exists in three distinct solid states, two of which are crystalline: carbon is found also in three solid states, two of them crystalline; and phosphorus occurs in two solid modifications, only one of which has yet been crystallized. These modifications of the same body, whether crystallized or not, in which different properties appear, are called allotropic modifications, and the phenomenon is called allotropism.

When sulphur is melted by heat, and allowed to cool, it forms small rectangular four-sided prisms. But when dissolved in bisulphuret of carbon, it forms large and broad crystals, which are oblique octahedrons. In its third allotropic state, sulphur, instead of being yellow, crystalline, and brittle, is brown, amorphous (that is, destitute of crystalline form), and tough.

Carbon, in the diamond, forms transparent regular octahedrons. In graphite it is opaque, black, and crystallized in scales or prisms. In charcoal, lamp-black, and anthracite, it is black and amorphous.

Phosphorus, in its ordinary state, is translucent, nearly colourless, very fusible, and it takes fire when heated to rather short of 100° Fahr. In its allotropic state, it is of a deep brownish-red, amorphous, much less fusible, and very much less inflammable.

In attempting to explain these remarkable facts, we must suppose, either that in one state the molecules contain more or fewer ultimate atoms than in another, or else that these atoms, if equal in number, are differently arranged. Hence the terms allotropic and allotropism, signifying that the atoms or the molecules are turned another way.

The occurrence of allotropic modifications of elementary bodies illustrates that hypothesis of the transmutation of elements formerly alluded to, but not explained. If, it is said, the ultimate atoms of an element, when grouped into molecules in one manner, exhibit certain properties, and when differently grouped (whether the difference consist in the number of atoms which go to form a molecule, or simply in their arrangement), acquire new properties, is it not possible, nay, even probable, that by some such difference of grouping one elementary body may be transformed into another? It has been stated by Dr Samuel Brown, that he succeeded in converting carbon into silicon, and iron into rhodium. And in the former case he supposes the molecule of silicon to be formed of three times as many atoms as that of carbon, the atoms being the same.

It must be admitted that such results are quite within the limits of possibility, and that the phenomena of allotropism, up to a certain point, favour the notion. But, in the first place, it must be remembered, that in the case of allotropic modifications, it is the physical properties which are chiefly affected, while the element retains its chemical characters. Thus, the physical properties of the diamond are as different as can well be imagined from those of lamp-black or charcoal. But the chemical characters are the same. In all its forms, carbon, when heated in oxygen or in air, burns, and is converted into carbonic acid gas; 6 parts of carbon invariably yielding 22 parts of carbonic acid. Moreover one allotropic form may, in general, be easily converted into another. Even the intractable diamond, under certain circumstances, passes into black, amorphous charcoal. Any one of the forms of sulphur may be converted into the other two. Ordinary phosphorus is converted, by a certain degree of heat, into the red variety; and this, when still more strongly heated, is reconverted into the ordinary form.

Now, if silicon be an allotropic form of carbon, it cannot be at pleasure either produced from carbon or reconverted into carbon. In the experiments where silicon is supposed to have been formed, only a fractional part of the carbon

at best underwent the change, and this, as it were, accidentally, if the change really occurred. Chemistry.

But, secondly, the experiments of Dr Brown have not yielded in the hands of other chemists any such result; and chemists in general are of opinion, that the silicon found by Dr Brown must have been derived from the substances employed, in which it may have been accidentally present as an impurity.

Lastly, we may again point out that if an element is intended to perform any function in nature—and carbon, for example, has a most important function as the chief element of all organized tissues, as well as of all products of organic life—such element must possess a degree of stability and permanence, altogether inconsistent with the possibility of its being, under ordinary circumstances, transmutable into an element of different properties. Without this stability, neither compounds, nor analysis, nor chemistry could exist.

Various methods are employed in order to obtain bodies in the form of crystals, since crystallization is one of the best and most convenient means of purifying any substance. During crystallization the molecules of the same body attract each other, and seem to repel all others, except such as are isomorphous. When several salts are present in a solution, and it is evaporated till crystals appear, especially on cooling, the crystals of the different salts can be easily distinguished and separated.

Many substances crystallize best when their solution is boiled or evaporated down to the point at which it deposits its crystals on cooling, most substances being more soluble in hot than in cold liquids. But some, such as common salt, which are almost equally soluble in hot and in cold water, are best crystallized by boiling down the solution, while the crystals form in the hot liquid. Others crystallize when left to spontaneous evaporation; others, such as sulphur, are best crystallized, in one form at least, by fusion, and allowing the melted mass to cool slowly till half consolidated. The part still fluid being poured off, the interior is found lined with crystals projecting inwards.

Some bodies are crystallizable by sublimation, their vapour assuming at once the solid state.

A large proportion of those substances which crystallize from their solution in water, combine in the act of crystallizing, with a greater or less amount of water, which is called water of crystallization, being essential to these crystals. One eq. of dry or anhydrous alum takes up, in forming the ordinary crystals of alum, 24 eqs. of water. The carbonate, sulphate, and phosphate of soda, when crystallized, all contain more than half their weight of water. Such crystals are apt to lose part of their water on exposure to air, and to become opaque or fall to powder. This is called efflorescence. Salts containing no water of crystallization are called anhydrous. Such are the carbonate, sulphate, and nitrate of potash, common salt or chloride of sodium, and many others.

We have been led to consider briefly the subject of crystallization, from the relation of crystalline form to the atomic or molecular constitution of matter. There remain two other subjects connected with this, namely, combination by volumes in the gaseous state, and what is called the atomic volumes of different substances.

COMBINATION BY VOLUMES.

When we compare the quantities of different bodies which combine together, in the solid or liquid state, we cannot trace any simple or obvious relation between their volumes, such as exists between their weights. But when we compare the same substances in the gaseous form, the most simple relations at once become manifest.

Eight parts by weight of oxygen, as we know, combine with

Chemistry. 1 part of hydrogen to form water. Now 8 parts of oxygen are in volume exactly equal to half of the 1 part of hydrogen. Or, in other words, 2 volumes (2 cubic inches, for example) of hydrogen unite with 1 volume of oxygen to form water; and the water thus formed, if measured in the state of gas or vapour, is equal in volume to the hydrogen, or 2 volumes. Two volumes of steam, therefore, contain their own bulk, or 2 volumes of hydrogen, and half their bulk, or 1 volume, of oxygen; so that 3 volumes of the gases, 2 of hydrogen and 1 of oxygen, when combined, are contracted into 2 volumes of steam or gas of water.

In all cases where two gases combine, we can trace some such simple ratio. The commonest are, 2 volumes to 1, the 3 volumes condensing into 2; 1 volume to 1, without condensation, and yielding, therefore, 2 volumes of the compound, as when 1 volume of chlorine and 1 volume of hydrogen unite to form 2 volumes of hydrochloric acid; and 1 volume to 3, the 4 volumes being condensed into 2. This is seen when 1 volume of nitrogen and 3 volumes of hydrogen unite to form 2 volumes of ammonia.

It will be seen that the doctrine of combination by volumes in the gaseous state confirms that of combination by weight, and equally establishes the fact that the combining proportions are fixed and invariable. The reason why we can trace relations so simple between the combining volumes of bodies in the state of gas must be connected with the molecular constitution of gases, and with the distance between their particles.

In solids, the force of cohesion preponderates over that inherent repulsive force which tends to remove the particles of matter farther apart. In liquids, these two forces are exactly balanced, so that the particles move freely in all directions. But in gases, the repulsive force or elasticity, which is derived apparently from the heat present in all matter, has entirely overpowered cohesion, and removed the particles to a much greater distance. Thus, in steam, the gas of water, the particles are so far asunder, that a given weight of water in that state occupies about 1400 times the volume it did in the liquid form.

Indeed, cohesion is so effectually overpowered in gases, that the particles would go on separating still further, in virtue of their mutual repulsion or elasticity, were it not for the force of gravitation, and also for the pressure of the atmosphere; which forces, in gases, hold an exact equilibrium with their elasticity. Heat enables elasticity to prevail, and thus causes gases to expand enormously in volume. Cold, on the contrary, contracts them. Diminished pressure has the same effect as increase of temperature; and increased pressure has the effect of cold on gaseous substances.

It appears probable, that if we could compare all gases under parallel circumstances as to heat and pressure, say, for example, at a hundred, or any other number of degrees above their respective boiling points (that is, the points at which they respectively assume the form of gas, overcoming cohesion), we should find that their particles are at equal distances; or, in other words, that equal volumes contain an equal number of atoms or of molecules. Or, if not, there would at least be a very simple ratio between them.

We have already assumed that the atoms of different elements have different weights; and on this supposition, if equal gaseous volumes contain an equal number of atoms, the weights of equal gaseous volumes must be to each other as the atomic weights. In that case, also, the densities of gases, if we adopt the same standard of reference, must coincide with the atomic weights.

Now this is actually, to a great extent, the case; and where the atomic weights and densities of gaseous bodies, both referred to hydrogen as unity, do not coincide, they at least exhibit some very simple multiple ratio. This is seen in the following table, in which the density as well as the atomic weight of hydrogen is made = 1:—

The densities of gases are generally, however, referred to that of atmospheric air as unity, which conceals the relation between densities and atomic weights.

But it must not be overlooked that, although these facts may depend on the circumstance that equal gaseous volumes contain equal numbers of atoms, which are consequently at equal distances, and differ in the weight of the individual atoms; yet it is also quite possible that the difference in weight of equal gaseous volumes may depend on this, that equal volumes contain unequal numbers of atoms, or that the atoms are united into molecules of unequal size and weight, and placed at different distances in different bodies. It is even possible that the atoms of all bodies might be, individually, of equal weight, and that the difference in their combining proportions might depend on the number of atoms grouped in one molecule, and therefore on differences of size and weight in these complex molecules, not in the ultimate atoms. These are questions which cannot be resolved with certainty; but, if we assume that in gases equal volumes contain equal numbers of atoms, then the facts of combination by volumes follow naturally from the hypothesis.

In some substances, such as oxygen and sulphur, the density in the form of gas is such, that the combining weight is represented, not by a whole volume, but by a fraction, which in sulphur is $\backslash frac\{1\}\{2\}$th of a volume. One volume of hydrogen unites with $\backslash frac\{1\}\{2\}$th of a volume of the vapour of sulphur to form hydrosulphuric acid. If water be regarded as composed of 1 atom of oxygen and 1 of hydrogen, the atom of oxygen is represented by $\backslash frac\{1\}\{2\}$ volume; whereas the atoms of hydrogen, chlorine, bromine, iodine, and nitrogen, are each represented by an entire volume. The table already given exhibits one or two instances, besides those of oxygen and sulphur, where the atom occupies only part of a volume. It is not easy to give a reason for this; but it is possible, that in the case of sulphur, for example, it depends on the existence of allotropic modifications. If sulphur, in one of its three allotropic forms, has a vapour or gas six times denser than in another, it is evident that $\backslash frac\{1\}\{6\}$th of a volume of the former will have the same weight as 1 volume of the latter. And it is certainly probable that each allotropic solid form has a density of vapour peculiar to itself; for there is reason to think that the different allotropic forms of an element differ in the number of atoms grouped in each molecule—a character which is likely to belong to such an allotropic modification, whether it be in the solid, liquid, or gaseous state.

ATOMIC OR EQUIVALENT VOLUMES.

The relation between the atomic weight and the specific gravity of bodies in the gaseous form has been briefly indicated in the preceding section. But the subject admits of being considered under different points of view, according to the notions entertained of the atomic constitution of

Chemistry. gases. On the supposition, for example, that the atoms, or ultimate particles of all elementary gases, with their surrounding spheres of heat, possess the same volume, all such gases would contain, in equal volumes, the same number of atoms. But as it is certain that compound gases do not, in all cases, contain the same number of atoms in equal volumes, it is quite possible that elementary gases may also differ in this respect; and, as above stated, the combining volumes of sulphur and of some other elements agree with this conclusion. It is therefore generally admitted that equal volumes of different elementary gases contain different numbers of atoms; that, for example, 1 volume of oxygen contains twice as many atoms, and 1 volume of sulphur (in the form of gas) six times as many atoms, as 1 volume of hydrogen, 1 volume of nitrogen, or 1 volume of chlorine.

This obviously implies that the atoms, with their spheres of heat, are of different sizes; and, to take the cases above mentioned, that the atoms of oxygen gas are $\backslash frac\{1\}\{2\}$ the size, and those of sulphur $\backslash frac\{1\}\{6\}$ the size of the atoms of hydrogen, nitrogen, chlorine, &c. This is what is called the atomic volume of gases. It is not meant that we can ascertain the absolute volume of the atoms, but the relative or comparative volume of the atoms or particles of two or more gases.

Now, since the specific gravity of a gas depends on the number of atoms in a given volume, and on the weight of these atoms, it is evident that the atomic weight, divided by the specific gravity, must give the (relative) atomic volume.

For example, let hydrogen be taken as the standard for the specific gravity of gases, as it is for their atomic weights; then the atomic weight of hydrogen, = 1, divided by its specific gravity, = 1, will yield the quotient 1 for the atomic volume of hydrogen. Again, the atomic weight of oxygen, = 8, divided by its specific gravity, = 16 (that of hydrogen = 1), gives the quotient 0.5, or $\backslash frac\{1\}\{2\}$, as the atomic volume of oxygen; and the atomic weight of sulphur, = 16, divided by its specific gravity as gas, = 96 (that of hydrogen = 1), gives the quotient 0.1666 or $\backslash frac\{1\}\{6\}$, as the atomic volume of sulphur.

We thus see that, on the supposition above adopted that the atoms of different gases differ in size, we can prove that, whatever be the size of an atom of hydrogen gas, an atom of oxygen gas must be half, and that of an atom of sulphur gas one-sixth that size.

It is further obvious, that the number of atoms in equal volumes must be inversely as the atomic volume; or that the specific gravity of a gas, divided by its atomic weight, will give the number of atoms in a given volume. Hydrogen being retained as the standard, then we have $\backslash frac\{1\}\{1\}\; =\; 1$ = the number of atoms in 1 volume of hydrogen; $\backslash frac\{1\}\{2\}\; =\; 2$ = the number of atoms in 1 volume of oxygen; and $\backslash frac\{1\}\{6\}\; =\; 6$ = the number of atoms in 1 volume of gas of sulphur.

More briefly, the atomic volume and the number of atoms are the inverse of each other: so that we have $\backslash frac\{1\}\{6\}$ and 6, $\backslash frac\{1\}\{2\}$ and 2, 1 and 1.

If, while we make hydrogen the standard of atomic weights, we make air the standard of the specific gravity of gases, then we obtain, as quotients, a series of numbers equally comparable among themselves, but less simple and easy to retain than the above. We should have, for example, $1\; \backslash div\; 0.0694\; =\; 14.409$ for hydrogen; $8\; \backslash div\; 1.1026\; =\; 7.2554$ for oxygen; and $16\; \backslash div\; 6.9000\; =\; 2.3188$, for the atomic volumes of hydrogen, oxygen, and sulphur respectively; and these numbers are to each other as 1, $\backslash frac\{1\}\{2\}$, and $\backslash frac\{1\}\{6\}$.

In the case of solids and liquids, the relation between atomic weight and specific gravity is far from being so simple, in consequence of the force of cohesion interfering with and disturbing the results. We cannot ascertain whether the atoms of solid bodies have the same size in different bodies or not; and we cannot tell whether the difference of specific gravity depends on a difference in the number

of the atoms in an equal volume, a difference in the size of the atoms, or a difference in the size of the interstices between the particles, or possibly on two or more of these causes.

Some chemists assume that there are no interstices, but that the atoms wholly fill up the space within the circumference of the body. On this supposition, the atomic weight, divided by the specific gravity (in solids and liquids), must give the atomic volume. It is difficult, however, to admit the absence of interstices or pores in solids and liquids, if we consider them formed of atoms; and it is perhaps better to use the term equivalent volume, instead of atomic volume.

The equivalent volume, then, of a solid or liquid is obtained by dividing the atomic weight (or rather equivalent number) by the specific gravity in the solid or liquid state. Water, the standard for the specific gravity of liquids and solids, may be made the standard of equivalent volumes.

Thus the atomic weight of water, = 9, divided by its specific gravity, = 1, gives the quotient 9 as its equivalent volume. The atomic weight of potassium, 39.26, divided by its specific gravity, 0.865, gives 45.387 for its equivalent volume; and the atomic weight of carbon, 6, divided by its specific gravity in the form of diamond, = 3.5, the quotient 1.717 for the equivalent volume of the diamond.

On the other hand, the specific gravity, divided by the atomic weight, gives the relative number of atoms in a given volume, and in the case of potassium this is $0.865\; \backslash div\; 39.26\; =\; 0.0220$; in the case of carbon it is $3.5\; \backslash div\; 6.04\; =\; 0.5794$. Finally, in the case of water, the relative number of atoms in a given volume, which may be made the standard, is $1\; \backslash div\; 9\; =\; 0.1111$. If, for convenience, the number for water is made 1000, then that for potassium becomes 198.0, and that for carbon becomes 5215.

Assuming, likewise for convenience, the equivalent volume of water (the standard) to be (instead of 9) 1000, the equivalent volume of potassium becomes 5043, and that of carbon 191.666.

We thus perceive that the equivalent (or atomic) volume of carbon is about twenty-five times less than that of potassium, and that the number of atoms of carbon contained in a given volume is about twenty-five times greater than in the case of potassium. This compression of so large a number of atoms into a given volume may be the cause of the great hardness of the diamond.

The whole subject of equivalent volumes is full of interest; but, as chemists have only recently begun to study solid and liquid bodies in this point of view, our knowledge on the subject is still very imperfect and limited. For what has lately been done, we are chiefly indebted to Kopp and to Schroeder.

Playfair and Joule have very lately published the first part of an elaborate investigation into the volumes occupied by bodies both in the solid form and when dissolved in water; and they have obtained results of an unexpected nature as well as of very great value.

The reader is referred to their paper in the Memoirs of the Chemical Society. Here we have only space to allude to the subject, and to mention that, among other curious results, these chemists have found that many salts, when dissolved in water, do not add to the bulk of the water more than is due to the water actually present in the salts. Thus, for example, alum, 1 eq. of which contains 23 equivalents of the elements potassium, aluminum, sulphur, and oxygen, besides 24 eqs. of water, dissolves in water without increasing its bulk more than the addition of the 24 eqs. of water must necessarily do; so that the 23 eqs. above mentioned occupy no additional space, and must either be contained in the pores or interstices of the water, or disappear altogether as far as the occupying of space is concerned, if water be supposed to have no pores.

They have further shown that when salts do add to the bulk of the water in which they are dissolved, the increase of the bulk corresponds to that of a volume, or some multiple of a volume, of water. It is evident that these and similar researches must soon greatly extend our knowledge of the mechanical constitution of matter.

There is another circumstance connected with chemical combination which must be briefly noticed. We have seen that bodies of different composition may have the same crystalline form, if they only differ by the substitution of one element for another isomorphous with it. But we find, also, that bodies have the same composition, that is, the same relative quantities or proportions of their elements, while their properties are totally different. In fact, this phenomenon in compound bodies is closely analogous to allotropism in elementary ones. It is not very frequent in inorganic chemistry, but very common in organic compounds. Thus cyanogen (a compound radical) forms with oxygen three compounds, cyanic acid, fulminic acid, and cyanuric acid, all three of which have exactly the same proportions of cyanogen and oxygen, but differ entirely in properties. This is accounted for by the fact, that in the first 1 eq. of cyanogen is united to 1 eq. of oxygen; in the second, 2 eqs. of cyanogen to 2 eqs. of oxygen; and in the third, 3 eqs. of cyanogen to 3 eqs. of oxygen. Such compounds are said to be polymeric, and correspond to those allotropic modifications of elements in which the molecules contain a different number of atoms.

But there are also cases in which not only the proportion but the absolute number of the elements is the same, while yet the properties are different. Aldehyde consists of 4 eqs. carbon, 4 eqs. hydrogen, and 2 eqs. oxygen. Acetic ether contains 8 eqs. carbon, 8 eqs. hydrogen, and 4 eqs. oxygen. It is, therefore, polymeric with aldehyde; and besides the difference in the number of atoms, there is an obvious difference in their arrangement. But butyric acid contains, like acetic ether, 8 eqs. of carbon, 8 eqs. hydrogen, and 4 eqs. oxygen. In these two compounds, then, the absolute number of atoms is the same, as well as their relative proportion; but the two bodies are totally different. This can only be explained by a difference in the arrangement of the same number of the same elements. Accordingly, in acetic ether they are arranged in two groups, acetic acid and ether, combined together; while in butyric acid two other groups, namely, dry butyric acid and water, are united. Such compounds are said to be isomeric, and sometimes metameric.

It is quite evident that both isomerism and polymerism are natural corollaries from the atomic hypothesis. If elements combine by atoms, it is obvious that a compound which contains twice or thrice as many atoms as another, or in which the same number or a multiple of it is arranged differently, might be expected, a priori, to exhibit different properties; and when we consider that, while there is no limit to the number of atoms which may combine, the slightest difference in arrangement will produce new properties as surely as a difference in number, we can see how vast is the number of different compounds producible on these principles from a few elements. This, as we shall see, is remarkably the case in organic chemistry.

Having now briefly noticed the most important laws of chemical combination, as well as various circumstances which are connected with it, it might be expected that we should offer some explanation of these phenomena. But we must confess that we are unable to do this. In the ordinary language of science, combination is attributed to a force called chemical attraction or affinity. But it is easy to see, that to say that two bodies combine together in consequence of chemical attraction or affinity between them, while it has the appearance of an explanation, really explains nothing, and amounts to no more than saying, that these

bodies combine because they combine. For when we ask Chemistry, what is chemical attraction, we can obtain no other answer than that it is the force or cause in virtue of which different bodies unite; just as cohesion is the cause in virtue of which two or more portions of the same body are held together. But in neither case have we any knowledge of the real nature of this force, nor can we say with certainty that an attraction exists. The same is true of gravitation, and of all natural forces. We know not their nature, and we think that we explain them when we call them attractions, and speak of the attraction of gravitation, the attraction of cohesion, and chemical attraction. All that we really know is that a compound is formed, but how, or in what manner, we cannot tell; just as we see that particles cohere to form a mass, and that all bodies possess weight, without being able in the least to explain either phenomenon.

So true is this, that while some admit as many forces as there are different phenomena, others conceive that gravitation, cohesion, and affinity, as well as other so-called attractions, are all the result of one and the same force, acting under different circumstances, or at different distances. This, however, for the present, is purely hypothetical. We know nothing of natural forces except their effects, just as we know nothing of matter except its properties.

One hypothesis, however, concerning chemical attraction has enjoyed considerable popularity, and, as it has affected the language of the science, it is necessary to notice it.

It has long been known that bodies charged with electricity either attract or repel each other; and it has been found that there are two kinds or forms of electricity, which are called positive and negative, or vitreous and resinous. It is still a disputed point whether these are really distinct, or whether the positive be merely the excess, the negative the deficiency, of the same agent. However this may be, when two bodies are charged with the same kind of electricity, they repel; when charged with opposite kinds, they attract each other; that is, in the former case they move away from each other, in the latter they move towards each other.

Now, according to the electrical hypothesis, all bodies are naturally charged with one or the other electricity, and such as are negatively charged are supposed to attract such as are positively electric. Oxygen is said to be strongly negative, and such bodies as hydrogen and potassium as strongly positive. These form the extremes of an electric series of all the elements, which, when placed in their proper position, are found to be negative in regard to all lying between them and the positive end; and positive in regard to all between them and the negative end of the scale. Thus sulphur is positive with regard to oxygen, chlorine, bromine, iodine, and fluorine, all of which lie nearer the negative end; but negative to all the metals, and to hydrogen, which lie towards the positive end.

Now it is found that the strongest affinities, that is, the strongest tendencies to combine, really do exist between the most negative and the most positive elements, and that two bodies lying contiguous in the scale exhibit usually feeble affinity for each other, and when combined are very easily separated.

According to this electro-chemical hypothesis, every compound, however complex, consists of two constituents, one positive, the other negative, which unite because they are so.

One circumstance which seems to favour this hypothesis is this, that when any compound is decomposed by a current of electricity, the positive element always appears at the negative pole, and the negative element at the positive pole, which is supposed to be due to the fact, that unlike electricities attract each other, while like electricities repel each other.

On the other hand, there are circumstances which do not so easily admit of explanation. The elements, in their un-

Chemistry. combined state, exhibit no electricity of any kind, such as is supposed to be inherent in them. Again, when two oppositely electrified bodies attract each other, as soon as they touch, the electricity of both is neutralized and disappears, or becomes insensible. Now, if two elements combine, in virtue of their opposite electricities, when combined these electricities must be neutralized and disappear. What, then, retains the elements in combination, since we cannot suppose them, after contact, to continue powerfully negative and positive? This difficulty renders the electro-chemical hypothesis very doubtful.

Nevertheless, there is a very decided relation between electricity and chemical action. For every case of chemical action produces electricity, and this is the foundation of the galvanic pile and battery. Moreover, electricity, in certain circumstances, promotes chemical action, while in others it causes decomposition. It has been shown by Faraday, that the electricity of a galvanic arrangement may be accurately measured by the amount of decomposition it effects. We cannot tell what is the real nature of the connection between chemical action and electricity, but that relation exists. Indeed, it would seem that force, using that term in its most general sense, may take the form either of chemical force or of electricity, and may also be transformed into mechanical force, or into heat, and that any one of these, as also light and magnetism, is capable of taking the form of any of the others.

This seems to indicate that all these forms of energy are modifications of one and the same power. This is a subject of much interest, both theoretically and practically. It has been much studied of late, and promises to clear up our views on these important points.

In the meantime, the electro-chemical doctrine is the foundation of the prevalent or dual view of chemistry, and of the arrangement usually followed, as will be more fully explained when we come to treat of the elements in their order.

Before doing so, however, it is necessary to explain the system of notation employed by chemists, without a knowledge of which all chemical works would be utterly unintelligible. This notation is a system of abbreviation, the symbols employed being simply the names of the elements contracted into one or two letters. No other signs are used except ciphers, and the signs +, -, and =, in their usual acceptance, signifying addition, subtraction, and equality.

The symbols for the elements are the first letters of their Latin names; and where two or more have the same initial letter, a second letter is added to distinguish them. Thus O is the symbol for oxygen, H for hydrogen, C for carbon; while osmium is represented by Os, mercury (hydrargyrum) by Hg, chlorine by Cl. These symbols are all given in the second column of the table of equivalents, p. 438.

The symbol of an element, by itself, signifies an equivalent of that element. O stands not for oxygen abstractly, but for 1 eq., or 8 parts by weight of oxygen (hydrogen = 1).

A cipher subjoined to the symbol multiplies it. Thus O₂ means 3 eqs. of oxygen, Cl₂ 5 eqs. of chlorine, &c.

Two symbols placed side by side express a compound of 1 eq. of each element. Thus, HO means water, a compound of 1 eq. of hydrogen and 1 of oxygen. HCl, hydrochloric acid, 1 eq. of hydrogen and 1 of chlorine; KCl, chloride of potassium, 1 eq. of potassium (kalium) and 1 of chlorine, &c. &c. Two symbols joined by the sign +, signify the same thing; but for the sake of brevity the former notation is preferred for binary compounds.

If a cipher be attached to the right of and below one of the symbols, it multiplies that symbol only. Thus, SO₃ is sulphuric acid, 1 eq. of sulphur and 3 of oxygen; PO₅ is phosphoric acid, 1 eq. of phosphorus and 5 of oxygen; Cu₂O is suboxide of copper, 2 eqs. of copper and 1 of

oxygen; Fe₂O₃ is sesquioxide of iron, 2 eqs. of iron and 3 eqs. of oxygen.

When two binary compounds, that is, compounds of two elements, are placed side by side, or separated only by a comma, or united by the sign +, this means a compound of the two. Thus HO, SO₃, is hydrated sulphuric acid, composed of 1 eq. of water and 1 of sulphuric acid; and it may be written either HO, SO₃, or HO + SO₃; but the comma is preferred to indicate the union of two binary compounds.

When three or more binary compounds are combined, it sometimes becomes advisable to use the sign + in addition to the comma. Thus KO, HO, 2 SO₃, or KO, SO₃ + HO, SO₃, equally represent the bisulphate or acid sulphate of potash; which is viewed as either composed of 1 eq. of potash (KO), 1 of water, and 2 of sulphuric acid, or 1 eq. of neutral sulphate of potash, and 1 eq. of hydrated sulphuric acid.

When a large cipher is prefixed to the symbol of a compound, or to those of several compounds, it multiplies all to the next comma or + sign; as in the first formula for bisulphate of potash above given, in which 2 SO₃ means 2 eqs. of sulphuric acid. If written thus, 2 SO₃, KO, HO, the 2 multiplies only the SO₃. When it is required to multiply the whole of a complex formula, as often happens in the chemical equations to be presently described, the symbols to be multiplied are included within brackets. Thus 2 (KO, SO₃) means 2 eqs. of neutral sulphate of potash; while 2 KO, SO₃, would signify a compound of 2 eqs. of potash and 1 of sulphuric acid.

Such are the whole of the rules for the use of our chemical notation in expressing the view we entertain of the constitution of chemical compounds. This notation, or these formulae are very simple, very brief, and thoroughly clear and unmistakable. They are mere abbreviations, and, as such, are of the utmost value, as enabling us to express, in very small space, and in a form very obvious to the senses, a number of facts concerning the relative weights and the supposed arrangement of the combined elements, which would, if written fully, occupy pages of print. To take an example, alum, a complex salt, is represented by the formula—

This tells us that chemists consider alum as composed of 1 eq. of neutral sulphate of potash, 1 eq. of neutral tersulphate of alumina (composed of 1 eq. of alumina, a sesquioxide, and 3 eqs. of sulphuric acid), and 24 eqs. of water. This is seen at a glance, but we see also much more. We see that in the sulphate of potash the oxygen of the acid is 3 times that of the base; and that in the sulphate of alumina, where the base contains 3 eqs. of oxygen, the oxygen of the acid is still 3 times as much, since there are 3 eqs. of the acid to 1 of the base. We see that 1 eq. of alum contains 1 eq. of potassium, 2 of aluminum, 4 of sulphur, 24 of hydrogen, and 40 of oxygen; that all the hydrogen is in the form of water, &c. &c. And all this, and much more, is to be easily seen in a formula which occupies but half a line.

We see, then, that our symbols and formulae enable us to express, in the most compendious form, the fullest and most exact account of what we believe to be the constitution of any compound. Should we require to express a different view, it is done with the utmost facility, and the different or opposite views are presented to the eye in the most distinct and intelligible manner. Thus, oil of vitriol, if considered as being hydrated sulphuric acid, a compound of the dry acid and water, is represented by HO, SO₃. But if considered as a compound of hydrogen and not of water, its formula becomes H, SO₃, which means that hydrogen is here supposed to be combined with the hypothetical compound radical SO₃; a view which is regarded

as more probable than the other. In like manner, sulphate of potash may be viewed either as $KO,\; SO\_3$, or $K,\; SO\_3$. It is impossible for us to ascertain with certainty which of these views is the true one, because in both cases the resulting composition is the same. It is only the arrangement or constitution which differs. But the two opinions are quite as clearly and far more briefly expressed in the formulae than they can be in words. Whatever view can be imagined as to the constitution of any compound, however complex, it is just as easily expressed by symbols, and they are therefore in constant use. The student must, therefore, in the outset, make himself familiar with them, which is very easily done.

These, however, are not nearly the whole of the advantages derived from the use of symbols and formulae. They enable us, besides, to express, in by far the most convenient and the clearest manner, the results, real or supposed, of any chemical change among substances of known composition. This is done by means of equations, which are just as simple as the formulae themselves.

The first half or section of the equation contains the formula or formulae of the substance or substances which undergo change. The second consists of the formulae of the substances formed or liberated by the change. These must of course be equal, otherwise the explanation is false or imperfect.

The action of potassium on water is thus represented:—

This shows that potassium seizes the oxygen of water; while the hydrogen is liberated. Again, the action of hydrochloric acid on oxide of silver is thus represented:—

Here the hydrogen of the acid and the oxygen of the oxide unite to form water; while the chlorine of the acid and the metal of the oxide combine to form chloride of silver.

The more complex the example, the more useful is the equation, as placing the whole clearly before the eye. When hydrochloric acid acts on peroxide of manganese the equation is as follows:—

This shows that for 1 eq. of peroxide of manganese 2 eqs. of hydrochloric acid are required; that 1 eq. of chlorine unites with the metal, forming chloride of manganese; that the 2 eqs. of hydrogen form water with the 2 eqs. of oxygen; and that 1 eq. of chlorine is set free. In this way chlorine is prepared.

Another method for obtaining chlorine consists in the action of sulphuric acid on chloride of sodium (sea-salt) and peroxide of manganese. The equation is

which tells us that 1 eq. of salt, 1 of peroxide of manganese, and 2 of sulphuric acid, yield 1 eq. of sulphate of soda, 1 eq. of sulphate of protoxide of manganese, and 1 eq. of chlorine. This is the manufacturing process.

Such equations offer another great advantage, that, namely, of enabling us to calculate precisely how much of the materials we ought to use, and how much of the products we ought to obtain.

Nitric acid is prepared by the action of oil of vitriol, or hydrated sulphuric acid, or nitrate of potash, according to the equation—

Here 2 eqs. of oil of vitriol act on 1 eq. of nitrate of potash, yielding 1 eq. of bisulphate of potash and 1 eq. of hydrated nitric acid.

Now since the equivalent of a compound is the sum of those of its elements, the equivalent of nitrate of potash must be (in round numbers)

Nitrate of Potash.

Oil of Vitriol.

Hence we must use, for 102 parts of nitrate of potash, 98 of oil of vitriol. The products are—

Bisulphate of Potash.

Nitric Acid (hydrated).

On the one hand the materials employed, 102 parts of nitrate of potash, and 98 of oil of vitriol, amount to 200 parts. On the other, the products, 137 parts of bisulphate of potash, and 63 of hydrated nitric acid, also amount to 200 parts. Any other proportion of the materials would imply waste or loss of a part.

Should any theoretical view of a process require to be tested, it may be done by comparing the amount actually obtained of any one or all of the products with that indicated by the proper equation. Should they differ very much, it is a proof, if nothing have been lost in the process, that the equation, and consequently the theory on which it rested, is erroneous.

The use of these equations, and the calculations connected with them, enable us to see clearly how it happens, that when two neutral salts decompose each other, the resulting salts are also neutral. Thus, if sulphate of potash and nitrate of baryta, two neutral salts, act on each other, they yield nitrate of potash and sulphate of baryta, which are also neutral, according to this equation—

Here the equivalents are, before the change,

After the change, we have

Now it will be seen that the quantity of potash which neutralizes 40 parts of sulphuric acid, namely 48 parts, is exactly sufficient to neutralize 54 of nitric acid; and that the quantity of nitric acid which neutralizes 76 of baryta, namely 54 parts, exactly suffices to neutralize 48 parts of potash. In short, as before stated, the equivalent numbers are proportional, and this is seen in the equations without the trouble of calculation.

From the more complex nature of organic compounds, the formulae which express them are more complex, but in principle precisely the same. Many organic compounds contain 3 or 4 elements: thus, oxalic acid is $C\_2O\_2HO$, or, according to the most recent researches, double this, $C\_4H\_2O\_4$, which implies, that in the first formula it is a monobasic acid, containing 1 eq. of basic water, replaceable by bases; and in the second that it is dibasic, that is, has 2 eqs. of replaceable water. Acetic acid is $C\_2H\_3O\_2HO$; butyric acid, $C\_3H\_5O\_2HO$; benzoic acid $C\_7H\_5O\_2HO$. Oxide of ethyle or ether is $C\_2H\_5O$; alcohol, its hydrate, is $C\_2H\_5O,\; HO$; all these consist of 3 elements, carbon, hydrogen, and oxygen, the number of eqs. of which is often much larger. Thus stearic acid is $C\_\{18\}H\_\{32\}O\_2HO$; Melissic acid is $C\_\{10\}H\_\{16\}O\_2HO$. Then we have hydrocyanic acid, which is $C\_2NH$; cyanic acid, $C\_2NO,\; HO$; cyanuric acid, $C\_6N\_3O\_3,\; 3HO$; urea, $C\_2H\_4N\_2O\_2$; gly-

Chemistry. cocaine, $C\_8H\_\{17\}NO\_4$; leucine, $C\_\{12\}H\_\{21\}NO\_4$; ethylamine, $C\_4H\_9N$; phenylamine, $C\_6H\_5N$; quinine, $C\_\{20\}H\_\{24\}NO\_4$; indigo, $C\_\{15\}H\_\{10\}NO\_4$; gelatine, $C\_\{19\}H\_\{28\}N\_2O\_\{12\}$; albumen, $C\_\{12\}H\_\{20\}N\_2O\_4$ &c., all of which contain nitrogen, which in hydrocyanic acid only is without oxygen, and in albumen is also associated with sulphur. We have given these organic formulae to show both their somewhat complex nature, and the extreme facility and conciseness of the mode of notation. It will be seen that all these organic compounds are formed from a very small number of elements, never, if we exclude the incombustible part or ash, exceeding 5, and most frequently only 3, carbon, hydrogen, and oxygen, or 4, these with the additions of nitrogen. The same four elements form innumerable organic compounds.

The most complicated changes in organic chemistry are just as easily expressed. When sugar undergoes fermentation it yields alcohol and carbonic acid, as in the equation—

In many organic compounds, and, indeed, in entire series of compounds, we admit the existence of certain permanent groups, which are called compound radicals, and which play exactly the part of elementary bodies. It is often convenient to adopt a single symbol for each such compound radical; for this gives to its compounds the simple formulae of inorganic bodies. The following are some of the admitted compound radicals:—

We do not assert that all so-called compound radicals actually exist in a separate state, although cyanogen and several others do so. But even if none of them did, we derive, from assuming their existence, the advantage of being able to represent a series of compounds related together in a very simple manner. Thus the compounds of cyanogen above named may be written $HCy$, $CyO$, $HO$, $Cy\_2O\_3$, $3HO$. Those of ethyle (ether and alcohol) become $AeO$ and $AeO,\; HO$. Acetic acid becomes $AcO\_2,\; HO$, and benzoic acid $BzO,\; HO$. Ethylamine takes the formula $NH\_2Ac$, or $AdAe$ ($Ad=NH\_2$ stands for amide); perchloride of formyle or chloroform, $C\_2HCl\_3$, may be also written $FCl\_3$. In this way, even very complex compounds may be expressed as simply as the simplest inorganic bodies. Compare hydrocyanic acid, $HCy$ with hydrochloric acid, $HCl$; oxide of ethyle, $AeO$, with oxide of potassium; acetic acid (hydrated) $AcO\_2,\; HO$, with hydrated sulphuric acid, $SO\_3,\; HO\_2$; and hyduret of benzoyle, $C\_\{14\}H\_9O\_2H$ or $BzH$, with sulphuretted hydrogen $SH$.

Organic acids and bases are sometimes written simply with the initial letter of their names, above which is placed
+ for a base, and - for an acid. Thus $\backslash overline\{Q\}$ means quinine, $\backslash overline\{M\}$, morphine. $\backslash overline\{T\}$ stands for tartaric, and $\backslash overline\{C\}$ for citric acid.

CHEMICAL NOMENCLATURE.

Before proceeding to the description of the elements and of their compounds, it is necessary to say a few words on the nomenclature employed. This is far from being in a satisfactory state. It was first proposed about the time of Lavoisier's discoveries, and was founded on his theory of combustion. But the science has made, of late years, such rapid progress, that we can no longer employ the same principles of nomenclature, at least in many cases; and yet, as no better system has hitherto been proposed, the old one retains its place, with a large number of heterogeneous additions.

The elements are named on various principles. Such as have long been known retain their old names, as iron, sul-

phur, &c.; those of more recent discovery have been named either from some property, or, in the case of some metals, after some of the planets, &c. Thus, oxygen is so named because it was supposed to be the cause of acidity in compounds; hydrogen, from its producing water; nitrogen, from producing nitric acid; chlorine and iodine, from their colour; bromine, from its smell, &c. Of the metals, potassium, sodium, magnesium, calcium, strontium, from occurring in potash, soda, magnesia, lime (calx), strontia, &c.; barium, from the weight of its compounds; cerium, mercury, palladium, selenium, tellurium, and uranium, after the heavenly bodies—Ceres, Mercury, Pallas, the Moon, the Earth, and Uranus. The remaining elements are named on similar principles. The Latin names of all metals end in um, as aurum, argentum, cuprum, plumbum; for gold, silver, copper, and lead. Only one non-metallic body, selenium, ends in um, but this is because it was at first supposed to be a metal. On the whole, the names of the elements are arbitrary, and the less significant they are the better. Oxygen, for example, is no longer considered as the only producer of acids, for many acids are known which contain no oxygen, and the name would be almost better applied to hydrogen, which forms many acid compounds.

In naming binary compounds, we first observe whether they are acid or not. If acid, they are called acids, and the name of one of the elements is prefixed, with the termination ic or ous. Thus we have sulphuric acid, carbonic acid, phosphoric acid, nitric acid, and many others.

When there are two acids containing the same element united to oxygen, that which contains less oxygen has the termination in ous. Thus sulphurous and nitrous acids contain less oxygen than sulphuric and nitric acids.

Should other acids containing the same elements be discovered, the prefix hypo is employed in addition. Thus, after sulphurous and sulphuric acid had long been known, other acids were discovered, one of which is called hyposulphurous, another hyposulphuric acid; which means that the former contains less oxygen than sulphurous, the latter less oxygen than sulphuric acid. We have also hyponitrous, hypophosphorous, hypochlorous acids, and hypochloric acid.

When a compound of oxygen is not acid, it is called an oxide of the element which is combined with oxygen. Water is an oxide of hydrogen, and we have oxides of nitrogen, carbon, and all the metals.

An oxide consisting of 1 eq. of each element is called a protoxide. Water is protoxide of hydrogen, and there are protoxides of almost all the metals, as protoxide of lead, of iron, of manganese, &c.

When the proportion in an oxide is that of 1 eq. of the other element to $1\backslash frac\{1\}\{2\}$ of oxygen, that is, 2 to 3, it is called a sesquioxide, as, sesquioxides of iron, aluminum, chromium, manganese.

When we find 1 eq. to 2 of oxygen, it is called a deuteroxide or binoxide, and in some cases peroxide, meaning the oxide having most oxygen; as deuteroxide of hydrogen, of nitrogen, of tin, of manganese, of lead; the two latter, as well as the first, being often called peroxides.

When the proportion is 1 to 3, the compound is called a teroxide, as teroxide of antimony.

When a compound, instead of oxygen, contains chlorine, bromine, iodine, or fluorine, it is called a chloride, bromide, iodide, or fluoride; and we have protochlorides, protobromides, deutochlorides, &c., terchlorides, and perchlorides or perbromides, periodides, &c., just as with oxides.

When the compound contains, instead of oxygen, chlorine, &c., sulphur, phosphorus, carbon, selenium, &c., it is called a sulphuret, phosphuret, carburet, selenuret, &c.

Examples—Chloride of sodium, bromide of carbon, iodide of lead, deutochloride or bichloride of mercury, protochloride of mercury, deutiodide of mercury, terchloride of antimony, terfluoride of silicon, perchloride of manganese,

Chemistry. sulphuret of potassium, phosphuret of lead, carbonet of iron, seleniuret of copper, bisulphuret of carbon, bicarburet of nitrogen, tersulphuret of arsenic, protosulphuret of iron, pentasulphuret of potassium, &c. &c.

Such is the method employed in naming binary compounds.

When two binary compounds combine, this is expressed in the name. If one of them be acid, as commonly happens, and the other basic or alkaline, the name of the acid goes first, with the termination in ate for an acid in ite, that in ite for an acid in ous. Thus sulphuric acid and potash (oxide of potassium) form sulphate of potash; nitric acid and oxide of lead, form nitrate of (oxide of) lead. Sulphurous acid and ammonia form sulphite of ammonia.

When there are 2 or 3 eqs. of acid, or more, to 1 eq. of base, we prefix bi, ter, quadri, &c. Thus we have bisulphate of potash, tersilicate of lime, quadroxalate of potash.

When there are 2, 3, or more eqs. of base to one of acid, the prefixes di, tri, &c., are employed; as dinitrate of mercury, trisilicate of potash.

When we wish to express, in general, excess of acid over base, we use the prefix super, as supercarbonate of lime; for excess of base, generally, we use sub, as subnitrate of mercury, subsulphate of mercury. In double salts we express the names of both bases, as sulphate of alumina and potash, oxalate of chromium and potash.

Observe, that instead of sulphate, nitrate, &c., of oxide of lead, oxide of iron, &c., we say, for shortness sake, sulphate of lead, nitrate of silver, &c. But it is always understood that the compound contains an oxide of the metal united with the acid. The oxides of potassium, sodium, lithium, barium, strontium, calcium, magnesium, aluminum, yttrium, glucinum, zirconium, and thorium, are also called potash, soda, lithia, baryta, strontia, lime, magnesia, alumina, yttria, glucina, zirconia, thorina, and we speak of the salts of potash, soda, &c., instead of saying the salts of oxide of potassium, or of potassium.

It is in organic chemistry that the chief difficulties of nomenclature occur. In the case of organic acids they are named like inorganic acids, as acetic, butyric, oxalic, tartaric, citric acids, &c., but generally from the source which yields them. The organic bases are made to terminate in ine, as morphine, quinine, nicotine, glycine, methylamine, ethylamine, dimethylamine, triethylamine, &c., as will be fully explained when we come to treat of them.

Compound organic radicals generally terminate in yle, as ethyle, methyle, cetyle, acetyle, formyle, benzoyle, and their compounds, are named accordingly; as oxide or chloride of ethyle, terchloride of formyle, hyduret of acetyle, hyduret of benzoyle, cyanide of ethyle, &c.

But with these and a few similar exceptions, the enormous number of organic compounds have been named without system, and constitute a perfect chaos, which will continue to annoy chemists until a rational and consistent method of nomenclature shall be devised. An attempt has been made by Gmelin; but the names, though systematic, are so uncouth, and in many cases so liable to be confounded together, that no one has adopted Gmelin's system.

We are now prepared to enter on the consideration of the elements individually, and of the compounds formed by their union. In order to facilitate the statement and comprehension of the facts, some arrangement must be followed; and, although the science is not sufficiently advanced to admit of a perfectly consistent classification, yet, by attending to the observed analogies, we may obtain a very convenient working arrangement.

It is generally founded on the electro-chemical relations of the elements, as already explained; but it coincides very closely with the obvious and natural division of the elements into metals and non-metallic bodies or metalloids, in the first instance; and then the further subdivision of both, according to the degree of affinity for oxygen.

Chemistry. When any binary compound of oxygen is decomposed by an electric current, the oxygen invariably appears at the positive pole, while the other element goes to the negative pole. This proves that oxygen is always negative compared to all other elements. In like manner, hydrogen always goes to the negative pole, as do also such metals as potassium and sodium. Now, as there are no substances which have so strong an affinity for oxygen as these three, it is plain that the degree of affinity is measured by the degree of electric opposition between two elements. At one end of the electro-chemical scale, therefore, we place oxygen as being the most negative of all bodies; at the other, hydrogen, potassium, and sodium, as being the most positive. All the other elements are placed according to their relation to oxygen on the one hand, to hydrogen and the alkaline metals on the other.

Thus chlorine is positive in relation to oxygen, for when a compound of these elements is decomposed by the current, the chlorine appears at the negative pole; but it is negative in relation to all metals, and to all or nearly all the non-metallic bodies except oxygen. The true position of fluorine is unknown; but it is intensely negative, and must stand very near to oxygen and chlorine, perhaps between them, perhaps even outside of oxygen, as it may prove to be negative in regard to that element. But with the exception of this doubtful case, chlorine is negative to all but oxygen, and therefore ranks next to it. Bromine and iodine follow closely, and are positive to oxygen and chlorine, negative to all the rest, bromine being the more negative of these two. Carbon, sulphur, phosphorus, selenium, boron, and silicon, are all strongly negative with regard to hydrogen and metals, but yet even more strongly positive to oxygen; they exhibit, therefore, strong affinities on both sides. Nitrogen holds a nearly isolated position, having affinities of considerable energy to all the other classes of elements, but not belonging decidedly to any. This is a most important character, which is essentially necessary to fit nitrogen for the important part it has to perform as an indispensable element of all living organisms or tissues.

The metals, as a class, are positive to oxygen, chlorine, and its congeners, sulphur, and the like; but their positive energy varies from the highest in potassium, sodium, and lithium, to the lowest in gold, platinum, and iridium. Consequently these latter metals, and others like them, are negative to such as potassium. Such metals as mercury, copper, lead, zinc, iron, &c., hold an intermediate position, and are negative to such as potassium, positive to gold, &c. It is difficult to arrange the individual metals accurately, but they are easily divided into groups, as we shall presently see.

The only so-called non-metallic element which has a place among the most intensely positive is hydrogen. But there is much reason to think that hydrogen, which has hitherto been seen only as a gas, is in reality the gas or vapour of a very volatile metal. At all temperatures above 600°, the vapour of mercury, and at a white heat those of arsenic, zinc, cadmium, potassium, and sodium, are gases, just as hydrogen is at ordinary temperatures.

We shall commence with oxygen gas, beyond all question the most important element, and consider after it those which stand nearest to it. But, in consequence of the extreme importance for the understanding of what is to follow, and also in a practical sense of hydrogen and of nitrogen—which last has no well-marked place of its own—we shall interpolate these elements between oxygen and chlorine. After chlorine will come bromine, iodine, fluorine—the last placed here from the perfect analogy between its compounds and those of the three preceding elements; and after them, because its exact place is uncertain, since we are not acquainted with it in an uncombined state. This is the group of the negative, that is, highly negative, non-metallic ele-

Chemistry. ments, or supporters of combustion, with the addition of hydrogen and nitrogen.

The next group is that of the less negative or more positive non-metallic bodies, namely, carbon, sulphur, selenium, phosphorus, boron, and silicon. These elements are also called the combustible non-metallic bodies or metalloids. The non-metallic elements are 13 in number, but it seems probable that silicon will prove to be metallic, since, according to very recent statements, it has been deposited on metallic surfaces with a high metallic lustre. This, however, requires confirmation. If it prove true, silicon will then be called silicon.

The metalloids or non-metallic elements are bad conductors or non-conductors of heat and of electricity, and are destitute of the metallic lustre. At least, none of them combine the two properties of conducting power and metallic lustre. Selenium has a lustre approaching the metallic, and so has iodine, while carbon in one state conducts electricity well.

The metals, on the other hand, all possess both these characters; they are excellent conductors of heat and electricity, and they are distinguished by the metallic lustre when in a compact state, although in the state of powder or that of a spongy mass they may not exhibit this character. It will always appear, however, on burnishing, even in dull metallic powder.

We shall begin the study of the metals with the most highly positive, or those at the opposite end of the scale from oxygen, and proceed regularly to the more negative or less positive metals. We begin, then, among the metals, with potassium and sodium, pass on through lithium, which, with the two first, form the group of the alkaline metals, or metals of the alkalies proper, to those of the alkaline earths, barium, strontium, calcium, and magnesium; thence to those of the earths proper, aluminum, zirconium, yttrium, glucinum, thorium. The next group is that of those of the heavy or common metals, which form very strong bases with oxygen, iron, manganese, zinc, cadmium, cobalt, nickel, and tin. The next contains such metals as are remarkable for forming acids with oxygen, arsenic, antimony, chromium, vanadium, molybdenum, tungsten, titanium, columbium. The next consists of metals having a less powerful attraction for oxygen, yet still forming bases with it, as bismuth, copper, lead, mercury. And the last group contains the noble metals, or those which have the feeblest attraction for oxygen, such as silver, gold, platinum, iridium, palladium, rhodium, ruthenium, osmium. There are a few metals which have not been mentioned in this enumeration, because they are not as yet known in a state of purity, and therefore their precise place is not quite fixed. Cerium, lanthanum, didymium, erbium, and terbium seem to have their place between the third and fourth groups, but nearer the third if not within it. Pelopium and niobium belong apparently to the fifth or acidifiable metals. But the whole of these imperfectly known metals are of so little importance, being very rare, and as yet applicable to no purpose, that we shall do no more than indicate their existence.

Such is the arrangement we propose to follow. It will be found to a great extent natural, for most of the groups are strongly marked by nature, and indeed interesting from the extreme analogy between their constituent members.

When we have described the two first elements, oxygen and hydrogen, we shall then proceed, before taking up a third, to give an account of the compounds formed by the two first; when the third element, nitrogen, has been reviewed, we shall mention its compounds, first with oxygen, then with hydrogen; and, in short, under every element we shall describe its compounds with those previously considered. By this means we shall very soon become acquainted with the most important compounds, and thus acquire a much more extensive knowledge of chemistry in a

short time than we could in a far longer period, if we described all the elements first, before speaking of any compounds, which would be a more strict and regular plan.

Our limited space compels us to be brief, so that we shall only notice essential and practically important points, and allude shortly to the technical applications of the bodies described. It will be impossible to enter into any detail as to the processes by which these substances are prepared. For these the reader must consult larger works.

NON-METALLIC ELEMENTS, OR METALLOIDS.

(A.) NEGATIVE, OR SUPPORTERS OF COMBUSTION.

1. Oxygen.

Symbol O. Equivalent = 8.

Oxygen is the most abundant and the most important of all the elements. It constitutes $\backslash frac\{1\}\{4\}$th of the atmosphere, $\backslash frac\{3\}\{4\}$ths of all the water in our globe, and fully $\backslash frac\{1\}\{2\}$d of alumina and silica, $\backslash frac\{1\}\{2\}$d of lime, and $\backslash frac\{1\}\{2\}$th of potash; these being the chief ingredients of all the rocks in the earth's crust, and of the soils on its surface. It also forms a part of all the other ingredients of rocks, such as magnesia, oxide of iron, and carbonic acid, and of all abundant minerals, except only rock salt, and the sulphurets of a few metals. It is, moreover, an essential component part of all organized beings, of all tissues, and of all but a very few products of vegetable life. The quantity of it in our earth is prodigious beyond all conception, and its presence in the atmosphere is indispensable to animal life.

In its purest state, it is only known to us as a gas, which cannot be condensed into the liquid state by the most intense cold combined with the highest pressure that we can apply to it; that is, its boiling and melting points are beyond the reach of our appliances.

Oxygen gas is best obtained by applying heat to the chlorate of potash, a salt which yields, when heated, the whole of its oxygen, as is shown in the following equation:—

It is also obtained by heating peroxide of manganese, which loses part of its oxygen, and is reduced to sesquioxide. Thus:—

Oxygen may be obtained by several other processes, which we have not space to mention.

It is collected over water, which has hardly any action on it. It is a transparent, colourless, tasteless, and inodorous gas. It is distinguished from other gases by its power of supporting combustion. Any burning body introduced into it burns with increased intensity and brilliancy. A candle,

or a stick, or a string, with the smallest spark on it, bursts out into flame in this gas. Nay, a candle just blown out, and without even a spark, if introduced, while the wick is yet warm, into oxygen, bursts into a brilliant white flame in a short time.

Sulphur and charcoal burn very brightly in it; phos-

Fig. 1.

Chemistry. Phosphorus gives out a splendid white light of dazzling intensity; and even iron, if heated to redness in it, burns with bright sparks, especially in the form of steel, as a watch-spring. Many other metals may be burned in oxygen. In short, it combines with most of the elements, with the phenomenon of combustion, or evolution of heat and light, to be presently explained.

Oxygen gas is a little heavier than atmospheric air; that is, if a given bulk of air, at a certain temperature, and under a certain pressure, weigh 1000 parts (grains, ounces, or pounds), an equal volume of oxygen gas, at the same temperature and pressure, will weigh 1111 parts. According to some, this number is a little too high, the true number being 1102.6. But the first number is very near the truth, and it is easily remembered. The number 1111 is said to represent the specific gravity or density of the gas, compared to air as a standard, 1000 being the number adopted as the specific gravity of air. The reader will understand that specific gravity or density means the relative weights of equal volumes, and is always, therefore, a relative, not an absolute property. For gases, air is made the standard of density; for liquids and solids, water.

An animal confined in oxygen at first feels little inconvenience; but after a time it appears to stimulate too powerfully, and would ultimately cause death, although it is not, like some gases, irremediable.

Oxygen enters into combination with all the other elements, except only fluorine, which has not yet been made to combine with oxygen. The compounds of oxygen are of very great importance, as has been already stated in various places. Many compounds of oxygen, especially such as contain three or more, and occasionally two eqs. of oxygen for one eq. of the other elements, possess acid properties; and it was formerly supposed that every acid must contain oxygen, and that oxygen was the cause of acidity. But we are now acquainted with a large number of acids containing no oxygen; and if any element can be said to be the cause of acidity, it is rather hydrogen, which is found in many acids without oxygen, and in most of those which contain oxygen. But, in truth, acidity is a property belonging to the compounds, and not derived in any peculiar manner from either oxygen or hydrogen, both of which form many compounds which are not only not acid, but basic or alkaline; so much so, that all protoxides of metals are powerful bases, and neutralize acids.

The uses of oxygen are most important. Diluted with nitrogen in the atmosphere, it becomes not only respirable by animals, but indispensable to their existence. It is also essential to all such processes of combustion as are carried on in atmospheric air, and therefore assists in producing artificial heat and light. It is also a necessary agent in the important process of the decay of dead vegetable and animal matter, that process by which these are not only prevented from accumulating and proving injurious, but are at the same time converted by oxidation into those compounds which form the food of a new generation of plants. In fact, decay is a slow combustion, without the evolution of light, and with a very slow and hardly sensible development of heat.

It was at one time believed that oxygen was indispensable to every combustion; that every combustion was an oxidation. But it is now seen that it is only in such combustions as occur in our atmosphere or in pure oxygen that oxygen is essential. There are many combustions in which oxygen has no share. Thus phosphorus, antimony, and most of the metals in a state of fine division, take fire spontaneously, and burn in chlorine gas, or in the vapour of bromine or of iodine. Many metals, when heated, burn in the vapour of sulphur; but, as all ordinary and useful combustions take place in air and depend on oxygen, the term combustion is still commonly applied to those cases in which oxygen is concerned. The strict definition of the term, how-

ever, is this, chemical combination, attended by the evolution of heat and light. Chemistry.

In reference to combustion, oxygen is often called the supporter of combustion, and the bodies which burn in it or in air are called combustible. In like manner, chlorine and its congeners are also called supporters of combustion; but, if we reflect on the definition of combustion just given, we shall see that the two combining bodies are equally supporters of the combustion, and equally combustible. The common language is founded on the fact, that the heat and light appear to proceed from the so-called combustible (such as a candle) in the air or oxygen. But this is an illusion, depending on the fact that one of the two bodies (the air or oxygen) is a gas; and the other—whether solid, liquid, or gaseous—is placed in the middle of it, and surrounded by it. In these circumstances, combustion can only take place where the two bodies meet, which is only at the surface of the central body or combustible, whether it be a coal, or oil, or a jet of gas. If we reverse the conditions, and cause, for example, a jet of oxygen to escape into an atmosphere of coal gas, and apply a light to it, the oxygen appears to take fire, as the coal-gas did in air, and continues to burn, the heat and light appearing at the surface of the jet of oxygen, because there only action can take place. In this form of experiment we might call oxygen the combustible, and coal-gas the supporter of combustion. But, as before stated, both bodies are alike supporters of the combustion, and both alike combustible, and the appearances depend on the arrangement of the experiment. In all ordinary cases, oxygen appears to be the supporter of combustion, and the other body the combustible, so that practically these terms are so applied without leading to misconception.

The binary compounds of oxygen with the other elements are generally very important, and are of three kinds; 1st, Acid, as sulphuric, nitric, and chromic acids; 2d, Alkaline or basic, as the protoxides of potassium, sodium, calcium, iron, lead, silver, &c., and sesquioxides, such as those of aluminum, iron, chromium; 3d, Neutral bodies, that is, neither acid nor basic, but sometimes playing the part of one or the other; as water, deutoxide of manganese, deutoxide of lead. The nomenclature of all oxides has been already explained.

The acid oxides are negative compared with the basic oxides, which are positive. Hence they tend to combine together, and when such a ternary compound is decomposed by the electric current, the acid always appears at the positive, the base at the negative pole. Such compounds of a negative with a positive oxide are saline bodies, although when they are insoluble in water the usual saline characters are not seen. Examples, sulphate of potash, $KO,\; SO\_3$; nitrate of soda, $NaO,\; NO\_3$; these and many others have all the characters of salts. Sulphate of baryta, $BaO,\; SO\_3$; carbonate of lime, $CaO,\; CO\_2$; these are insoluble, but yet are true salts.

These ternary saline compounds of oxygen are undoubtedly formed when the acid and the base meet. Thus $KO$ and $SO\_3$ form $KO,\; SO\_3$, sulphate of potash. But we do not know that in these salts the base and acid continue to exist as such; and it is now considered probable that they do not, but that the acid and base, in forming the salt, undergo a change, whereby the metal of the base forms one constituent, and all the other elements together, that is, the acid + the oxygen of the base form the other. It is only a question of the arrangement of the elements which are certainly present, for nothing is added, and nothing taken away. The old view is expressed in the formula $KO,\; SO\_3$, for sulphate of potash. The new one, which is exactly equal to it, is expressed by $K,\; SO\_3$, and the group $SO\_3$ is supposed to form a compound radical, analogous in properties to chlorine, and like it forming salts by combining with metals. We shall return to this question when treating of acids, such as hydrochloric acid or sulphuric acid. Mean-

Chemistry. time, the reader should render himself familiar with both views of the salts which contain oxygen.

We now, for the reasons formerly given, deviate from the strictly natural order, and proceed to describe hydrogen, as being, next to oxygen, perhaps the most important of the elements, especially with regard to its compounds.

2. Hydrogen.

Symbol H. Equivalent = 1.

This element is, like oxygen, very abundant in nature, but almost invariably in some form of combination. It is said to occur uncombined among the gaseous products of volcanoes, which is not improbable. But it is chiefly found in union with oxygen, in water, of which it constitutes $\backslash frac\{1\}\{8\}$th part by weight. As the quantity of water in the sea, rivers, lakes, marshes, and streams, in the atmosphere, suspended as vapour, or separating in the form of clouds, rain, snow, hail, and dew, is prodigiously great; the actual amount of hydrogen is very large. Besides the sources of water just mentioned, all animals and vegetables contain from $\backslash frac\{1\}\{4\}$ to $\backslash frac\{2\}\{3\}$ths of their weight of water; and hydrogen is also an essential constituent of the animal and vegetable tissues, and of all animal and vegetable products, more especially of all such as are oily or resinous, and of such bodies as wood, starch, sugar, gum, and the vegetable acids and bases of alcohol, ether, and similar compounds; of coal, bitumen, asphalt, petroleum, and fire-damp, the explosive gas of coal mines, in the mineral kingdom.

Hydrogen is easily prepared by the action of zinc or iron filings, or clippings, in diluted sulphuric and hydrochloric acids. The change is represented in the following equations.

Fig. 1.

It will be seen, that in both cases the metal (and iron acts precisely as zinc does) takes the place of the hydrogen, which is set free. The sulphate of zinc and chloride of zinc formed are salts as analogous to each other as the acids were. The two processes are therefore in fact the same; but yet the equations differ. This depends on the view taken of the constitution of sulphuric acid or oil of vitriol, which in the first equation is represented as composed of water and dry acid. But if we take the newer view of the constitution of the acid, and consider it as a compound of hydrogen with the hypothetical compound, radical SO₃, if we represent it by H, SO₃, instead of the equivalent formula HO, SO₃, then the two equations become as like each other as the two operations are. Thus—

The only difference now is, that in one case the zinc and hydrogen are united to a simple radical, chlorine, in the other to the compound radical SO₃. But it must be remembered that chlorine is only called simple or elementary because we cannot prove it to be compound, not because we know absolutely that it is simple. Indeed, it is considered probable that chlorine will one day prove to be a compound; and, in that case, the two equations would be exactly of the same kind. As it is, if we use the newer expression for sulphuric acid and sulphate of zinc, the analogy in all essential points is complete. Both acids are compounds of hydrogen, and both the salts formed are compounds of the metal with the radicals Cl and SO₃. The

fact, that while the older view of sulphuric acid (which regards it as a compound of water with dry acid) represents these two similar processes by different equations, the newer view gives them the same form; is a very strong argument in favour of the latter, according to which sulphuric acid, and all acids which, like it, contain hydrogen along with oxygen, are compounds of hydrogen and of water, like hydrochloric acid and all acids analogous to it, of whose constitution only one view is possible. What used to be two series of acids are thus reduced to one series, and the salts of both may be regarded as forming part of the same series as their respective acids, if we define acids and salts under the name of saline compounds as formed of hydrogen and metals (which have many points of analogy) with radicals, whether simple or compound.

Let R stand for any radical capable of forming an acid and salts, any salt-forming radical; and let X represent any metal or hydrogen; then the general formula of all such acids as sulphuric and hydrochloric acids, and all such salts as sulphates and chlorides—that is to say, such acids as nitric, phosphoric, selenic, silicic acids, &c., as well as hydrobromic, hydriodic, hydrofluoric, hydrosulphuric acids, &c., and the nitrates, phosphates, seleniates, silicates, &c.—and the bromides, iodides, fluorides, and sulphurets, &c.—the universal formula for all such compounds is XR.

For X we may substitute the symbol of any metal, or of hydrogen; and for R, that of any salt forming radical, simple, or compound; and thus we obtain such special cases of the general formula, as HCl; HF; HS; H, SO₃; H, NO₃; H, PO₃; KCl; NaBr; CaF; PbS; Ba, SO₄; Ag, NO₃; &c. The reader, by referring to the table of symbols, will easily discover what are the elements whose symbols are used, and will see that the series of which the general formula, as above explained, is XR, includes a very large number of acids and of salts, perfectly analogous in properties, which were formerly, and even still are, placed in two different series.

We have taken the opportunity of the process for making hydrogen to explain the principle on which this great simplification is effected, and so large a number of compounds of hydrogen, namely, all the important acids, are classed together, instead of separately. The reader will find frequent occasion to avail himself of what has now been explained.

To return to hydrogen. When prepared as above explained, it appears as a gas, and is collected over water. Like oxygen, it is known only in the form of gas, never having been yet liquefied by the most intense cold and pressure. When pure it is colourless, tasteless, and inodorous, but when prepared from zinc, and especially from iron, it has a peculiar smell, arising from the presence of an oil formed by impurities in the metals. Hydrogen prepared from water by the electric current has no smell. It is the lightest body known, its specific gravity being 69.4 compared to air as 1000. It is exactly 16 times lighter than oxygen.

A burning body introduced into hydrogen is extinguished for want of oxygen; but the hydrogen, being heated by the flame, and in contact with the oxygen of the air at the mouth of the vessel, takes fire and burns away very rapidly from its lightness, if the mouth of the vessel be upwards; and very slowly, for the same reason, when the mouth of the jar is turned downwards, which prevents it from readily mixing with the air. The flame of burning hydrogen is very feebly luminous, but intensely hot, that is, much heat and little light are evolved in its combination with oxygen. When hydrogen is mixed with oxygen, or even with air, and a light applied, explosion ensues, and both gases disappear, water being the only product. The mixed gases are also exploded by the electric spark, and by contact with platinum, in the form of sponge or of powder, as we shall see presently.

Chemistry. Hydrogen is highly positive, and has a very strong affinity for oxygen, with which it forms at least two compounds, water, $\backslash text\{HO\}$, and deutoxide of hydrogen, $\backslash text\{HO\}\_2$; and

probably a third, a teroxide or peroxide, $\backslash text\{HO\}\_3$, if recent statements to that effect shall be confirmed. By reason of this attraction for oxygen, hydrogen is much used by chemists as a deoxidizing agent, especially when aided by a red or white heat. When the gas is passed through a red-hot tube containing the oxide to be reduced, water is formed, and the substance which was combined with oxygen is left in a state of purity.

It has also been used from its lightness for filling balloons; and a balloon of very moderate size filled with hydrogen has a great ascending power. But coal-gas is so much cheaper, and, although heavier than hydrogen, yet with a large balloon has so much ascending power, that it has supplanted hydrogen for this purpose.

It is used also, when burned in a jet with oxygen, which gives what is called the oxy-hydrogen blow-pipe, to produce the most intense heat that is known, except perhaps that of a powerful galvanic battery, and that of the sun's rays collected in the focus of a large burning-glass. This will be explained in treating of the combination of oxygen and hydrogen.

Lastly, hydrogen is made, in this country, the standard or unity of equivalent or atomic weights; for which purpose it is well adapted as having by far the lowest equivalent among the elements.

Hydrogen with Oxygen.

(1.) Water $\backslash text\{H\}\_2\backslash text\{O\}$.

We have already mentioned the various circumstances which cause these two gases to combine. If mixed and kept at the ordinary temperature, or in any heat short of a strong red heat, they have no action on each other. But the contact of flame, or of any other red-hot body, the passage of the electric spark, which is intensely hot, and the contact of platinum, cause the combination to take place with explosion. The flame and the electric spark act by their intense heat; but the action of platinum is more obscure. Spongy platinum, and the fine powder of that metal called platinum black, although cold, cause the mixed gases to explode as readily as flame does. Even polished slips of platinum, if perfectly clean, will cause them to combine, though more slowly; and it is then seen that the contact of the cold metal first causes a part of the gases to unite; this produces warmth; the metal being warmed by it, acts more vigorously; more heat is developed, so that by degrees the metal becomes red-hot, and if any of the mixed gases be still uncombined, it causes them to explode. In the case of the powder or the sponge, especially the former, all this takes place so rapidly from the enormous surface of the metal, that it becomes red-hot as soon as it is introduced, and fires the mixture as rapidly as a flame.

But all this does not explain how the platinum causes the gases to combine at first. There are two views on this point, both suggestions, and neither established. One supposes that the surface of the metal attracts the particles of

both gases, because there is not the same repulsion between Chemistry. a solid and a gas as between two gases, or the particles of the same gas; that consequently the particles of the two gases come on the surface of the metal nearer to each other than elsewhere, and near enough for affinity to act. According to the other view, proposed by Doebereiner, the pores of the spongy or powdery metal are filled with oxygen absorbed from the air, and condensed with so great a force as to occupy only $\backslash frac\{1\}\{50\}$th part of its former bulk. This condensation he ascribes to a peculiar molecular attraction; and he states, that the powder when heated gives off a large amount of oxygen, although there is certainly no combination between the metal and any part of the oxygen. As this condensed oxygen is denser than if it were liquid, although still gaseous, its particles come near enough to those of the hydrogen to combine with them. Either explanation, if true, still leaves unexplained how the platinum attracts the gases, or condenses the oxygen. Doebereiner constructed a lamp for instantaneous light on this principle. It is very ingenious, but the spongy platinum is apt to lose its efficacy, from the vapours of various substances adhering to it. Other metals, and even other porous bodies, exhibit the same property, though in an inferior degree, and usually only when aided by heat.

When oxygen and hydrogen, from whatever cause, combine, either by combustion or explosion, it is always in the proportions to form water; that is, invariably 2 volumes of hydrogen gas to 1 volume of oxygen gas. Any excess of

either is left uncombined. As the two gases both disappear, it is plain that, if we measure the volume that has disappeared, that is, the contraction in volume, by comparing the residue with the original volume, $\backslash frac\{2\}\{3\}$ds of that loss of volume must be hydrogen and $\backslash frac\{1\}\{3\}$d oxygen. This enables us to use hydrogen to determine the amount of free oxygen in air, or in any gaseous mixture, as will be explained under atmospheric air.

The composition of water has been proved in many different ways, both synthetically and analytically. It is a point of great importance with reference to the analysis of other bodies.

Synthetically, oxygen and hydrogen, carefully measured, are made to burn together in a jet, and the water produced is collected and weighed. The weight of the gases consumed is known from their volume and densities, and that of the water is exactly their sum. 8 lb. of oxygen and 1 lb. of hydrogen yield 9 lb. of water. $\backslash text\{O\}\; +\; \backslash text\{H\}\; =\; \backslash text\{HO\}$.

Or hydrogen gas is passed over a weighed portion of oxide of copper in a tube heated to redness. (See Fig. 5.) The oxygen of the oxide combines with hydrogen, forming water, which is carried onward by the current of gas into a small weighed apparatus containing chloride of calcium, which retains all the water. The increase of weight in this

Chemistry. vessel gives the amount of water formed; the loss of weight in that containing the oxide of copper now reduced to a metal, gives the amount of oxygen, and the difference is the hydrogen. 39.7 grains of oxide of copper yield 9 grains of water, and 31.7 of metallic copper. The loss, 8 grains, is oxygen, and the difference, 1 grain, is hydrogen. The equation is $\backslash text\{CuO\}\; +\; \backslash text\{H\}\; =\; \backslash text\{H\}\_2\backslash text\{O\}\; +\; \backslash text\{Cu\}$.

Analytically, the composition of water is proved by passing a weighed quantity of its vapour over a red-hot metal, such as iron, excluding air. The increase of weight in the metal, which is oxidized at the expense of the water, gives the oxygen, and the difference is hydrogen; or the

hydrogen may be collected and measured or weighed. This process, however, is difficult, and only used as an illustration. Potassium placed in contact with water, under the surface is oxidized, disengaging hydrogen, which may be collected, the oxide dissolves in the remaining water, and this may be evaporated, and the amount of potash determined in the form of a salt, such as the sulphate.

Or, water may be decomposed by the electric current, and the two gases separately collected and measured. They are invariably in the proportion of 2 volumes of hydrogen to 1 of oxygen, and as oxygen is 16 times heavier than hydrogen, 1 volume of oxygen must weigh 8 times as much as 2 volumes of hydrogen.

In these different ways the composition of water has been proved. Its formula is $\backslash text\{H\}\_2\backslash text\{O\}$, at least in this country. On the Continent, where atoms and volumes are believed to agree, the formula of water is $\backslash text\{H\}\_2\backslash text\{O\}$. This is merely a question of what is the weight of 1 atom of hydrogen. We consider that 1 atom of hydrogen weighs the eighth part of 1 atom of oxygen. The French chemists say that it weighs only $\backslash frac\{1\}\{8\}$ of 1 atom of oxygen, so that 2 atoms are required to yield the proportion of 1 to 8. The same remark applies to the volume. We consider the atom of hydrogen as represented by twice the volume of 1 atom of oxygen; they regard 1 atom of hydrogen as having the same volume as 1 atom of oxygen. But they agree with us as to the equivalent, for they regard the equivalent of hydrogen, in water, as formed of 2 atoms.

The properties of water are well known. It is of all compounds the most important, being essential to both animal and vegetable life. It has neither colour, taste, nor smell, melts at $32^\backslash circ$ Fahr., and boils at $212^\backslash circ$ Fahr. It is therefore solid at all temperatures below $32^\backslash circ$, and gaseous at all above $212^\backslash circ$.

It has a remarkable power of dissolving solid matters, and on this its use chiefly depends.

Water is the standard of specific gravity for liquid and solid bodies, and its specific gravity is made 1, 10, 100, or 1000, according to the writer's choice.

It is remarkable that the point of greatest density in water is not, as might be expected, about $33^\backslash circ$, when it is about to freeze, but about $39^\backslash circ.5$, and it becomes lighter both above and below that degree of heat. This has a most important practical effect, for when deep water is cooled

by frost, the water keeps sinking as it cools till its temperature falls below $39^\backslash circ.5$ when it becomes lighter, and remains at the surface till frozen, and then it is lighter still. From that point, therefore, there is no further mixture of the colder with the warmer water, and below the crust of ice and the upper stratum of water, there remains a mass of water at $39^\backslash circ.5$, which is only cooled very slowly by conduction. This is one reason why deep lakes are never entirely frozen, and why in the hardest winters the ice never extends far from the surface. Were it otherwise, the freezing would begin at the bottom, the whole mass, however deep, would soon be frozen, and the summer might not suffice to melt it all again.

There is another reason why freezing and melting are slow operations. When a body melts, a large amount of heat disappears or becomes latent, and is employed in giving the liquid form to the solid body, without raising its temperature above the melting point till all is melted. In congelation again, this latent heat reappears and prevents the temperature from falling below the freezing point, however intense the cold applied, till the whole is frozen. For these reasons, the freezing of large masses of water, and the melting of large masses of ice, are both very slow operations.

The same thing occurs when water is boiled or converted into vapour. This takes place at $212^\backslash circ$; but so great is the amount of heat that disappears in forming steam, that if a vessel of water be placed on white-hot coals, the water will boil away slowly, but will never rise above $212^\backslash circ$, while any water retains the liquid form. And, in like manner, when steam is condensed by cold, all this heat becomes sensible and keeps up the temperature at $212^\backslash circ$, whatever cold be applied, till the vapour is entirely liquefied, when the temperature begins at once to fall.

A familiar proof of the enormous amount of latent heat in steam is obtained in the fact, that boiling water at $212^\backslash circ$, and steam at $212^\backslash circ$, produce totally different effects on the skin. The scald from steam is very greatly more severe than that from boiling water at the same temperature. Again, if we add 4 oz. of boiling water to 16 oz. of water at $60^\backslash circ$, the mixture is barely tepid; but if we force 4 oz. of steam at $212^\backslash circ$ into 16 oz. of water at $60^\backslash circ$, the whole 20 oz. will be found to boil briskly. Hence the use of steam as a heating agent, which has the advantage that it cannot heat any substance in open vessels beyond $212^\backslash circ$, and cannot therefore char or injure them.

Besides boiling at $212^\backslash circ$, under the ordinary atmospheric pressure, water is slowly converted into vapour, or it evaporates at all temperatures. In this way water rises into the air from the sea, lakes, rivers, &c., and falls again as rain, snow, and dew. Some heat is required for evaporation as well as for boiling; the necessary heat is taken from the surrounding bodies, and cold is the result. Water, at a temperature not very far above the freezing point, is so much cooled by its own evaporation, that part of it is frozen by the evaporation of the rest. Ice is actually thus obtained on cool nights in hot climates. The same result is obtained at ordinary temperatures, if the pressure be diminished, which accelerates evaporation. This is well illustrated by Wollaston's cryophorus.

Under diminished pressure water boils at temperatures below $212^\backslash circ$. For this reason it boils lower on the top of a mountain than at its base. $212^\backslash circ$ is the boiling point of water at the sea level, under the average pressure, or with the barometer at or near 29 to 30 inches. In ascending mountains, the boiling point falls $1^\backslash circ$ Fahr. for every 440 feet of ascent. The height of mountains may be thus pretty accurately measured, provided the state of the barometer be noted below, at the level of the sea, as well as on the hill top.

Under increased pressure, as when the steam is not allowed to escape freely, water boils at temperatures above

Chemistry. 212°, and higher, in proportion as the pressure is higher; as is well shown by Dr Marcot's machine. This occurs in the high-pressure steam engine, in which the steam cannot escape till its elasticity is so far increased by heat, as to overcome the pressure on the piston or on the safety-valve. When such increased pressure is suddenly taken off, by allowing the steam to escape, while the temperature is far above 212°, the steam rushes out with great force, and the temperature within rapidly sinks to 212°. This is the principle of the high-pressure engine; while in the low-pressure form, the steam is simply allowed to enter the cylinder below the piston, and after it has forced up the piston-rod, is condensed by a jet of cold water, steam being at the same moment admitted above the piston, and so on alternately. For full details on these matters the reader must refer to works on mechanics and on the steam engine.

Water, when frozen, increases in volume to a considerable extent, so that ice is lighter than water, and floats on its surface. This expansion takes place with irresistible force; and hence the freezing of small portions of water which has filled the spaces between the layers of the hardest rocks, bursts them asunder. For this reason hard frost is perhaps the most powerful agent in the disintegration of rocks.

The chemical characters of water are very important. It combines with dry or anhydrous acids, producing the hydrated acids, which, as has been explained, may be viewed also as hydrogen acids. Thus hydrated sulphuric acid may be represented either as $\backslash text\{HO\}\_2\backslash text\{SO\}\_3$ or $\backslash text\{H\}\_2\backslash text\{SO\}\_4$. Water combines also with anhydrous bases, forming hydrates of the bases. Thus anhydrous lime, $\backslash text\{CaO\}$, combines with water, forming hydrate of lime or slaked lime, $\backslash text\{CaO\}\backslash cdot\backslash text\{H\}\_2\backslash text\{O\}$. Thirdly, water combines with neutral salts, forming hydrates. Thus sulphate of magnesia, $\backslash text\{MgO\}\backslash cdot\backslash text\{SO\}\_3$, combines with 1 eq. of water to form the hydrated salt, $\backslash text\{MgO\}\backslash cdot\backslash text\{SO\}\_3\backslash cdot\backslash text\{H\}\_2\backslash text\{O\}$. Lastly, water combines with salts, both anhydrous and hydrated, in another form, which is called water of crystallization. The hydrated sulphate of magnesia, $\backslash text\{MgO\}\backslash cdot\backslash text\{SO\}\_3\backslash cdot\backslash text\{H\}\_2\backslash text\{O\}$, takes up 5 eqs. more of water to form the usual crystallized salt, which is represented as follows, $\backslash text\{MgO\}\backslash cdot\backslash text\{SO\}\_3\backslash cdot\backslash text\{H\}\_2\backslash text\{O\}\; +\; 5\; \backslash text\{aq.\}$ It is easily shown that these 5 eqs. are in a different state of combination from the first eq.; for a gentle heat expels the five, but a red heat is required to expel the sixth. Again, this equivalent of water may be replaced by a neutral salt or by sulphate of potash, yielding the double salt, $\backslash text\{MgO\}\backslash cdot\backslash text\{SO\}\_3\; +\; \backslash text\{K\}\_2\backslash text\{SO\}\_4$, which cannot be done with the five others. Here we see that water plays the part of a neutral salt; whereas in hydrated acids it plays that of a base, and in hydrated bases that of an acid; being replaceable in the former case by bases, in the latter by acids, and, as we have seen, in the case of its combining with neutral salts, by neutral salts.

But there is a fourth form in which water combines with other bodies, that, namely, in which it dissolves them. Here we have not the same distinct evidence of its combining in definite proportions; inasmuch as, although there be a limit to the quantity of any solid substance water can dissolve, there is none to the quantity of water that may be made to combine with a given weight of any such body. In other words, aqueous solutions may be diluted to any extent. It is probable that water forms with each substance certain definite liquid

compounds, characterized by their special densities, boiling points, &c.; and that these are miscible in any proportion. Professor Graham, who has already investigated the diffusion of gases, has long been engaged in profound researches on the mutual diffusion of aqueous solutions. He has obtained very interesting results; but our space forbids our entering on these, especially as the investigation is still in progress.

Heat, as a general rule, increases the solvent power of water, while cold diminishes it. There are a few exceptions. Thus common salt is not materially more soluble in hot than in cold water; and hydrate of lime is less soluble in hot water than in cold. In general we can obtain crystals of substances soluble in water by boiling them with that fluid till it is saturated, when, on cooling, as its solvent power diminishes, it deposits in crystals what it cannot retain in solution. Crystals are also obtained by the slow or spontaneous evaporation of aqueous solutions. Some substances crystallize best in the one method, some in the other.

In consequence of its great solvent power, water is never found pure in nature. Even rain-water dissolves gases, and minute portions of solid matters which it meets with in falling through the atmosphere. We can always detect ammonia, carbonic acid, and sea-salt in rain-water, even when collected in clean vessels at a distance from towns. The salt is no doubt carried into the air from the sea by high winds, and is therefore more abundant during or immediately after a strong gale blowing from the sea. As soon as rain reaches the earth, it begins to dissolve a part of almost everything it meets with in the rocks or soil through which it filters, such as sea-salt, gypsum, silicate of potash, carbonate of lime (in virtue of the carbonic acid already in the rain-water, as well as of that it takes up from the soil), carbonate of magnesia, carbonate of iron, compounds of iodine or bromine, if present; also fluoride of calcium, phosphate of lime (soluble in solution of carbonic acid), and organic matters. According to the amount of dissolved matter, which varies exceedingly, spring and river water is hard or soft, or becomes mineral water. When the solid matter dissolved does not exceed from 1 to 6 or 8 grains per gallon, the water is soft, especially if the proportion of supercarbonate of lime be small. But when more solid matter is present, especially supercarbonate of lime and gypsum, when there are from 10 to 15, 20, 30, 50, 80, 100, or even, as sometimes happens, 150 grains of solid matter in a gallon, the water is more or less hard, the salts of lime decomposing soap, and rendering necessary a large consumption of it to obtain detergent effects. Moreover, the supercarbonate of lime is decomposed on standing, or when boiled, and deposits a crust of neutral carbonate of lime, which renders hard water totally unfit for use in steam boilers, and in fact ruinous to the boilers by the effects of the crust, to the presence of which many explosions have been justly referred. The methods of detecting the presence of these impurities, and of improving the quality of hard water, will be mentioned under the head of the substances named.

When the amount of foreign matter exceeds a certain proportion, and especially if it consist of salts of soda and magnesia, of iron, of sulphurets of metals, or of sulphuretted hydrogen, of alkalies, or of carbonic acid, the water is called a mineral water, although all water is mineral. Sea-water is a true mineral water. Such as are charged with carbonic acid are called acidulous or sparkling waters; such as contain saline matters are saline waters; those containing sulphur, iron, or alkalies, are respectively sulphureous, chalybeate, or alkaline waters.

Pure water can only be had by distillation; and even in distilled water there are often traces of ammonia and carbonic acid.

Chemistry. The importance of water to man cannot be over-estimated. It is essential to both animal and vegetable life, the best soil being barren if no rain fall. It is almost equally essential to almost all chemical operations, among which vegetation may be included. It is through water that plants are supplied with their whole food, namely, carbonic acid, ammonia, silicate of potash, sulphate of lime, sea-salt, carbonates of iron, magnesia, lime; phosphates of lime and magnesia; iodides, bromides, and fluorides; the carbonates and phosphates, which are insoluble in pure water, being dissolved in water containing free carbonic acid, which also contributes powerfully to the disintegration, and to the rendering soluble of the useful constituents of felspar and of clay, which is half-decomposed felspar.

In the animal body, every part, solid or liquid, consists chiefly of water, which forms at least $\backslash frac\{2\}\{3\}$ths of the weight of all the soft solids, and about $\backslash frac\{1\}\{3\}$ths of that of the bones.

We shall see that, besides its use as a solvent, and as a constituent of organized tissues, water takes a share, by its elements, in the formation of almost all organic compounds whatever. It is certainly of all compounds the most valuable and useful.

(2.) Deutoxide of Hydrogen, $\backslash text\{HO\}\_2$, = 17.

Water can take up an additional eq. of oxygen, and is thus converted into the deutoxide. This compound is obtained, in a diluted form, by the action of hydrochloric acid on successive portions of deutoxide of barium, which sets free an eq. of the deutoxide; thus, $\backslash text\{BaO\}\_2\; +\; \backslash text\{HCl\}\; =\; \backslash text\{BaCl\}\; +\; \backslash text\{HO\}\_2$. The deutoxide of hydrogen and the chloride of barium both dissolve in the water in which the process is carried on. Sulphuric acid, cautiously added, removes the barium as the insoluble sulphate, leaving free hydrochloric acid as before; thus, $\backslash text\{BaCl\}\; +\; \backslash text\{HO\}\_2\; +\; \backslash text\{SO\}\_3\; =\; \backslash text\{BaO\}\_2\; +\; \backslash text\{HCl\}$. In the filtered liquid, a second portion of deutoxide of barium is dissolved, and the whole process is repeated till the liquid is sufficiently charged. It is thus concentrated by evaporation in vacuo, and when pure has the consistence of a thin syrup. Its preparation is a tedious and delicate process; and when made it can only be preserved in a freezing mixture for a time, as it undergoes spontaneous decomposition, and that very rapidly, at ordinary temperatures. Phosphoric acid, and even sulphuric acid, may be substituted for the hydrochloric acid.

The deutoxide of hydrogen readily parts with half its oxygen, and is reduced to water by contact with organic matters. It disorganizes the skin, causing a white spot. It gives off oxygen spontaneously, and the presence of various powders hastens the change. When oxide of silver, for example, is introduced into it, rapid effervescence ensues, and there is given off, not only the second eq. of oxygen of the deutoxide, but also the oxygen of the oxide of silver, which is thus reduced to the metallic state, or deprived of oxygen, by one of the most powerful oxidizing agents, which we should rather expect to yield oxygen to it. The explanation appears to be, that the motion of the particles of decomposing deutoxide is mechanically communicated to those of the oxide of silver, and the equilibrium of that compound being thus destroyed its elements separate. Several other oxides are decomposed in the same way.

The deutoxide of hydrogen has been occasionally used to oxidize certain substances in chemical research, and has been proposed as a remedy. In both ways it may probably prove useful, but the difficulty of preparing it and preserving it will, for the present, very much limit its employment.

(3.) Ozone.

This name has been given, on account of its pungent smell, to a substance formed under several circumstances; as when electric sparks are passed through dry oxygen gas;

or better, when water is decomposed by the electric current, when it (ozone) is found in the oxygen collected at the positive pole; and, finally, when phosphorus is slowly oxidized in atmospheric air, which thus acquires the odour of ozone.

By whatever method it is formed, its quantity is always singularly small, so that it has hitherto been found impossible to obtain it pure, or to analyse it quantitatively. But its odour is very powerful, resembling that of chlorine, and also that which is observed in thunderstorms. As to its other properties, it is a most energetic oxidizing agent, and therefore contains oxygen, probably in large quantity. Its presence is easily detected either by the smell, or by its power of decomposing iodide of potassium, setting free the iodine, and of oxidizing the salts of protoxide of manganese, so as to form peroxide.

From these characters, and from its occurring where oxygen is in the nascent state, as at the positive pole of the battery, and in presence of water, it is supposed, on good grounds, to be either an isomeric (or allotropic) form of deutoxide of hydrogen, or a teroxide of hydrogen. Recent researches tend to show that there are probably two compounds included under the name ozone, and that the ozone formed in the electrolytic decomposition of water is really teroxide of hydrogen, while that formed in dry oxygen gas is an allotropic modification of oxygen gas.

It is probable, from the facility with which it is formed, and the equal facility with which it is decomposed, that ozone is very often produced in the atmosphere, and acts powerfully on other bodies. It destroys organic substances, even when diluted with much air or oxygen, so that neither cork nor caoutchouc can be used to connect the apparatus in which it is prepared. Hence it probably plays an important part in hastening the oxidation or decay of dead organic matter.

3. Nitrogen.

Symbol N. Equivalent = 14.

This element, like hydrogen, is here introduced out of its strict place, on account of its great importance, and especially of the importance of its compounds.

Nitrogen occurs, mixed with oxygen, in our atmosphere, of which, when dry, it constitutes about $\backslash frac\{4\}\{5\}$ths. It is also found in all organized tissues, and in the juices of plants and animals, as an essential constituent. In the crust of the earth it occurs in certain spots, in the form of the nitrates of potash and soda, that is, compounds of these bases with nitric acid. It is also an ingredient of ammonia, which exists in the atmosphere, and is produced from volcanoes.

Nitrogen is best obtained from air, by removing the oxygen by means of phosphorus, which, if made to burn under a bell gas, inverted over water, combines with the oxygen, forming phosphoric acid, and this acid is dissolved by the water, leaving the nitrogen pure. It may also be obtained by the action of chlorine on a solution of ammonia.

However prepared, nitrogen always appears as a transparent, colourless, tasteless, and

Fig. 11.

Fig. 12.

Chemistry. inodorous gas. Water absorbs only a very minute portion of it. It is rather lighter than air, in the proportion of 972.2 to 1000. This must be so, since a mixture of it with oxygen, which is rather heavier than air, has the density of air. It extinguishes the flame of any burning body, and does not take fire itself as hydrogen does. It cannot support animal life when respired, but it is not poisonous, as some gases are. Animals soon die in pure nitrogen gas, but this is simply from the want of oxygen. For at all times we breathe air, which is only a mixture of oxygen with four times its volume of nitrogen, not only without injury, but with advantage, for it serves to dilute the oxygen and render it fit for respiration. It will be seen that nitrogen is characterized entirely by negative properties.

It is one of those gases which has hitherto resisted all attempts to condense it into the liquid state.

Nitrogen has very remarkable and important chemical relations. Its affinities for oxygen and for hydrogen are both considerable, and pretty nearly equal, and it is capable of combining both with the positive and the negative elements. But its compounds, from the fact that it has affinities in every direction, and those not the strongest, are in general easily decomposed, and frequently with explosive violence. To those complex organic compounds in which it is an essential ingredient, it gives a tendency to undergo transformations of all kinds, by which they are fitted for the functions they have to perform; and it is such compounds alone that undergo the transformation called putrefaction, and another similar one, by which they become ferments, or excitors of fermentation.

Nitrogen and Oxygen.

Nitrogen forms several compounds with oxygen, which have been already alluded to as an example of multiple proportions. Besides these, there is atmospheric air, a mixture, not a compound, of these gases, which we shall consider after them.

(1.) Protoxide of Nitrogen, $\backslash text\{NO\}\; =\; 22$.

This compound is obtained by the action of heat on nitrate of ammonia, thus:—$\backslash text\{NH\}\_4\backslash text\{NO\}\_3\; =\; 4\backslash text\{HO\}\; +\; 2\backslash text\{NO\}$. Here all the hydrogen is oxidized to water, and the remaining oxygen is just sufficient to convert into protoxide the nitrogen, both of the acid and of the base.

The protoxide is, at ordinary temperatures, a gas, transparent and colourless, having a sweetish taste and faint smell; it is absorbed by water to some extent, but may be collected over that liquid, although it cannot be long kept in contact with it. It supports the combustion of burning bodies pretty much as oxygen does, evidently because it contains half its volume of that gas. It may be breathed, but cannot be thus taken for more than a short time, because its action paralyses the muscles of the mouth, which cease to grasp the tube, and common air enters. An animal confined in the gas soon dies, after exhibiting symptoms of excitement.

When respired by man, it first produces a sensation of thrilling and warmth in the chest, spreading to the extremities, followed, as we have stated, by paralysis of the muscles of the mouth, which puts a stop to the further breathing of it. Then, usually after a very short period of quiet and almost of stupor, the patient becomes excited, sings, laughs, leaps, dances, sports, and begins to indulge in violent muscular actions, to which an irresistible tendency is felt. The laughter which occurs in most cases is entirely without object, and as it excites laughter among the bystanders, the patient is apt to take offence and to threaten them. He is generally, however, good humoured, unless force be roughly applied to restrain him, when he becomes violent. In the course of a minute or two all has passed suddenly off, and the patient returns to full consciousness with a bewildered stare, having either no recollection, or a very confused one, of what he has done and felt. He usually describes

his sensations as agreeable, and states that at a certain time he became unconscious or nearly so. In some cases this excitement either does not appear or is only brief and transient, passing into complete unconsciousness and apparent stupor. In this state, and frequently also in that of excitement, insensibility to pain is present, as we have often ascertained. In fact, the action of the gas, as well as its taste and the sensation it produces in the chest, are the same as those of the vapour of ether and of chloroform. The reason why it produces, in general, excitement, and rarely complete unconsciousness, is simply this; that being a gas, it must be breathed from a bag through a tube; that, by paralyzing the muscles of the mouth, it puts an end to the inhalation of the gas before enough has been taken to produce full coma and anesthesia, except in a few individuals who are more easily affected. Ether and chloroform, being volatile liquids, can be poured on a sponge or cloth, and held to the mouth and nose of the patient, so as to insure a full dose. But where by chance an insufficient dose of them is given, the stages of excitement, laughter, singing, &c., appear just as with the laughing gas. We have repeatedly produced entire coma and insensibility to pain by this gas in persons easily affected; and as we have repeatedly inhaled all three substances, we can testify to the identity of the effects, bearing in mind the impossibility, in most cases, of giving a full dose of the gas.

This gas is formed of 2 volumes of nitrogen and 1 volume of oxygen the three volumes after combination occupying the space only of two. This condensation to the amount of $\backslash frac\{2\}\{3\}$ renders the gas a heavy one. For we have—

which yield 2 vols. protoxide of nitrogen, weighing 3055
Consequently 1 vol. of protoxide, weighs 1527
and this number 1527, represents its specific gravity.

By a pressure of upwards of 50 atmospheres at $32^\backslash circ$ Fahr., or by a less pressure at lower temperatures, this gas is liquefied. The condensed gas is a very mobile liquid, which, on the tube being opened, assumes the form of gas with explosive rapidity, producing intense cold by its vaporization. The most intense cold yet produced has been obtained by means of this gas in vacuo. It has also been solidified by the cold produced by its own evaporation.

(2.) Deutoxide of Nitrogen, $\backslash text\{NO\}\_2\; =\; 30$.

Prepared by the action of moderately strong nitric acid on copper.

Copper. Nitric Acid. Nitrate of Copper. Deutoxide of Nitrogen.
$3\backslash text\{Cu\}\; +\; 4\backslash text\{NO\}\_3\; =\; 3(\backslash text\{CuO\},\; \backslash text\{NO\}\_3)\; +\; \backslash text\{NO\}\_2$

It is a gas, transparent and colourless, not absorbed by water. It cannot be tasted, smelled, nor inhaled, on account of its action on common air, with the oxygen of which it forms red, suffocating, corrosive vapours of nitrous acid, $\backslash text\{NO\}\_2$. This character distinguishes it from all other gases.

It is formed of equal volumes of oxygen and nitrogen united without condensation. Hence its specific gravity is the mean between those of oxygen and nitrogen.

Fig. 15.

Chemistry. It is, therefore, very little heavier than air. As it contains, like the preceding gas, half its volume of oxygen, it supports the combustion of some burning bodies, especially of phosphorus, if introduced into it in full combustion, when the phosphorus burns nearly as brightly as in oxygen.

This gas is absorbed by a solution of sulphate of protoxide of iron (green vitriol), which it turns black. The black liquid absorbs oxygen powerfully.

The attempt to inhale this gas is most dangerous, because, meeting with air in the mouth and air passages, it forms nitrous acid, which is corrosive. The gas itself appears to be poisonous.

When mixed over water with half its volume of oxygen, that is, as much as it already contains, there is instantly formed the red gas of nitrous acid, which is quickly absorbed by the water, the gases entirely disappearing. The action is $\backslash text\{NO\}\_2\; +\; \backslash text\{O\}\_2\; =\; \backslash text\{NO\}\_3$.

(3.) Hyponitrous Acid, $\backslash text\{NO\}\_2\; =\; 38$.

Hardly known in a pure state. It seems to be a liquid, blue at ordinary temperatures, colourless at $32^\backslash circ$, and very volatile, its vapour being red, like that of nitrous acid. When the vapour of this acid is passed through nitric acid, it gives it either a blue colour or an olive colour, according to the quantity, nitric acid being probably also formed, and its orange colour mixing with the blue, produces the olive. Hyponitrous acid forms some salts, and enters into some compounds derived from organic substances. Its vapour is obtained by heating starch with nitric acid, but is not free from nitrous acid.

(4.) Nitrous Acid, $\backslash text\{NO\}\_2\; =\; 46$.

Obtained by mixing oxygen and deutoxide of nitrogen as already explained, or by heating nitrate of lead. The change in the latter case is—

It is a volatile liquid, colourless when cold, straw-yellow when somewhat warmer, and orange-yellow or orange-red when warm. Its vapour is deep red. It is very corrosive. It has a remarkable action on the solar spectrum, which Sir D. Brewster has described. It forms salts, called nitrites, and in organic chemistry it is frequently substituted for its equivalent of hydrogen, producing what are called nitro-compounds, such as nitro-benzoic acid, nitraniline, and others, to be afterwards described; gun-cotton is one of these, being woody fibre, or cellulose, in which a certain amount of hydrogen has been replaced by nitrous acid.

Fig. 16.

(5.) Nitric Acid, $\backslash text\{NO\}\_3\; =\; 54$.—Hydrated Nitric Acid, $\backslash text\{HO\}\_2\backslash text\{NO\}\_3$ or $\backslash text\{H\}\_2\backslash text\{NO\}\_3$.

This acid is obtained by heating nitrate of potash with its own weight of sulphuric acid. The action is as follows:—

The acid collects in the receiver as a colourless fuming liquid in the middle of the process, but is coloured by a little nitrous acid at the beginning and end. When coloured, it is easily purified by redistilling, when the red vapours of nitrous acid pass off first, and the colourless acid then distils. By collecting separately the first tenth or twentieth part in the original process, all impurities adhering to the neck of the retort are washed away, and the

rest is quite free from all traces of sulphuric acid or of Chemistry. potash.

Nitric acid, when pure, has the specific gravity 1.520, compared to that of water as 1.000. It is highly corrosive, and stains the skin yellow. It readily yields part of its oxygen to bodies having an attraction for it, being itself reduced to nitrous or hyponitrous acid, or to deutoxide of nitrogen. Its action on metals and on organic substances, especially oils, is very violent. It combines with bases to form salts, called nitrates, which at a red heat oxidize all oxidizable matter, often with explosion. Nitrate of potash, or nitre, is the oxidizing agent in gunpowder.

Fig. 15.

The presence of free nitric acid is detected by its power of decolorizing solution of indigo, and by its causing solutions of the salts of protoxide of iron to become nearly black; and when it is in the form of a nitrate, sulphuric acid is first added to set it free, and then the salt of iron.

Nitric acid is much used as an oxidizing agent, both in chemistry and the arts. It is employed, somewhat diluted, to corrode copper in etching. By means of nitric acid sugar and starch are converted into oxalic acid. Nitric acid is also used in medicine.

The acid we have described is the hydrated acid, and may be viewed either as composed of water and dry acid, $\backslash text\{HO\}\_2\backslash text\{NO\}\_3$, or as formed of hydrogen, with the hypothetical radical, $\backslash text\{NO\}\_3$; thus, $\backslash text\{H\}\_2\backslash text\{NO\}\_3$. It was long supposed that the anhydrous acid did not exist in a separate form, and that the 1 eq. of water in $\backslash text\{HO\}\_2\backslash text\{NO\}\_3$ could not be removed. But it has recently been shown, that the dry acid $\backslash text\{NO\}\_3$ may be obtained by the action of chlorine or dry nitrate of silver; thus, $\backslash text\{AgO\}\_2\; \backslash cdot\; \backslash text\{NO\}\_3\; +\; \backslash text\{Cl\}\; =\; \backslash text\{AgCl\}\; +\; \backslash text\{O\}\; +\; \backslash text\{NO\}\_3$. Anhydrous nitric acid forms crystals which are volatile and easily decomposed by heat or otherwise. But the hydrate, as in other cases, is the true active permanent acid.

Atmospherical Air.

Our atmosphere, as already mentioned, consists chiefly of nitrogen and oxygen gases. But these, although present in atomic proportion, or very nearly so (for their proportion is very close to $\backslash text\{N\}\_2\backslash text\{O\}$) are not combined, but only mixed together. This is proved by the fact that air has no new properties, but only those of oxygen diluted by nitrogen, and also by this, that a mixture of the two gases, in due proportion, is found to have all the properties of air. The circumstance that two gases of different densities are found uniformly mixed, is explained by the diffusion of gases. When two vessels, one full of carbonic acid gas, the other of hydrogen, gases which do not combine, are made to communicate by a tube, the hydrogen being uppermost, they are found in a very short time equally and uniformly mixed, although the lower gas is more than 20 times heavier than the upper one. The force by which this is effected is the same which affects the perfect and uniform mixture of oxygen and nitrogen in the atmosphere, and is called the force of diffusion.

The proportions of these gases in air is 4 volumes of nitrogen to 1 volume of oxygen; and, by weight, about 97 parts of nitrogen to 21 of oxygen, which is very nearly in the proportion of 2 eqs. N to 1 eq. O, or $\backslash text\{N\}\_2\backslash text\{O\}$.

Besides these gases, air contains also, as essential ingredients, watery vapour in variable amount, and carbonic acid and ammonia in very small proportion. It contains also traces of all volatile substances in quantities too small to be ascertained.

The uses of the air are well known. It is essential to

Chemistry. the life of animals, which respire it, consuming its oxygen, and replacing it by carbonic acid gas. It is equally essential to plants, which consume its carbonic acid, replacing it by oxygen, and which also consume its ammonia. By its oxygen it supports combustion and the decay of dead organic matter, both of which also replace the oxygen they consume by carbonic acid.

It is of great importance to be able to ascertain the amount of oxygen in air, because that gas is constantly consumed by respiration, combustion, and decay. This is called eudiometry, and is done in various ways. The oxygen of a measured portion is removed by phosphorus, or by copper clippings moistened with acid; or the air is mixed with a known volume of hydrogen, not less than $\backslash frac\{1\}\{3\}$ths, or half the volume of the air, and the electric spark passed through the mixture, or spongy platinum introduced. In either case the hydrogen unites with the oxygen, both gases disappear, and water is formed, leaving the nitrogen with any excess of hydrogen. The loss of volume, divided by 3, gives the volume of the oxygen. Thus, if 100 volumes of air are mixed with 50 of hydrogen, and exploded over mercury by the electric spark, the 150 volumes are found reduced to 90, while water is deposited. The loss of volume here is 60 volumes. But in water there are 2 volumes of hydrogen to 1 volume of oxygen; consequently 60 volumes of the gases which have disappeared consist of 40 of hydrogen and 20 of oxygen; or $60\; \backslash div\; 3\; =\; 20$ volumes—the amount of oxygen in 100 volumes of air.

Air is thus found, where it has perfect freedom of motion and mixture, to contain everywhere 20 volumes of oxygen in 100, whether it be examined in towns, in the country, at the level of the sea, or on the highest mountains. But in confined and ill-ventilated places the proportion of oxygen is found to be smaller, while that of carbonic acid is larger, and the air in consequence unfit for respiration.

The proportion of oxygen and that of carbonic acid in air, although the former amounts to 20 volumes in 100, the latter only to at most 1 volume in 1000, have been found uniform in all parts of the world, and at all times and periods. Air, hermetically sealed up 2000 or 3000 years ago, in Herculaneum, and in the Egyptian catacombs, has been found the same as at the present day. This alone would indicate that there is a relation between these two gases, oxygen and carbonic acid. But since we know that animals consume the oxygen replacing it by carbonic acid, that plants consume carbonic acid, replacing it by oxygen, and that carbonic acid contains its own volume of oxygen, we see that there is a balance between animal and vegetable life, which are mutually dependent, each restoring to the air what the other has removed, and consuming what the other has produced, and thus preserving constant the composition of the air; each while living in it rendering it fit for the life of the other. Should any cause suddenly increase the amount of one of them—and some causes, such as volcanic action, and the combustion of fuel in manufactures, &c., do tend to increase that of carbonic acid—the vegetable kingdom instantly seizes on it, more luxuriantly, purifies the air, and at the same time produces more food for animals, so that an increase of the food of plants (carbonic acid) causing an increase of vegetation, is followed by an increase of food for animals and of animal life, and thus the balance is kept up between the animal and vegetable worlds by means of oxygen and carbonic acid, the atmosphere being the scene of action.

Air contains a variable amount of water in the form of

vapour. The quantity which it can take up depends on the temperature, and when it is saturated with vapour at a given temperature, cooling even to the extent of 1 degree causes a deposition of moisture, and thus gives rise to dew, rain, snow, and hail. If the air at a given temperature be not saturated with moisture, it does not deposit any until cooled down below that point at which the vapour present is sufficient to saturate it. This is called the Dew Point, and when we know the dew point at any temperature, or what is the same thing, the difference between the temperature of the air and the dew point, we can calculate, from tables constructed for the purpose, the amount of water in the air at the time. Instruments for ascertaining this are called hygrometers, and the most accurate is the dew point hygrometer, founded on the principles just explained. Some vessel or apparatus is cooled below the temperature of the air, till dew appears on its surface, and its temperature is noted. It is then allowed to become warmer spontaneously, and the temperature again noted at the moment the dew again disappears. The mean between the two temperatures is taken as the true dew point.

Air is the standard of specific gravity for gases, and its specific gravity is made 1000. 100 cubic inches of air weigh about 31.5 grains, and by weighing the same, or any known volume of another gas, its specific gravity is ascertained by a simple proportion. 100 cubic inches of hydrogen weigh only about 2.25 grains, while 100 cubic inches of oxygen weigh 34.5 grains nearly. Hence we obtain the specific gravities of—

Air has, in perfection, all the physical properties of permanent gases. It is perfectly elastic, that is, its volume varies inversely with the pressure to which it is subjected. 100 cubic inches of air under the ordinary pressure, or that of 1 atmosphere, become 50 cubic inches under 2 atmospheres, and 200 under half an atmosphere of pressure. This is supposing the temperature unchanged; for air, like all elastic fluids, expands much when heated, and contracts when cooled. The amount of change due to heat is $\backslash frac\{1\}\{320\}$th of the volume, at 32° Fahr. for every degree of Fahr. while the pressure is the same.

The pressure of the atmosphere depends on its weight, which, from the enormous extent of the atmosphere, is very great, amounting to about 15 lb. on every square inch of surface at the level of the sea. As we ascend, on a hill, for example, this pressure gradually diminishes, because there is less air above than before. For this reason, as before mentioned, water boils at a lower temperature as we ascend higher, the boiling point falling nearly 1° Fahr. for every 440 feet of ascent.

The pressure of the atmosphere is measured by the barometer, which consists of a long tube, first filled with mercury, and then inserted with the open end in a cup of that liquid. The mercury falls to about 29 or 30 inches, and remains stationary at that point, the weight of the column of mercury in the tube being counterpoised by that of the air, pressing on the surface of the mercury in the cup.

By means of this instrument, the details of which belong to mechanical philosophy, the pressure of the atmosphere is found to be constantly varying, in the same place, between certain limits, from about 28 inches of mercury to 31 inches. This depends on the varying amount of air over any point, which again depends on the motions caused in the air by changes of temperature and other causes, such as the rotation of the earth on its axis. These changes of pressure are the causes in part, and in part also the effects of winds, which are the motions of the air. A sudden fall in the barometer, indicating a sudden diminution of pressure,

Chemistry shows the existence of a partial vacuum over the spot where it is observed. The surrounding air rushes into this vacuum with violence proportioned to its degree, and thus restores the former pressure, or a greater. Hence, a sudden fall of the barometer is invariably followed by high winds; while a steady barometer indicates steady weather. The effect of diminished pressure on evaporation has been already mentioned.

The presence of carbonic acid gas in air is easily detected by lime or baryta water, which attract it, forming the insoluble carbonates of lime or baryta. Ammonia, though always present, can hardly be detected, on account of its minute quantity. But it is easily shown to be present in rain-water, which, in passing through the air, dissolves it; by adding a drop of sulphuric acid, evaporating nearly to dryness, and adding lime, when the smell of ammonia is at once perceived. This ammonia is the source whence plants derive probably the greater part of their nitrogen, and while plants absorb it greedily, animal life, and still more the decay of dead animal and vegetable matter, restores to the air the ammonia it has lost, as fast as it is consumed; so that here also a balance exists between plants and animals in regard to a constituent of the atmosphere.

Nitric acid is formed in the air, especially during thunderstorms, partly by the oxidation of ammonia, partly, it is believed, by the direct oxidation of nitrogen. But it is not to be detected in the air, being instantly removed by water. It appears to contribute to the supply of nitrogen to plants.

Nitrogen and Hydrogen.

(1.) Ammonia. $\backslash text\{NH\}\_3\; =\; 17$.

This very important compound, as has just been mentioned, is produced during the decay of organic matters containing nitrogen. It is formed artificially by the action of heat on such organic compounds; and it seems to be given out occasionally from volcanoes.

It is best obtained from sal-ammoniac, hydrochlorate of ammonia, or chloride of ammonium (for the salt has all these names), by heating it with slaked lime, when the ammonia is given off as a gas. The change is—

Hydrochlorate of Ammonia. Slaked Lime. Chloride of Calcium. Ammonia.

Ammonia is a gas, transparent and colourless, of a very pungent and peculiar odour, and a burning taste. It must be collected over mercury, or by displacement, being lighter than air; for water instantly absorbs it, acquiring its taste and smell. It extinguishes burning bodies without itself taking fire, although a jet of it may be set fire to in oxygen gas. It is fatal to animals when inhaled. Ammonia is a very powerful base or alkali; neutralizing the strongest acids; it is, in fact, the type of all volatile organic bases, a numerous class, and belongs strictly to organic chemistry. We shall, therefore, postpone to that section the consideration of its principal relations, viewing it here as a compound of nitrogen and hydrogen.

It consists of 3 volumes of hydrogen and 1 volume of nitrogen, which form, not four, but two volumes of ammonia. It is lighter than air, its specific gravity being 590-2. Cold water absorbs about 600 times its volume of the gas, becoming thereby lighter, and acquires all its pungency, being a powerful rubefacient and diffusible stimulant. Its salts, except the carbonate, have no smell.

Although a very powerful base, it is expelled from its salts by almost all fixed bases, on account of its volatility.

Ammonia is reduced to the liquid state by a pressure of about 17 atmospheres at the ordinary temperature.

Its presence is recognised by its smell when free, and by its forming thick white fumes of sal-ammoniac when a rod dipped in hydrochloric acid is brought near it. When

combined, it is first set free by the addition of lime or potash, and the tests are then applied.

The uses of ammonia are numerous. It is an important part of the food of plants, and a most valuable ingredient, therefore, in manures. It is much used by chemists in their researches, and also in medicine and pharmacy. It is commonly employed in the form of solution in water, called aqua, or liquor ammonia, which is made by causing a current of the gas to pass through water kept cool, till it is saturated.

Large quantities of ammonia, formed by the action of heat on coal, are now obtained from the water of gas-works in the form of sulphate.

Fig. 18.

(2.) Ammonium. $\backslash text\{NH\}\_4\; =\; 18$.

This is a hypothetical compound, or, at least, has not yet been obtained in a separate form; but there are good reasons for admitting its existence in the salts of ammonia. It is believed to have the properties, at all events the chemical properties, of a metal, and to be closely related to potassium. The arguments in favour of the existence of ammonium, are as follows:—

1. When a salt of ammonia is decomposed by the electric current in contact with mercury, the mercury is converted into a soft semisolid mass many times the volume of the mercury, which resembles entirely the compounds of mercury with metals, such as potassium, sodium, &c. Hence it is believed to contain a metal, and is called the amalgam of ammonium; the compounds of mercury with other metals being called amalgams. 2. This amalgam, left to itself, is soon decomposed, and yields nothing but mercury, ammonia, and hydrogen. Hence, if there be a metal combined with the mercury, that metal is formed of ammonia and hydrogen. 3. The salts of ammonia with oxygen acids are isomorphous with those of potash, provided they contain 1 eq. of dry acid, 1 eq. of ammonia, and 1 eq. of water. The salts of ammonia with hydrogen acids are isomorphous with those of potassium, without this 1 eq. of water. Now let us compare the formulae of the two classes of isomorphous salts, which, according to the doctrine of isomorphism, ought to have an analogous constitution. We find—

Here, at first sight, we perceive no analogy in either case. But if we assume that the salts of ammonia are really salts of ammonium, the analogy is at once evident, especially if we use a single symbol for ammonium, for example, Am. We have then—

Here we see that ammonium, $\backslash text\{NH\}\_4$ or Am, and oxide of ammonium, $\backslash text\{NH\}\_4\backslash text\{O\}$ or Am O, can replace potassium and potash (oxide of potassium) without changing the form of the compound. Now oxide of ammonium, $\backslash text\{NH\}\_4\backslash text\{O\}$, is the

Chemistry, same thing as ammonia plus water, $\backslash text\{NH\}\_3\; +\; \backslash text\{HO\}$, and this explains why 1 eq. of water exists in addition to ammonia in all the salts of oxygen acids with ammonia which are all likewise isomorphous with those of potash. If this eq. of water be excluded we obtain different compounds, not true salts of ammonia. For every salt of potassium there is a corresponding salt of ammonium, of like form, and perfectly analogous properties; and wherever oxide of potassium is present, it is replaced, not by ammonium, but by oxide of ammonium, in other words, by the elements of ammonia and water. There is no way known in which this remarkable analogy and isomorphism can be explained, except the hypothesis of ammonium, and it explains all the facts perfectly. The only difference between the salts of ammonium on this hypothesis and those of potassium, beyond what exists between the salts of any two analogous metals is, that while potassium is elementary, ammonium is compound. But it must be remembered that potassium, like all the elements, is not absolutely elementary but only cannot be shown to be compound, and that this may one day be done. We have been thus particular in explaining the doctrine of ammonium, because it is the type, like ammonia, of a numerous class of organic compounds, in which the analogy to potassium comes out still more strongly.

The salts of ammonia, then, on this hypothesis, now almost universally admitted, are salts of ammonium. Sulphate of ammonia, $\backslash text\{NH\}\_4\backslash text\{HO\},\; \backslash text\{SO\}\_4$ is considered to be sulphate of oxide of ammonium, $\backslash text\{NH\}\_2\backslash text\{O\},\; \backslash text\{SO\}\_4$ or $\backslash text\{AmO\},\; \backslash text\{SO\}\_4$. When a hydrogen acid such as hydrochloric acid, $\backslash text\{HCl\}$, acts on ammonia, it is believed not to combine with it, as expressed in the old formula of sal-ammoniac, $\backslash text\{NH\}\_3,\; \backslash text\{HCl\}$, but to react on it, the hydrogen of the acid forming with the ammonia ammonium, with which the chlorine combines. $\backslash text\{NH\}\_3\; +\; \backslash text\{HCl\}\; =\; \backslash text\{NH\}\_4\backslash text\{Cl\}\; =\; \backslash text\{AmCl\}$.

The reason why oxide of ammonium does not exist uncombined, seems to be, that the attraction of the oxygen for the fourth eq. of hydrogen, held as it must be by a feeble attraction than the three others, is sufficient to break up the molecule, forming water, and of course, ammonia. $\backslash text\{NH\}\_4\backslash text\{O\}\; =\; \backslash text\{HO\}\; +\; \backslash text\{NH\}\_3$. This result is also promoted by the very strong tendency of nitrogen and hydrogen to form ammonia.

Since ammonia plus water is equal to oxide of ammonium, which is isomorphous with dry or anhydrous oxide of potassium or potash, it is obvious that, to form a body analogous to and isomorphous with the hydrate of potash (caustic potash), ammonium must take up 2 eqs. of water. For we have—

• Oxide of potassium, dry, $\backslash text\{KO\}$
• Oxide of ammonium, $\backslash text\{NH\}\_2\backslash text\{O\}$
• Hydrated oxide of potassium, $\backslash text\{KO\},\; \backslash text\{HO\}$
• Hydrated oxide of ammonium $\backslash text\{NH\}\_4\backslash text\{O\},\; \backslash text\{HO\}\; =\; \backslash text\{NH\}\_3\; +\; 2\backslash text\{HO\}$

The amalgam of ammonium is best made by passing a melted amalgam of sodium into a warm solution of chloride of ammonium, when the mercury swells up till it rises out of the liquid. The amalgam of sodium should contain no more than 1 part of sodium to 10 of mercury, and it is quite liquid at little more than $100^\backslash circ\; \backslash text\{Fahr\}$.

(3.) Amide. $\backslash text\{NH\}\_2\; =\; 16$.

This compound, like ammonium, is not yet known in a separate state, probably on account of its strong attraction for a third eq. of hydrogen or for other bodies. But there are, especially in organic chemistry, many compounds which appear to contain it, and are of considerable interest. They are called in general amides, and individually are named from the acids which yield them; as oxamide from oxalic acid, benzamide from benzoic acid, &c. They are always formed from a salt of ammonia (ammonium) by the separation of water, one eq. of which is formed by oxygen from the acid and hydrogen from the ammonia which is thus

reduced to amide. To take an example—benzoate of ammonia, $\backslash text\{NH\}\_4,\; \backslash text\{HO\},\; \backslash text\{C\}\_6\backslash text\{H\}\_5\backslash text\{O\}\_3$ or $\backslash text\{NH\}\_2\backslash text\{O\},\; \backslash text\{C\}\_6\backslash text\{H\}\_5\backslash text\{O\}\_3$, losing 2 eqs. of water becomes benzamide, $\backslash text\{NH\}\_2,\; \backslash text\{C\}\_6\backslash text\{H\}\_5\backslash text\{O\}\_3$. Some mineral acids seem to yield amides, and amide certainly combines with metals, as with potassium, sodium, &c., and forms various inorganic compounds. White precipitate, a medicinal compound of mercury, contains mercury, chlorine, and amide. With platinum and some other metals, amide forms remarkable basic compounds, analogous to ammonia.

Indeed, there is every reason to believe that ammonia itself is not a binary compound of nitrogen and hydrogen, but is really composed of amide and hydrogen. We shall return to amide under organic chemistry, to which its most important compounds belong.

4. Chlorine.

Symbol $\backslash text\{Cl\}$. Equivalent = 35.5.

Having considered, somewhat out of the natural order, the very important elements hydrogen and nitrogen, we now resume the arrangement we have adopted, and next to oxygen we find chlorine, which has many points of analogy with it, but at the same time forms part of a group in which a still more striking analogy prevails. We have entered into considerable detail in regard to the preceding elements, a knowledge of which is essential to the understanding of the principles of chemistry, and which are also of the highest practical importance; but our space will not permit us to describe so minutely the remaining elements, nor is it necessary to do so, since the laws already explained apply to all.

Chlorine is found in vast quantities, combined with sodium in sea and rock salt, a compound which is present in every natural water, more or less, and also in all plants and animals. A few other metals such as potassium, calcium, magnesium, lead, mercury, and silver, occur in combination with chlorine; the three first in sea-water, and chloride of potassium in the ashes of plants, and in the animal juices.

It is prepared by the action of hydrochloric acid on peroxide of manganese, or by that of sulphuric acid and chloride of sodium on the same oxide. The first process is—

Fig. 19.

The second, which is the manufacturing process, is thus represented.

Chlorine is a greenish-yellow gas (hence its name), which may be collected over warm water, or by displacement, being heavier than air. It is absorbed both by cold water and by mercury. It has a very pungent and suffocating smell, and irritates the air passages dreadfully, unless much diluted. Its specific gravity is 2.500. Under a pressure of about 4 at-

Chemistry, mopheres it is liquefied. Water absorbs several times its volume of the gas, acquiring its odour.

In chlorine gas, a candle burns with a feeble smoky flame, the hydrogen of the tallow alone combining with it, while the carbon is separated as smoke. A jet of hydrogen, when heated by a flame, readily burns in chlorine with a pale light, forming hydrochloric acid. Phosphorus, and most metals in a state of fine division, take fire spontaneously in chlorine, forming chlorides. Perhaps the most striking properties of chlorine are those of bleaching vegetable colours, and of destroying fetid or noxious effluvia. These effects it produces apparently by its attraction for the hydrogen, which is present in all such bodies. Chlorine is remarkable for the strength of its attractions for the more positive elements, such as hydrogen and the metals. With hydrogen it forms an acid, with oxygen several acids, with metals it forms salts.

Chlorine is much used for bleaching, and for disinfecting. Much diluted with air, it is also used with advantage for inhalation in pulmonary affections. Many of its compounds, such as hydrochloric acid, and chloride of sodium or sea-salt, are of great utility and value. In describing its compounds with the preceding elements, we shall take that with hydrogen first, as the most important.

Chlorine and Hydrogen.

Hydrochloric Acid. $HCl\_2\; =\; 36.5$.

Chlorine and hydrogen gases, when mixed, do not combine till exposed to the sun's rays, when they combine with explosion; or to diffused light, when the combination takes place more slowly; or when a flame is introduced or the electric spark passed through them, in both which cases explosion ensues, hydrochloric acid being formed.

The acid is best prepared by the action of oil of vitriol (sulphuric acid), aided by heat, on sea-salt, which is as follows:—

It is a colourless transparent gas, of a pungent and suffocating acid smell, and very sour burning taste, forming gray fumes with the moisture of the air. It has an intense attraction for water, which instantly absorbs it, and must be collected over mercury, or by displacement. It is rather heavier than air, being formed of

which yield 2 vols. hydrochloric acid, ... 2569.4
So that 1 vol. of hydrochloric acid must weigh 1284.7

This acid, like all similar ones, forms thick white vapours with ammonia. These vapours in this case are solid particles of sal-ammoniac or chloride of ammonium, and are soon deposited as a powder.

Hydrochloric acid gas is absorbed by water, which takes up if kept cool about 500 times its volume of the gas, increasing considerably in bulk and also in density. The saturated solution has the specific gravity 1.210, fumes strongly,

and is corrosive. This solution is the form in which the acid Chemistry is chiefly used, and is best made by heating 1 eq. of salt, with 2 eqs. of oil of vitriol, previously diluted with rather less than half its bulk of water. The gas is conducted by a bent tube into a bottle containing cold water, which is kept cool, the tube just dipping below the surface. However rapid the current of gas, not a particle escapes, if the water be kept cool, till it is saturated. The water becoming heavier as it absorbs the gas, descends, lighter particles taking its place, and thus a constant mixture is effected without external agitation.

This aqueous solution of the acid, commonly called liquid hydrochloric acid, is a most valuable solvent for mineral substances. It converts metals and metallic oxides into chlorides, most of which are soluble in water. If M be any metal, and MO any metallic oxide, then we have

The only insoluble chlorides are those of silver and mercury (protochloride).

Both in itself, and in its action on metals, and on their oxides, hydrochloric acid is the type of acids in general. For, as we have already explained under hydrogen, sulphuric acid and other oxygen acids may be viewed as compounds of hydrogen with compound radicals, instead of being regarded as consisting of anhydrous or dry acids and water. Let X stand for any acid radical or salt radical, simple or compound, M for any metal, H for hydrogen; we then have the following general formulae, examples of which are placed below each:—

We shall see that there are several other acids with simple radicals, analogous to hydrochloric acid. At present, our object is to show that hydrochloric acid is the type also of those acids which contain compound radicals, when viewed in this manner, and that chlorides of metals are the types of the salts of those acids, in point of constitution, just as sea-salt, (chloride of sodium), is the type of all salts in reference to properties.

Hydrochloric acid gas has been liquefied by high pressure and cold combined.

Chlorine and Oxygen.

Chlorine has no very strong affinity for oxygen, but can be made to combine with it indirectly, forming several compounds, which are remarkable in general for being easily decomposed, often with explosion, on account of the feeble attraction between these elements. These compounds are difficult to study, for this reason, and are not fully understood. We shall therefore notice them briefly, only dwelling a little on the most important. They form two well

Chemistry, marked groups, both acid; but those of the second group are the more permanent.

1st Group.—Bleaching and explosive compounds.

These seem to be three in number, namely, 1. Hypochlorous acid, $\backslash text\{ClO\}$; 2. Chlorous acid, $\backslash text\{ClO\}\_2$; and, 3. Hypochloric acid, $\backslash text\{ClO\}\_3$. They are all either gaseous, or extremely volatile liquids, all of a much deeper yellow colour than chlorine; all explode on very slight causes, separating into chlorine and oxygen; and all possess bleaching properties, due probably to the easy separation of the chlorine they contain. They are only to be obtained by indirect processes, such as the action of chlorine on peroxide of mercury, that of hydrochloric acid on chlorate of potash, and that of sulphuric acid on the same salt. They are very dangerous, exploding with violence on the approach of a flame, often from a very slight rise of temperature, or even from the pressure of an inch or two of mercury. They seem to form among themselves compounds of similar properties, some of which compounds have been described as chlorochlorous and chloroperchloric acids. But their composition is still rather doubtful, from the extraordinary similarity of properties in all. Only one of them is practically important, and that one, hypochlorous acid, only in the form of a somewhat complex substance, containing one of its salts, namely, the bleaching powder, or chloride of lime, as it is inaccurately called. As the formation of this compound is connected with that of chloric acid, we shall describe both processes together.

2d Group.—Colourless, permanent, strongly acid compounds.

These are two in number, namely, 4. Chloric acid, $\backslash text\{ClO\}\_3$; and, 5. Perchloric acid, $\backslash text\{ClO\}\_4$.

Chloric acid, $\backslash text\{ClO\}\_3$, is chiefly important and useful in the form of chlorate of potash, which is prepared as follows:—A current of chlorine gas is passed through water, in which slaked lime is suspended until the lime is entirely dissolved. In this first stage of the process, there is formed the bleaching liquor, or solution of bleaching powder. The powder is made by placing dry slaked lime in contact with chlorine gas till it refuses to absorb any more. The bleaching powder of commerce, thus prepared, contains a good deal of unaltered lime, which is left undissolved when it is acted on by water. The solution of bleaching powder thus made is the same as the bleaching liquor prepared by passing chlorine through a mixture of lime and water.

The change, in the formation of the bleaching compound, is as follows:—

The bleaching compound is thus a sort of double salt, formed of hypochlorite of lime and chloride of calcium, and contains the elements of 2 eqs. lime and 2 eqs. chlorine; hence the common name of chloride of lime.

This substance only bleaches in contact with acids. When it seems to do so alone, which it does very slowly, it is from the action of the carbonic acid of the air on it. The stronger acids at once disengage the whole 2 eqs. of chlorine, hence the very powerful bleaching action. The change is as follows, with sulphuric acid:—

Now, from the bleaching solution we can prepare chlorate of lime, and from that chlorate of potash. For, when the bleaching liquor is boiled, it loses its bleaching powers, and the following change takes place:—

If now we add to the solution 1 eq. of chloride of po-

tassium, and evaporate, chlorate of potash crystallizes, and Chemistry, only chloride of calcium remains dissolved.

In practice, the chloride of potassium is added to the bleaching solution before boiling it, when the object is to obtain chlorate of potash. This salt, being sparingly soluble in cold water, crystallizes readily when the solution is boiled down to a certain point, leaving in solution the very soluble chloride of calcium.

From chlorate of potash, chloric acid may be obtained by distillation with sulphuric acid, but when concentrated, a part is decomposed by the heat. The addition of a little water prevents this, and we obtain a solution of chloric acid nearly pure. When free from water, it forms crystals. Its solution is colourless and strongly acid, oxidizing organic matter, and forming with ammonia a dangerously explosive salt. It is only used in the form of chlorate of potash prepared as above. This salt, when heated, gives off all its oxygen quietly, as explained under oxygen. But when mixed with combustible matter, such as sulphur, phosphorus, charcoal, or sugar, the mixture deflagrates or explodes by the contact of a spark, by friction and percussion, and in the case of charcoal even spontaneously, so that it is too dangerous to be employed for gunpowder. But when duly mixed with sulphur or phosphorus, gum and water, with the addition of a little charcoal, the mixture is extensively used for lucifer matches, exploding and setting fire to the sulphur on the match, by friction on any hard body. A mixture of chlorate of potash with sugar takes fire on being touched with sulphuric acid, which, acting on the chlorate, produces hypochloric acid, and this at once sets fire to the sugar.

Perchloric acid, $\backslash text\{ClO\}\_4$, is easily obtained, in combination with potash, by heating the chlorate of potash till $\backslash frac\{1\}\{2\}$d only of its oxygen is expelled, which is known by the melted salt becoming quite thick and pasty, and requiring a stronger heat to expel the remaining $\backslash frac\{1\}\{2\}$d of the oxygen. The cooled mass consists of perchlorate of potash and chloride of potassium; thus—

It is dissolved in the smallest possible quantity of boiling water, and on cooling the very sparingly soluble perchlorate of potash crystallizes. This salt, distilled with sulphuric acid and a little water, yields a solution of perchloric acid; the acid when dry forms crystals. Perchloric acid is colourless and strongly acid. It produces in all solutions of potash a very sparingly soluble precipitate of perchlorate of potash, quite insoluble in weak alcohol. Hence the acid is used to distinguish and to separate potash from soda, the perchlorate of soda being very soluble even in alcohol.

Perchlorate of potash explodes feebly with combustible matter on heat being applied. It is not easy to see how an acid containing 7 eqs. of oxygen should be less disposed to yield oxygen to combustible matter than one with 5 eqs. only. But there is evidently something in the constitution of the molecule which renders it more permanent than that of chloric acid, which itself is far more permanent than those of hypochloric, chlorous, and hypochlorous acids.

Chlorine and Nitrogen.

When chlorine gas is placed in contact with a warm solution of chloride of ammonium (sal-ammoniac), it is absorbed, and oily drops are formed, which sink through the liquid. These are frightfully explosive, especially on simple contact with greasy, oily, or combustible substances. This explosive compound has been described as a compound of nitrogen with chlorine, $\backslash text\{NCl\}\_3$, or, according to some, $\backslash text\{NCl\}\_5$. But very recent researches have shown that this is not the case, and

Chemistry. that this compound contains also hydrogen. We do not therefore, at present, know any compound of these elements.

5. Bromine.

Symbol Br. Equivalent = 80.

Bromine stands unquestionably next to chlorine, and is so very analogous to it, that a very brief notice will suffice. It is found united to sodium, potassium, or magnesium, in very small proportion, in sea-water, and in saline springs. Those of Kreuznach are comparatively rich in bromides, as is also the water of the Dead Sea, which is simply a concentrated sea-water.

It is obtained from the mother liquor or brine which remains after salt-water has been evaporated to yield crystals of salt, and refuses to yield any more. This liquid contains much of the chlorides of potassium, calcium, magnesium, and a small proportion of some bromide or bromides. If we pass through it a current of chlorine, or, what is the same thing, add to it a little sulphuric acid and peroxide of manganese, which, with the chlorides, produce free chlorine;

Fig. 22.

the chlorine displaces the bromine, which, being set free, colours the liquid of a strong orange-yellow. Heat is applied, and the vapour of water, distilling over, carries with it the vapour of bromine, which collects in the receiver, partly as heavy drops of a deep red colour, which are pure bromine, partly as an aqueous solution of bromine of an orange colour, which floats above. Excess of chlorine must be avoided, as it combines with the bromine, forming a chloride of bromine. If the aqueous solution be shaken with a little ether, the ether dissolves the bromine and rises to the surface, of a deep red colour. This is added to the nearly pure bromine collected below, and the whole is then mixed with as much solution of caustic potash as entirely destroys the colour, by which the bromine is converted into two salts, bromate of potash and bromide of potassium. (Chlorine with potash undergoes a similar change, and chlorate of potash was formerly thus prepared.) The change is represented as follows:—

The solution is dried up and ignited, which expels the oxygen of the bromate, converting it also into bromide; so that we have $(\backslash text\{KO\}\_2\backslash text\{Br\}\_6)\; +\; 5\; \backslash text\{\; KBr\}\; =\; 6\; \backslash text\{\; KBr\}\; +\; \backslash text\{O\}\_2$. Lastly, the bromide of potassium is heated with sulphuric acid, a little water, and peroxide of manganese, when pure bromine passes over, with some water. The greater part of the bromine collects below an aqueous solution of a small part of it, and may be withdrawn by a pipette.

Bromine is a heavy dark red volatile liquid, of a most pungent odour, strongly affecting the mucous membrane of the nose and eyes, and producing a profuse catarrhal discharge. Its vapour must be carefully avoided. It is named, indeed, from its strong smell. Its vapour or gas is of the same red colour as that of nitrous acid, and very dense, its specific gravity being about 5500, rather more than twice as heavy as chlorine gas. Bromine is very poisonous; a drop

placed on the beak of a small bird soon proving fatal. It Chemistry. has feeble bleaching properties compared to chlorine.

Bromine is in all its relations so perfectly analogous to chlorine, that it is unnecessary to go into detail. What is true of chlorine is true of bromine, with regard to the other elements, in all cases where bromine combines with them. The chief difference is, that its equivalent is higher, and its affinities weaker, than those of chlorine, which displaces it from all its compounds.

With hydrogen it forms hydrobromic acid, $\backslash text\{HBr\}$; equivalent = 81; perfectly analogous to hydrochloric acid, but much more dense. It is absorbed by water with even greater energy.

With oxygen it forms bromic and perbromic acids, analogous in composition and properties to chloric and perchloric acids. Bleaching oxides or acids of bromine are not yet known.

To bromine and nitrogen the same remarks apply as to chlorine and nitrogen. The compounds of bromine with metals are so like those of chlorine, that they are hardly distinguishable from them.

Bromine will probably be found useful in medicine. At present its chief use is, with chlorine and iodine, in photography. It contributes to the formation of the most sensitive surfaces.

6. Iodine.

Symbol I. Equivalent = 127.

This element is the third of the remarkable group, of which the two other members have just been described. It is found in the ashes of marine plants, which form kelp, varec, and barilla. It is, no doubt, derived by the plants from the sea-water; but its proportion in that water is so small as hardly to admit of being detected directly. It is also found in the ashes of many land plants, especially such as grow near the sea; in some salt springs in small proportion, and in a few rare minerals.

It is obtained from kelp, after all the crystallizable salts have been separated by evaporation. The residual or mother liquid contains the iodide of sodium, potassium, or magnesium, mixed with chlorides, sulphurets, and sulphites, as well as various other impurities. It is extracted by adding first an excess of sulphuric acid, which disengages a vast amount of gases, hydrosulphuric and sulphurous acids, and causes a copious deposition of sulphur. On cooling, sulphates of soda, potash, and magnesia, are deposited, and the liquid, poured off from these, is heated with peroxide of manganese as long as purple vapours of iodine come off. In another method the iodine is precipitated as subiodide of copper, $\backslash text\{Cu\}\_2\backslash text\{I\}$, which is afterwards decomposed by sulphuric acid and peroxide of manganese. The vapours of iodine condense into crystals in the receiver, along with some water.

Iodine is a black solid, crystalline, very brittle, of a quasi-metallic lustre, and volatile. When dry it is converted into vapour, without melting, at $320^\backslash circ$ nearly; but if water be present the iodine passes rapidly over at $212^\backslash circ$. Its vapour is of a fine purple or violet colour, hence the name, and is very heavy, being nearly 9 times denser than air. Heated under pressure, iodine melts to a brown liquid. It is almost insoluble in water, 1 part requiring 7000 of cold water to dissolve it; but even this colours the water brown. It is very soluble in alcohol, ether, and similar liquids. Iodine hardly possesses bleaching properties. It stains organic matter deep yellow, and corrodes it (for example, paper) rapidly. Its distinguishing character is that of striking, with a cold infusion of starch or with starch paste, a deep blue. The blue substance seems not to be a definite compound, but to consist of starch, with minute particles of iodine mechanically diffused through and adhering to it. The violet-blue colour seems to be that of finely-divided iodine, as is seen in its vapour. Heat destroys the colour of the iodide

Chemistry of starch. This character distinguishes iodine from bromine, which colours starch brown.

Iodine is entirely analogous to chlorine and bromine; the three bodies forming a group in which bromine stands in every point, in form, density of vapour, strength of affinity, and atomic weight, precisely between chlorine and iodine. As bromine is perfectly analogous to chlorine in its compounds, so iodine is analogous to bromine.

With hydrogen it forms hydriodic acid, HI, which is so similar to hydrochloric and hydrobromic acids, that it cannot be distinguished from them, except by proving that it contains iodine. It is a colourless acid gas absorbed to a prodigious extent by water, forming gray fumes in moist air, and white fumes with ammonia. To obtain it, iodine is first placed in contact with phosphorus, in a tube filled with carbonic acid gas, to exclude oxygen. The two bodies combine with a flash of light, forming, according to the preparations used, either the triiodide of phosphorus, $PI\_3$, or the periodide, $PI\_5$. When cold, water is added, a conducting tube adapted, and heat applied. The action is as follows, taking the periodide; but it is the same, mutatis mutandis, for the other:—

Periodide of Phosphorus. Water. Phosphoric Acid. Hydriodic Acid.

The action of hydriodic acid on metals and on metallic oxides is exactly analogous to hydrochloric or hydrobromic acid. An iodide of the metal is formed; and in the one case hydrogen is liberated, in the other water is produced. The iodides of some metals have fine colours. Iodide of lead is bright yellow; periodide of mercury is of a bright scarlet.

With oxygen iodine is not known to form any bleaching compounds, but it forms iodic acid, $IO\_3$, and periodic acid, $IO\_5$, corresponding to chloric and perchloric acids. Iodate of potash is formed, along with iodide of potassium, when iodine is acted on by caustic potash, precisely as in the case of bromine and chlorine, $I\_2\; +\; 6\; KO\; =\; KO\_3IO\_3\; +\; 5\; KI$. The mixed salts being dried, alcohol dissolves the iodide, leaving the iodate, from which iodic acid may be obtained if required. Iodine may also be oxidized into iodic acid by boiling with the strongest nitric acid. Iodic acid is a crystalline solid, very soluble and very acid. Periodic acid is formed by a circuitous, indirect process. It is not applied to any use.

The compound, an explosive black powder, hitherto called iodide of nitrogen, appears from recent researches to have a different composition.

Iodine combines both with chlorine and bromine. With chlorine it forms several compounds, one of which is so like bromine, that when bromine was accidentally obtained by Liebig from the water of Kreuznach, which contains both chlorine and iodine, he took it for chloride of iodine, and thus missed the discovery of bromine, which was soon after made by Balard. The supposed chloride was then found to be pure bromine.

The compounds of chlorine both with bromine and iodine, as well as those of bromine and iodine, have nearly intermediate characters, and are not very precisely known. They are all more or less used in photography.

The iodides of metals, as well as the bromides, are entirely analogous to the chlorides, but are, those of iodine more particularly, less soluble in water.

Iodine is much used in medicine, both as iodine in tincture and ointment, and in the form of iodides, such as those of potassium, iron, zinc, lead, and mercury.

The extraordinary analogy between the three elements just considered seems to indicate something common to all three. But if they contain a common element, they must be really compounds, for their differences must depend on something peculiar to each. It is worthy of notice that the

relation between them as to physical properties, affinity, and atomic weight, is precisely that between three contiguous members of what is called in organic chemistry a series of homologous compounds. Thus methylc alcohol, ethylic alcohol, and propylic alcohol, form a precisely parallel group. Now, the first of these is converted into the second by the addition of $C\_2H\_2$, and the second into the third in the same way:—

It is therefore quite conceivable that there may be a similar relation between chlorine and bromine, bromine and iodine. But as to what is here the common difference, corresponding to $C\_2H\_2$ in the other case, we can say nothing. We can only point out the probability that such a relation exists, and leave the question to be decided by research, when our means of research shall have been improved; for hitherto all our efforts to convert chlorine into bromine, or bromine into iodine, and vice versa, have failed.

7. Fluorine.

Symbol F. Equivalent = 18.9.

This element is not yet known in a separate form; but from the character of all its compounds, it is obviously very analogous to the three preceding elements. The reason why we have not yet obtained it uncombined is, that its affinities are so powerful, that it corrodes all vessels in which the attempt has been made. It has been suggested that it might be isolated in vessels of fluor-spar; but no distinct results have yet been obtained. According to some it is a yellow gas like chlorine; but there is reason to think that chlorine was at all events mixed with the supposed fluorine.

Fluorine is found in nature combined with calcium as fluor or Derbyshire spar, and occurs also in small proportions in mica, topaz, cryolite, and a few other minerals. Fluoride of calcium, or fluor-spar, is also an essential ingredient, in small proportion, of bones, and is to be detected almost everywhere, in rocks, soils, springs, the sea, the ashes of plants, and the animal fluids, but in very minute quantity.

With hydrogen, fluorine forms its most important compound, hydrofluoric acid (often called fluoric acid), HF, = 19.9. This acid is obtained by distilling fluor-spar with sulphuric acid in vessels of lead, silver, gold, or platinum, for it corrodes glass and porcelain. The change is as follows:—

It forms a very volatile, fuming, and frightfully corrosive liquid, which produces intense heat when mixed with water; and a drop of which falling on the skin produces, even when instantly washed away, a very painful sore, which is extremely difficult to heal. The hands should be protected even against the vapour, for it penetrates under the nails, producing there sores which, from their confined position, cause intolerable pain.

It is in its relations very analogous to hydrochloric acid, &c., acting on metals and metallic oxides in the same way, and forming fluorides very similar to the chlorides. Its distinguishing character is its power of corroding glass and all silicious compounds, in virtue of the strong affinity of fluorine for silicon, with which it forms a gaseous compound. This property is not only applied to etching on glass, but to the detection of minute traces of fluorine or fluorides. The substance suspected to contain it, whether an ash, a mineral, the residue of any water, or the deposit from sea-water when boiled, is placed in a platinum vessel with pure sulphuric acid. A plate of glass is covered with melted wax, and when cold, marks are traced in the wax with a sharp point, which exposes the glass under these

Chemistry marks. The plate is then laid over the vessel with the materials, to which a very gentle heat is applied. The vapour of hydrofluoric acid, if a fluoride be present, rises and acts on the exposed lines. After a certain time, longer in proportion as the amount of fluorine is smaller, the wax is removed, and the lines are found indelibly etched on the glass.

The only use to which hydrofluoric acid is applied is that of etching on glass, and dissolving silicious minerals for analysis, by which means the whole silica is dissipated as gas, and the other elements remain. The presence of fluoride of calcium in bones is probably connected with their hardness and toughness. It is conjectured that the presence of the fluoride prevents the phosphate of lime from crystallizing, as it has a strong tendency to do, and thereby becoming brittle. Fossil bones and coprolites, rich in bone earth, invariably contain a small amount of fluoride of calcium. It has been said that the proportion is greater in fossil than in recent bones; but it seems rather that, from the absence of animal matter, it is more readily detected in the former.

No compounds are yet known of fluorine with oxygen, nitrogen, chlorine, bromine, or iodine. Its affinities are so very powerful for the positive elements, that it is probable it may prove to be more highly negative than oxygen, which would account for its exhibiting little or no attraction for that element.

(B.) POSITIVE OR COMBUSTIBLE METALLOIDS.

8. Carbon.

Symbol C. Equivalent = 6.

This most important element occurs in nature in several forms, more or less pure. Crystallized, it is found in two allotropic forms; one transparent, hard, and colourless, is the diamond, the other black and soft, is plumbago or black lead, or graphite, the crystalline form of which is quite different. A third, amorphous form, is found in anthracite or blind coal. It occurs also, combined with oxygen, as carbonic acid gas, in the atmosphere, in waters, in the chock-damp of mines, in volcanic districts; and the same acid, combined with lime, forms marble, limestone, chalk, shells, calcareous deposits, and calcareous or Iceland spar. Other carbonates occur in the mineral kingdom, such as those of magnesia, iron, lead, copper, &c. Carbon is the chief ingredient of all animal and vegetable substances; also of coal, which is vegetable matter in an altered state, and of peat, lignite, brown-coal, wood-coal, jet, asphalt, bitumen, &c., all of which have a similar origin.

Pure carbon, in the form of diamond, which forms octahedral crystals, is distinguished by its extreme hardness and by its action on light, giving it the fine play of colours for which it is valued. Graphite occurs in micaceous scales, which are short prismatic crystals. It is black and opaque, with somewhat of a metallic lustre. Anthracite has no regular form, and is dull, black, and opaque. So also are wood, charcoal, and lamp-black; the latter being pure carbon from oils or resins, the former, like anthracite, containing, as ashes, the mineral elements of the plants which yielded them. In all its forms it is a bad conductor of heat, but when dense and compact it conducts electricity. It is totally infusible by any heat we can produce. When heated to redness in air or oxygen, it burns without flame, frequently with sparks, forming carbonic acid gas. Diamond can easily be burned in oxygen, or even in air, by the heat of a glass-house furnace. No liquid is yet known which can dissolve carbon as such.

Carbon has the power of attracting, in a manner somewhat obscure, both gaseous and dissolved matters, and collecting them in its pores. Thus it absorbs large quantities of most gases; and when introduced into a liquid, generally removes from it any organic colouring matter that may be present, and very frequently saline substances also. It ab-

sorbs and renders innocuous all offensive effluvia. Animal Chemistry. charcoal has the greatest power in this way, either because it is more porous, or because it contains nitrogen, and is not in fact pure carbon.

Carbon and Oxygen.

1. Carbonic Oxide, CO = 14.

This is a gas formed when carbon burns with an insufficient supply of oxygen, or when carbonic acid gas, CO₂, is passed over red-hot charcoal. It may be formed also by heating with excess of sulphuric acid, oxalic acid and its salts, formic acid and its salts, or ferrocyanide of potassium. When oxalic acid is employed, it is resolved into equal volumes of carbonic acid and carbonic oxide, while water combines with the sulphuric acid; thus—

Formic acid consists of C₂H₂O₂, or C₂HO₂, HO; and yields precisely in the same way water and pure carbonic oxide gas. C₂H₂O₂ = 2HO + 2CO. The ferrocyanide of potassium first yields hydrocyanic acid, which, by the action of mere sulphuric acid reacts on water, producing formic acid and ammonia, and the formic acid is decomposed as above. When mixed with carbonic acid, the gas is purified by means of lime or potash, which absorb the carbonic acid, leaving the carbonic oxide. It is collected over water.

It is a colourless gas of specific gravity 972.1, without taste or smell, which, when heated in air, takes fire and burns with a pure blue lambent flame, being oxidized into carbonic acid. In every large coal fire, the air entering below forms carbonic acid, which, passing through the deep mass of red-hot charcoal, is converted into carbonic oxide, thus, CO + C = 2CO. The carbonic oxide rising to the surface, there burns with its peculiar blue flame, as may be seen in every large fire after it has burned so far that no more gas issues from the coal, and the whole is in a red glow. The blue flame is supposed to indicate frost, but it is evident that it indicates simply a large red-hot fire, which, of course, is more frequent in frosty weather than in summer.

This gas occurs in mines, along with carbonic acid and carburetted hydrogen. It is poisonous when inhaled.

Carbonic oxide, or a substance polymeric with it, seems to exist in some organic compounds as a radical. It is not applied to any use, although it has a share in the reduction of metal, such as iron, being formed in large quantity in the deep smelting furnaces, and having, from its tendency to take up an additional eq. of oxygen, a considerable deoxidizing power.

2. Carbonic Acid, CO₂ = 22.

This is a much more important compound. It is a gas which occurs abundantly in nature, as has been stated under carbon, both free and combined with lime and other bases. It is also abundantly formed in various natural and artificial processes, in the respiration of animals, in the decay of dead organic matters, in fermentation, and in combustion. It is from these sources principally that the carbonic acid of the atmosphere, so essential to vegetation, is supplied; and it is consumed by plants as fast as it can be produced, since its amount never exceeds, in free air, about $\backslash frac\{1\}\{250\}$th of the bulk or volume of the air.

To obtain it pure, carbonate of lime, that is, marble limestone or chalk, is acted on by diluted hydrochloric acid. The action is very simple. CaO, CO₂ + HCl = CaCl + HO + CO₂. Chloride of calcium and water are left, while carbonic acid gas passes off and is collected over water, which absorbs a certain amount of it, but not so much as to

Chemistry. prevent us from thus collecting it. It must not, however, be left to stand over water, or it will be absorbed.

It is colourless and transparent, has a sharp acidulous taste, and a certain pungency of smell, or rather a peculiar action on the organs of respiration, and not a true odour. It extinguishes all burning bodies, and is equally fatal to animal life, being poisonous even when diluted with air, and not merely negatively injurious like nitrogen. It is distinguished from all other gases by its rendering lime-water milky from the formation of the insoluble carbonate of lime. It is a heavy gas, its specific gravity being 1527.7. We do not know the volume of the carbon in it in the state of gas, but we know that it contains its own volume of oxygen, and that the whole weight of the carbon is added to that of the oxygen in the same volume. So that we have—

One volume carbonic acid, specific gravity..... 1527.7

If the carbon united to 1 volume of oxygen be, as gas, equal in volume to the oxygen, then 1 volume of gaseous carbon must weigh 416.6, when 1 of oxygen weighs 1111.1, and its specific gravity would be 416.6. But this would imply that the 2 volumes of the elements were condensed into 1 volume of the compound, of which we have no example. If, on the other hand, carbonic acid be formed of 2 volumes of oxygen and 1 volume of gaseous carbon, the 3 volumes condensed into 2, which is a very common occurrence, as in water, protoxide of nitrogen, &c., then the 416.6 will represent the weight only of half a volume of carbon, and the weight of an entire volume, or the specific gravity, will be $416.6\; \backslash times\; 2$, or 833.2. This latter view of the specific gravity of gaseous carbon, which cannot be directly determined, is considered the most probable. On the same supposition carbonic oxide will be formed of 1 volume of gaseous carbon and 1 volume of oxygen, united without condensation, or so as to form 2 volumes of the compound, which is also a very common occurrence, as in hydrochloric acid, &c. We have mentioned this here, to explain how chemists calculate the probable density in the gaseous form of a substance, such as carbon, which is not known in that form, except in compounds.

From its weight, carbonic acid gas may be poured out of one vessel into another, like water; and if a lighted candle be first placed in the lower vessel, it will be quickly extinguished, as the invisible gas flows down to it. The gas may even be poured in the open air so as to fall in a narrow stream on the candle and extinguish it. For the same reason this poisonous gas is apt to accumulate in deep pits, wells, or mines, where it is very dangerous. A candle let down first will indicate, according as it burns brightly or dimly, or is extinguished, the purity or impurity of the air in such places, before any one descends; and this precaution should never be neglected. The gas often accumulates in the huge vats of the brewers, and has frequently caused fatal accidents there as well as in mines or pits. It is the choke-damp of miners.

Carbonic acid is liquified by a pressure of 36 to 40 atmospheres, according as the temperature varies from $32^\{\backslash circ\}$ to about $60^\{\backslash circ\}$. The condensed acid is very dangerous, having

often, by its enormous pressure, burst very strong vessels and caused fatal results. It is best condensed by being forced by means of a very powerful forcing pump into a strong copper vessel, which, if it should at any time give way, tears instead of bursting. When allowed to escape through a small aperture, it assumes the gaseous form with almost explosive force, but is manageable, and if the current be made to circulate through a brass box, the evaporation of part produces a cold so intense as to solidify the rest. The solid acid is exactly like snow, and evaporates much more slowly than the liquid. But if mixed with ether, the mixture evaporates so rapidly, that by its means several pounds of mercury may be frozen hard in a minute or two. The liquid acid expands by heat to a remarkable degree.

Carbonic acid gas is absorbed by water to a small extent under the ordinary pressure, but under an increased pressure the quantity absorbed increases in proportion to the pressure; the water always absorbing about its own volume, whatever the pressure, but the gas being of course more and more condensed as the pressure increases. The solution sparkles, and has a pleasant acidulous taste, as observed in many mineral waters. The addition of a little carbonate of soda (or potash) causes water to absorb much more, and when this is combined with high pressure, a very large quantity is taken up, which escapes on the pressure being removed. This is the nature of soda water. Champagne is wine bottled before the fermentation is complete, and therefore charged with carbonic acid under pressure of the gas itself, which escapes with violence when the cork is drawn. All effervescing wines or other beverages are in the same way more or less charged with carbonic acid.

The solution of carbonic acid in pure water, or in water containing a little potash or soda, under the ordinary or a slightly increased pressure, occurs as a mineral water in many springs, especially in volcanic districts, even in such as, like Auvergne and the Eifel, have not been the scene of active volcanic action since the appearance of man on the earth. In the Eifel the soil is everywhere impregnated with the gas, which of course appears in all the springs. The Grotta del Cane, near Vesuvius, is a cave containing a small lake or pool, through which carbonic acid constantly bubbles, and from its weight remains near the floor of the cave, so that a dog, whose head is near the ground, falls down insensible, while men, whose heads are in pure air, perceive no effect. The animal recovers, if at once removed into pure air and thrown into the lake outside. This mode of applying the cold affusion is effectual in causing instinctive inhalations of pure air, and indicates the dashing of cold water on the face and chest of those poisoned by the gas as one of the most efficient remedies.

The solution of the gas in water reddens vegetable blues like other acids; but on standing, or after boiling, the blue colour returns, the acid being dissipated. This serves to distinguish carbonic acid from most other acids.

The same solution added to lime water renders it milky, by forming insoluble carbonate of lime, $\backslash text\{CaO\},\; \backslash text\{CO\}\_2$. But the addition of more carbonic acid clears all up again, forming the soluble bicarbonate, $\backslash text\{CaO\},\; 2\; \backslash text\{CO\}\_2$. Such a solution containing carbonate or bicarbonate of lime, dissolved in excess of carbonic acid, is exceedingly common in all districts where limestone occurs. The amount of lime present varies very much, but even a few grains per gallon render the water very hard. Such, indeed, is the usual cause of hardness in water, although the presence of sulphate of lime or gypsum also contributes to it in many cases. The rain dissolves carbonic acid as it passes through the air, and more in filtering through the soil, and when it now meets with chalk or limestone it dissolves it and becomes hard. Some waters contain 10 grains of carbonate of lime in the gallon, others a good deal more; very hard waters frequently 70 or 80 grains, and in some cases so much as 150 grains

Chemistry, and upwards. Such waters are useless for the purposes of washing, and generally for culinary purposes, and above all unfit for steam-boilers; for they deposit, on being boiled, a crust of carbonate of lime which is most injurious, and is often the cause of explosions. It is singular that hard waters—not, however, the hardest, but such as that of the Thames, and some still harder near Edinburgh, where from the abundance of the carboniferous limestones they are very common—are the best for the purposes of the brewer of ale or porter. On the small scale the carbonate of lime in hard water of this kind may be separated by boiling, but this is impracticable on the large scale. Professor Clark's process consists in adding to the water as much lime, dissolved in water (lime water), as it already contains in the form of bicarbonate, and more if there be an excess of carbonic acid beyond this. The added lime converts the soluble bicarbonate into the insoluble neutral carbonate, which is thus explained, $\backslash text\{CaO\}\; +\; 2\backslash text\{CO\}\_2\; +\; \backslash text\{CaO\}\; =\; 2(\backslash text\{CaO\},\; \backslash text\{CO\}\_2)$.

The chief use of carbonic acid is to form the principal part of the food of plants, which derive from it, and therefore from the atmosphere, the whole of the immense quantity of carbon they contain. The carbon of the animal world being derived directly or indirectly from plants, for carnivorous animals feed on such as feed on plants, is also derived from the same source; and the carbon of dead animals and vegetables again takes, in the process of decay, the form of carbonic acid, in which it becomes the food of a new generation of plants and animals. The other uses of carbonic acid, as a remedy and in various beverages, are familiar, and have been already mentioned. The salts of this acid are characterized by effervescing with strong acids.

Some other compounds are frequently described as compounds of carbon and oxygen, such as oxalic acid, melitic acid, &c. But in fact they all contain hydrogen also, and belong to organic chemistry.

Carbon and Hydrogen.

The compounds of these two elements are extremely numerous, far more so than those of any two elements whatever. Indeed there seems hardly any limit to their number. They are all of organic origin, and belong to organic chemistry, under which we shall describe them more particularly. Here we shall only say, that, being formed of two combustibles, they are all eminently combustible, and are used as combustibles. They occur in all forms, solid, liquid, and gaseous; and of all different proportions of the two elements. They form various series, some of which are homologous, others polymeric. In one homologous series, that of olefiant gas, the proportion is that of 1 eq. of each, but the absolute amount varies from $\backslash text\{C\}\_2\backslash text\{H\}\_2$ and $\backslash text\{C\}\_3\backslash text\{H\}\_4$, which are gaseous, to $\backslash text\{C\}\_\{14\}\backslash text\{H\}\_\{28\}$, &c., which is liquid, and to $\backslash text\{C\}\_\{31\}\backslash text\{H\}\_\{64\}$ and $\backslash text\{C\}\_\{63\}\backslash text\{H\}\_\{128\}$, which, with many others, are solid. In another exactly parallel series, that of marsh gas, the proportion is such that there are always 2 eqs. of hydrogen more than of carbon. It begins with $\backslash text\{C\}\_2\backslash text\{H\}\_4$, $\backslash text\{C\}\_3\backslash text\{H\}\_6$, and so on to $\backslash text\{C\}\_\{29\}\backslash text\{H\}\_\{58\}$, $\backslash text\{C\}\_\{37\}\backslash text\{H\}\_\{74\}$, $\backslash text\{C\}\_\{45\}\backslash text\{H\}\_\{90\}$, &c.; those low in the scale being gaseous, those higher liquid, and higher still solid. We find similar characters in the series of methyle and ethyle, in which the hydrogen always exceeds the carbon by 1 eq.; thus we have methyle $\backslash text\{C\}\_2\backslash text\{H\}\_3$, ethyle $\backslash text\{C\}\_3\backslash text\{H\}\_5$, amyle $\backslash text\{C\}\_4\backslash text\{H\}\_7$, cetyle $\backslash text\{C\}\_5\backslash text\{H\}\_9$, melissyle $\backslash text\{C\}\_6\backslash text\{H\}\_\{11\}$, &c. As an example of another kind of series of carbo-hydrogens, we may mention certain essential or volatile oils, such as oil of lemons $\backslash text\{C\}\_5\backslash text\{H\}\_8$, oil of turpentine $\backslash text\{C\}\_\{10\}\backslash text\{H\}\_\{16\}$, another oil $\backslash text\{C\}\_\{20\}\backslash text\{H\}\_\{32\}$, &c. All these matters belong to organic chemistry, and therefore we only indicate them very briefly, that the reader may see how very peculiar is the relation of carbon to hydrogen, which is evidently connected with the fitness of

these elements to constitute organic, that is, animal and vegetable compounds. Coal gas, naphtha, paraffine, and many volatile oils, belong to this group of compounds, which therefore admits of many useful applications. The fire-damp or explosive gas of mines is a mixture of some gaseous carbo-hydrogens with air.

Carbon and Nitrogen.

These elements, under certain circumstances, combine to form a very remarkable and important compound, namely, Cyanogen, $\backslash text\{C\}\_2\backslash text\{N\}$ or $\backslash text\{Cy\}$, which is a compound acid radical or salt radical, entirely analogous to chlorine in its chemical relations.

Cyanogen, $\backslash text\{C\}\_2\backslash text\{N\}\; =\; \backslash text\{Cy\}\; =\; 26$, is found, as hydrocyanic acid, in oil of bitter almonds; but as that is a product of fermentation, and is not present in the dry seeds, it is doubtful whether hydrocyanic acid exist ready formed in plants. But there must be some compound of cyanogen, which, when water is added to the bitter almonds, or to the kernels of stone fruit, produces hydrocyanic acid. As urea is formed from cyanic acid and ammonia, it is probable that the animal body also contains some compound of cyanogen.

It is formed artificially when organic matter containing nitrogen is exposed to heat in close vessels. But being gaseous, it is dissipated and lost, unless some means be taken to prevent this. If potash or carbonate of potash be added, cyanide of potassium is formed, and this salt is permanent at a red heat. When water is added, however, to the heated mass, consisting chiefly of animal charcoal and cyanide of potassium, that salt is decomposed, as follows: $\backslash text\{K\}\_2\backslash text\{C\}\_2\backslash text\{N\}\; +\; 5\backslash text\{HO\}\; =\; \backslash text\{KO\}\; +\; 2\backslash text\{CO\}\_2\; +\; \backslash text\{NH\}\_3\; +\; \backslash text\{H\}\_2$; that is, bicarbonate of potash is left, and ammonia and hydrogen are given off. To obtain cyanogen in a permanent form, there must be added, either before or with the water, iron, oxide of iron, or sulphuret of iron, all of which are dissolved by cyanide of potassium. Suppose iron to be used, then $3(\backslash text\{K\; Cy\})\; +\; \backslash text\{Fe\}\; +\; \backslash text\{HO\}\; =\; (2\backslash text\{K\; Cy\}\; +\; \backslash text\{Fe\; Cy\})\; +\; \backslash text\{KO\}\; +\; \backslash text\{H\}$. With oxide of iron no hydrogen is given off; and with sulphuret of iron sulphuret of potassium, $\backslash text\{KS\}$, is formed, instead of $\backslash text\{KO\}$; but in all these cases the compound $2\backslash text\{K\; Cy\}\; +\; \backslash text\{Fe\; Cy\}\; =\; \backslash text\{Cy\}\_3\backslash text\{K\}\_2\backslash text\{Fe\}$, which is ferrocyanide of potassium, is formed. On evaporation this salt forms large and fine yellow crystals, which are very permanent, and are manufactured on the large scale, and of great purity, being much used in dyeing and calico-printing.

This salt may be viewed either as a double salt, composed of 2 eqs. $\backslash text\{K\; Cy\}$ with 1 eq. $\backslash text\{Fe\; Cy\}$, or, as is now generally admitted, as formed of the compound radical ferrocyanogen $\backslash text\{Cy\}$, $\backslash text\{Fe\}\; =\; \backslash text\{Cfy\}$, which, being dibasic, takes up 2 eqs. of potassium, forming the yellow salt, $\backslash text\{Cfy\}\_2\backslash text\{K\}\_2$. The crystals contain in addition 3 eqs. water, and are $\backslash text\{Cfy\}\_2\backslash text\{K\}\_2\; \backslash cdot\; 3\backslash text\{HO\}$.

From this salt all the compounds of cyanogen and cyanogen itself are prepared. To yield cyanogen, it is heated with bichloride of mercury, which acts on the 2 eqs. $\backslash text\{K\; Cy\}$, that are present by their elements; $2\backslash text\{K\; Cy\}\; +\; \backslash text\{HgCl\}\_2\; =\; 2\backslash text\{K\; Cl\}\; +\; \backslash text\{Hg\; Cy\}\_2$. This forms bi-cyanide of mercury, but the heat employed decomposes this salt into cyanogen and mercury.

Cyanogen is a colourless gas, collected over mercury, of a very pungent smell, inflammable and burning with a pink flame. Water absorbs it, and the solution, if kept, undergoes a spontaneous change, by which numerous compounds are formed, a kind of putrefaction. Cyanogen and water contain the four elements, carbon, hydrogen, nitrogen, and oxygen, and there are formed compounds of these, and compounds of these compounds again, so as to give rise to very complex changes. Carbonic acid, ammonia, hydrocyanic acid, cyanic acid, cyanate of ammonia, urea, formic acid, and dark brown insoluble compounds containing cyanogen, but not fully studied, are formed.

Cyanogen is very analogous to chlorine in its relations. It forms with hydrogen hydrocyanic acid, $\backslash text\{HCy\}$, and with

Chemistry. metals cyanides, corresponding to the chlorides, as $K\backslash text\{Cy\}$ and $Hg\backslash text\{Cy\}\_2$, corresponding to $K\backslash text\{Cl\}$ and $Hg\backslash text\{Cl\}\_2$. With oxygen it forms three polymeric acids, cyanic, fulminic and cyanuric acids. But it is chiefly remarkable for forming with sulphur, and also with many metals, compound radicals, such as sulphocyanogen $\backslash text\{CyS\}\_2\; =\; \backslash text\{Csy\}$, ferrocyanogen $\backslash text\{Cy\}\_2\backslash text\{Fe\}\; =\; \backslash text\{Cfy\}$ and others, all of which, like cyanogen itself, form acids with hydrogen, and salts with metals.

Hydrocyanic acid, $H\backslash text\{Cy\}\; =\; 27$, is obtained by heating ferrocyanide of potassium with sulphuric acid and water. Ferrocyanide of potassium contains $\backslash text\{Cy\}\_2\backslash text\{K\}\_2\backslash text\{Fe\}$, and when the acid acts on it, there are first formed cyanide of potassium and cyanide of iron, the latter separating as an insoluble powder, $2\backslash text\{K\}\backslash text\{Cy\}\; +\; \backslash text\{Fe\}\backslash text\{Cy\}$. The $\backslash text\{Fe\}\backslash text\{Cy\}$ remain unchanged; but the $2\backslash text\{K\}\backslash text\{Cy\}$ act on sulphuric acid, thus, $2\backslash text\{K\}\backslash text\{Cy\}\; +\; 2\backslash text\{H\}\_2\backslash text\{SO\}\_4\; =\; 2(\backslash text\{K\}\_2\backslash text\{SO\}\_4)\; +\; 2\backslash text\{H\}\backslash text\{Cy\}$. The acid, $\backslash text\{H\}\backslash text\{Cy\}$, being volatile, distils over with water. It is obtained pure by adding chloride of calcium, which forms a heavy oily solution with the water, above which floats the pure dry acid, and may be drawn off by a pipette. It is very volatile, lighter than water, has a peculiar oppressive smell, and is very poisonous, even when smelled at, in its pure state. Before smelling it, it should be diluted. It is the well known poison prussic acid.

It is used in a diluted form in medicine, and the medicinal acid ought to contain in 100 parts not more than 3 of the pure acid. Its strength is ascertained by converting a measured portion into cyanide of silver, which is insoluble, and is formed on the addition of nitrate of silver, $\backslash text\{H\}\backslash text\{Cy\}\; +\; \backslash text\{AgO\},\; \backslash text\{NO\}\_3\; =\; \backslash text\{HO\},\; \backslash text\{NO\}\_3\; +\; \backslash text\{Ag\}\backslash text\{Cy\}$. As the cyanide of silver, $\backslash text\{Ag\}\backslash text\{Cy\}$, weighs very nearly five times as much as the hydrocyanic acid, $\backslash text\{H\}\backslash text\{Cy\}$, for $\backslash text\{H\}\backslash text\{Cy\}\; =\; 27$ and $\backslash text\{Ag\}\backslash text\{Cy\}\; =\; 134$, we can easily ascertain the weight of the pure acid by dividing that of the cyanide of silver by 5.

The presence of hydrocyanic acid is detected by converting it into Prussian blue, which is done by adding, first potash, then mixed sulphate of protoxide and sesquioxide of iron, leaving an excess of potash, and after a few minutes adding lastly hydrochloric acid, which dissolves all but the Prussian blue. If the quantity of hydrocyanic acid be very small, the liquid, after adding the hydrochloric acid, must be allowed to stand for 24 hours, when the blue will appear at the bottom of the vessel.

Or the hydrocyanic acid may be converted into sulphocyanide of iron, which is of a deep blood-red colour. To do this, add to the diluted acid a little sulphuret of ammonium with a little sulphur, and evaporate to dryness in a water bath. To the dry mass which now contains sulphocyanide of ammonium, add a salt of sesquioxide of iron, and the red colour will appear.

The three compounds of cyanogen and oxygen are $\backslash text\{CyO\}$, $\backslash text\{HO\}$, cyanic acid; $\backslash text\{Cy\}\_2\backslash text\{O\}\_2$, $2\backslash text\{HO\}$, fulminic acid, and $\backslash text\{Cy\}\_2\backslash text\{O\}\_3$, $3\backslash text\{HO\}$, cyanuric acid. The first and the last are both obtained from urea, and their relation to that compound and to each other is fully detailed under the head of polymerism and isomerism in organic compounds, and the transformations resulting from them. Fulminic acid, which is dibasic, and unknown in the free state, is obtained in combination with oxide of silver or oxide of mercury, forming the fulminating silver and fulminating mercury, the latter of which is the material used for percussion caps, by dissolving the metal in nitric acid and adding alcohol, when a very complex reaction occurs, and the fulminating salt is formed.

With sulphur, cyanogen forms a new compound radical sulphocyanogen, $\backslash text\{CyS\}\_2\; =\; \backslash text\{Csy\}$, unknown in the free state. It is obtained as sulphocyanide of potassium, by melting ferrocyanide of potassium with sulphur, when each equivalent of $\backslash text\{K\}\backslash text\{Cy\}$ takes up 2 eqs. of sulphur, forming the salt $\backslash text\{K\}\_2\backslash text\{CyS\}\_2\; =\; \backslash text\{K\}\backslash text\{Csy\}$. This is purified by solution in alcohol. The radical forms also an acid with hydrogen, $\backslash text\{H\}\_2\backslash text\{Csy\}$, hydrosulphocyanic acid. This acid and its soluble salts are recognised by striking a deep red colour with salts of sesquioxide of iron.

With the metals, cyanogen forms salts very similar to the chlorides. The cyanides of potassium, silver, and mercury can hardly be distinguished from the corresponding chlorides.

But cyanogen is remarkable for forming with certain metals remarkable compound radicals, which are monobasic, dibasic, and tribasic. Thus we have—

There are also cobaltocyanogen, $\backslash text\{Co\}\_2\backslash text\{Cy\}\_4$, tribasic, mangano-cyanogen, chromocyanogen, and iridocyanogen, and two or three different platinocyanogens. Our space forbids us to enter into minute details, but we may state that 2 eqs. of ferrocyanide of potassium, $2\backslash text\{Cy\}\_2\backslash text\{Fe\}$, $\backslash text\{K\}\_2\; =\; \backslash text\{Cy\}\_2\backslash text\{Fe\}\_2$, $\backslash text\{K\}\_2$ when acted on by chlorine yield 1 eq. chloride of potassium, $\backslash text\{KCl\}$, and 1 eq. ferricyanide of potassium $\backslash text\{Cy\}\_6\backslash text\{Fe\}\_2$, $\backslash text\{K\}\_2$. This is a beautiful red salt, also manufactured on the large scale. Prussian blue, a compound of cyanogen and iron, is formed either by the action of ferrocyanide of potassium on salts of sesquioxide of iron, or of ferricyanide on salts of protoxide of iron. It is probable that there are more blue compounds than one, and that the ferrocyanogen being dibasic, forms a different compound from that produced by the tribasic ferricyanogen. The reaction with the ferrocyanide is probably this:—

and with the ferricyanide—

But there is still some doubt as to the precise composition of the different forms of Prussian blue.

The variety of compound radicals formed by cyanogen, especially with iron and platinum, and the beauty of the salts thus formed, which generally crystallize with great facility, render these compounds very interesting. But it is in its relations to organic chemistry that cyanogen is most important, and we shall see, that in the decomposition of organic compounds containing nitrogen, compounds of cyanogen are constantly appearing, as well as compounds derived from them by transformation, such as urea. Cyanogen generally combines with the compound organic positive radicals, such as methyle, ethyle, benzoyle, and others. The most important of these compounds will be noticed in their proper place.

Carbon and Chlorine.

These elements do not directly combine; but when various organic compounds are exposed to the combined action of sun-light and chlorine, they are gradually destroyed, their hydrogen combining with chlorine, and finally their carbon also. There are two chlorides of carbon—$\backslash text\{C\}\_2\backslash text\{Cl\}\_2$, which has been called protochloride of carbon, a colourless liquid; and $\backslash text\{C\}\_4\backslash text\{H\}\_2$, which has been called sesquichloride of carbon, a crystalline solid. They are of no interest except as being the ultimate products of the substitution of chlorine for hydrogen in certain organic compounds.

The compounds of carbon with bromine and iodine are less known, but appear to be analogous to the chlorides. Carbon is not known to combine with fluorine.

9. Sulphur.

Symbol S. Equivalent = 16.

This element is found pure as a product of volcanic action, and vast quantities of it are exported from Etua, Lipari, &c. It occurs also frequently in combination with metals forming sulphurets; the principal ores of lead, copper, bismuth, antimony, and mercury, are sulphurets; and those of iron, cobalt, nickel, zinc, cadmium, tin, arsenic, molybdenum, and silver, also occur. In combination with oxygen it is found as sulphuric acid, in gypsum or sulphate of lime, and in the sulphates of baryta, strontia, and lead.

Chemistry. Sulphur combined with hydrogen, or hydrosulphuric acid, is found in some mineral waters.

Sulphur is a solid of a peculiar pale yellow colour. It crystallizes easily when melted, and is found often finely crystallized. It melts about 245°, and when heated beyond 300° becomes thick and viscid, but beyond 500° it becomes again fluid, though less so than at first. At about 600° it boils, forming a brown vapour of specific gravity about 6650, which condenses on a cold surface or in water into a light powder, called flowers of sulphur. Heated in the air, it burns with a blue flame, producing a very suffocating gas, sulphurous acid. It is insoluble in water, sparingly soluble in boiling alcohol.

It exists, as has been already mentioned, in three different solid states or allotropic modifications; one in which it forms 4-sided prisms, when melted and allowed to cool; another when dissolved by the aid of heat in sulphurets of carbon, when it forms oblique octahedrons; and the third, when heated to 500° or 600° and thrown into water, when it forms a dark brown, transparent, amorphous, viscid mass, which may be drawn into threads. It is probable that there are three corresponding liquid states, the first fluid at 240° to 300°, the second viscid, from 350° to 500°, and the third again fluid, from 500° to 600°. There are probably also three forms of the gas or vapour, of different densities; at least this seems to be the case in some of its compounds.

Sulphur has a strong affinity for oxygen, but in regard to metals and hydrogen it has equally strong affinities, being negative to them, and analogous in its relations towards metals to both oxygen and chlorine.

The uses of sulphur, as a combustible for matches, as an ingredient in gunpowder, in the manufacture of sulphuric acid, and in medicine, are well known and important.

Sulphur and Oxygen.

These elements combine in several proportions, all the compounds being acids. But two of them are of special importance, sulphurous acid and sulphuric acid. We shall first, therefore, describe these, and then briefly notice the others.

1. Sulphurous Acid, $\backslash text\{SO\}\_2\; =\; 32$.

This is a gas, formed whenever sulphur burns in oxygen or in air. It is colourless, transparent, of a peculiar unpleasant taste and a most suffocating smell. Its specific gravity is 2.247 nearly. It is prepared also by heating sulphuric acid with copper filings, mercury, or charcoal. In each case the sulphuric acid, $\backslash text\{SO\}\_3$, loses 1 eq. oxygen, which

Fig. 26.

unites with the metals or the carbon. In the case of copper we have, $2(\backslash text\{HO\},\; \backslash text\{SO\}\_3)\; +\; \backslash text\{Cu\}\; =\; (\backslash text\{CuO\},\; \backslash text\{SO\}\_3)\; +\; 2\backslash text\{HO\}\; +\; \backslash text\{SO\}\_2$. With charcoal, $2(\backslash text\{HO\},\; \backslash text\{SO\}\_3)\; +\; \backslash text\{C\}\; =\; 2\backslash text\{SO\}\_2\; +\; 2\backslash text\{HO\}\; +\; \backslash text\{CO\}\_2$. In the latter case, the gas is mixed with carbonic acid, and is therefore only proper for preparing the aqueous solution and the salts of sulphurous acid, the carbonic acid being excluded by the stronger sulphurous acid. Sulphurous acid gas must be collected over mercury—being absorbed by water, to which it communicates its suffocating odour.

The gas is easily liquefied by cold, under the ordinary pressure, and is kept in tubes hermetically sealed. It is a decided acid, forming salts with bases, and it has a peculiar action on vegetable blues, first reddening them, and then bleaching them. But the bleaching action differs from that

of chlorine, for the colour is not destroyed, and may be re-stored, reddened of course, by stronger acids.

Sulphurous acid is characterized by its tendency to combine with a third equivalent of oxygen, and thus to be converted into sulphuric acid, which is manufactured in this manner, since sulphur does not combine directly with more than 2 eqs. of oxygen. The salts of this acid, sulphites as they are called, possess a considerable deoxidizing power, depriving many substances of oxygen, and being converted into sulphates.

Sulphurous acid is used in bleaching wool and silk, for which chlorine cannot be employed.

2. Sulphuric Acid.

a. Anhydrous Acid, $\backslash text\{SO\}\_3\; =\; 40$.

Anhydrous or dry sulphuric acid is obtained by distilling the fuming or Nordhausen sulphuric acid, made by heating dried sulphate of iron. This fuming liquid is a compound or a mixture of anhydrous acid and of hydrated sulphuric acid, or oil of vitriol, which is the true active sulphuric acid; a gentle heat expels the anhydrous acid, which is very volatile, and collects in the receiver in the form of white crystals.

Anhydrous sulphuric acid is remarkable for its intense attraction for water. When thrown into water it hisses like red-hot iron, developing much heat, and forming oil of vitriol or hydrated acid. It is probably not a true acid, but readily passes into the true or hydrated form by the action of water. It melts at 68° and boils at 95°. It fumes strongly in the air, and attracts moisture with great rapidity. If a drop of water be allowed to fall into a bottle containing anhydrous sulphuric acid, there is a flash of light with explosion.

b. Hydrated Sulphuric Acid, $\backslash text\{HO\},\; \backslash text\{SO\}\_3$, or $\backslash text\{H\},\; \backslash text\{SO\}\_4\; =\; 49$.

This, the true, active sulphuric acid, also called oil of vitriol, because it is obtained by distilling green vitriol or sulphate of iron, and has an oily consistence, is formed whenever the anhydrous acid comes in contact with water, $\backslash text\{SO\}\_3\; +\; \backslash text\{HO\}\; =\; \backslash text\{HO\},\; \backslash text\{SO\}\_3$. It was formerly made by distilling green vitriol or protosulphate of iron; the action of heat on that salt, dried at a moderate heat, so as to leave 1 eq. of water, is as follows: $\backslash text\{FeO\},\; \backslash text\{SO\}\_3,\; \backslash text\{HO\}\; =\; \backslash text\{FeO\}\; +\; \backslash text\{HO\},\; \backslash text\{SO\}\_3$. The protoxide of iron, $\backslash text\{FeO\}$, is oxidized into sesquioxide $\backslash text\{Fe\}\_2\backslash text\{O\}\_3$, at the expense of half of the sulphuric acid; thus $2\backslash text\{FeO\}\; +\; \backslash text\{SO\}\_3\; =\; \backslash text\{Fe\}\_2\backslash text\{O\}\_3\; +\; \backslash text\{SO\}\_2$; so that finally sesquioxide of iron is left, and sulphurous acid is given off with the oil of vitriol.

The modern manufacturing process is far more economical and productive. Sulphur is burned in air, with the help

Fig. 25.

of a little nitre. The sulphurous acid gas thus formed is conducted into a large leaden chamber, where it meets with nitrous acid, obtained by the action of starch on nitric acid, and also with the vapour of water, introduced from a boiler. Water is also present on the floor of the chamber. The resulting action is rather complex. First, sulphurous acid, nitrous or hyponitrous acid, and water, combine to form the compound $2\backslash text\{SO\}\_3\; +\; \backslash text\{NO\}\_2\; +\; \backslash text\{HO\}$. This compound forms crystals which fall down in a shower like snow, but are decomposed as soon as they touch the water on the floor, when the sulphurous acid is oxidized into sulphuric acid, and the nitrous or hyponitrous acid reduced to deutoxide of nitrogen; thus, $2\backslash text\{SO\}\_3,\; \backslash text\{NO\}\_2,\; \backslash text\{HO\}\; =\; 2\backslash text\{SO\}\_3\; +\; \backslash text\{NO\}\_2\; +\; \backslash text\{HO\}$. The

Chemistry. sulphuric acid dissolves in the water; the deutoxide of nitrogen rises into the air of the chamber, in which a continual current of air is kept up, and with the oxygen of the air again forms nitrous acid, which forms, with a fresh portion of sulphurous acid and water, the same crystalline compound as before. This is again decomposed by the water, and again deutoxide of nitrogen rises, again forms nitrous acid, and again the crystals are formed and decomposed, and so on, continuously. In this way, a comparatively small amount of deutoxide of nitrogen, supplied at first as nitrous acid, oxidizes an almost unlimited quantity of sulphurous acid, acting as a carrier of oxygen from the air to the sulphurous acid. It is only because a little of the deutoxide is unavoidably lost, being carried away with the current of effluent air, that a little nitrous acid must be added from time to time to the original supply. The regular supply of air and of steam is also essential.

After this process has been continued for some time, the water on the floor of the chamber is found so strongly charged with acid, that it no longer thoroughly decomposes the crystals. It is withdrawn and replaced by fresh water. The acid liquid, which is free from nitrous acid provided an excess of sulphurous acid has been present before drawing it off, contains only sulphuric acid and water, with perhaps a little sulphurous acid. It is boiled down in vessels of glass or of platinum, to expel superfluous water, till acid begins to rise in vapour. At this point, all the water has been expelled except the one equivalent, essential, either as water or by its elements, to the existence of the hydrated acid. The liquid now boils at nearly $600^\{\backslash circ\}$, and has a specific gravity of 1.845, and an oily consistence. When pure it is colourless, but the smallest trace of any organic matter such as wood, straw, or even dust, colours it brown. It is very acid and corrosive, and has a most powerful attraction for water, the mixture of it with water developing so much heat as to break the vessel if suddenly effected. This hydrated sulphuric acid or oil of vitriol is a true acid, and in all its relations exhibits the characters belonging to hydrochloric acid, which we have called the type of acids. Like it sulphuric acid with metals forms salts, disengaging hydrogen gas, and with metallic oxides, forms the same salts, water being separated. Hence, as formerly stated, although it contains the elements $\backslash text\{SO\}\_3\; +\; \backslash text\{HO\}$, and may without inconvenience be regarded as a hydrate of the anhydrous acid, it is still more probable that it is really a compound of hydrogen, $\backslash text\{H,\; SO\}\_3$, analogous in constitution to hydrochloric acid, $\backslash text\{HCl\}$. All the phenomena which it exhibits with metals, oxides, &c. may be explained either way, but in the latter form its analogy with hydrochloric acid, so striking in the phenomena, becomes visible in the formula. Thus we have in the case of—

The older formulae in the second column indicate no analogy with hydrochloric acid and chlorides, but in the third and fourth columns this analogy is at once evident, especially in the fourth, where $\backslash text\{Su\}$ stands for $\backslash text\{SO\}\_3$, the hypothetical radical of sulphuric acid and sulphates. It must be borne in mind, however, that the body $\backslash text\{SO\}\_3$ is hypothetical and has not been proved to exist. Yet the assumption of it is generally agreed on, since it brings into one category hydrochloric acid and sulphuric acid with their respective congeners, otherwise separated. On this supposition the action of sulphuric acid on a metal and on a metallic oxide is as follows, which agrees perfectly with the action of hydrochloric acid:—

On a metal, $\backslash text\{M\}\; +\; \backslash text\{H,\; SO\}\_3\; =\; \backslash text\{M,\; SO\}\_3\; +\; \backslash text\{H\}$
On an oxide, $\backslash text\{MO\}\; +\; \backslash text\{H,\; SO\}\_3\; =\; \backslash text\{M,\; SO\}\_3\; +\; \backslash text\{HO\}$

In both cases, the metal simply replaces the hydrogen, Chemistry. and this is true of the older view also, for in that we have—

With metal, $\backslash text\{M\}\; +\; \backslash text\{HO,\; SO\}\_3\; =\; \backslash text\{MO,\; SO\}\_3\; +\; \backslash text\{H\}$
With oxide, $\backslash text\{MO\}\; +\; \backslash text\{HO,\; SO\}\_3\; =\; \backslash text\{MO,\; SO\}\_3\; +\; \backslash text\{HO\}$

We have explained this fully, because the same views apply to all hydrated acids.

Sulphuric acid forms salts with all bases, several of which occur in nature, as sulphate of lime, of baryta, of strontia, of magnesia, of lead. With 1 eq. of water it forms a hydrate, which crystallizes in cold weather. It also forms another hydrate with 2 eqs. of water, which does not crystallize so easily.

The acid is recognised by forming with baryta, whenever they meet in any mixture, the sulphate of baryta, absolutely insoluble, not only in water but also in nitric and hydrochloric acids, which dissolve many compounds of baryta insoluble in water. Any sulphate, moreover, if heated to redness with charcoal, yields, on the addition of water and acids, the odour of sulphuretted hydrogen.

The uses of sulphuric acid are numerous and important. It is employed in the manufacture of soda, bleaching powder, nitric acid, hydrochloric or muriatic acid, acetic acid, ether, alum, Epsom salt, in dissolving bone earth for agricultural purposes, in charging galvanic batteries for the electrolysis and telegraph, and lastly, in medicine and pharmacy. When we consider that many other arts and manufactures depend on these—as for example, the making of glass and of soap, on the manufacture of soda—we shall see that the value of sulphuric acid cannot be over-estimated, and that every improvement in its preparation, or everything that renders it cheaper, will create new manufactures and new arts hereafter, as has already occurred. It is impossible to calculate what might be the effect, for example, of a great reduction in the price of sulphur, for which substance we are dependent on Sicily, and whose cheapness at present enables us to obtain sulphuric acid at a mere fraction of its former price. Such a reduction would not only cheapen sulphuric acid, but also soda, bleaching powder, glass, soap, and many other articles, and through these a multitude of products connected with them. A rise in the price of sulphur, or an interruption in the supply, such as was threatened some years ago, would have a most disastrous effect, through its influence on the price of sulphuric acid.

3. Hyposulphurous Acid, S₂O₃ = 48.

This acid is hardly known in the uncombined state, as it speedily undergoes decomposition. But its salts are permanent. They are formed by boiling the salts of sulphurous acid with sulphur, when 1 eq. of sulphur is taken up by 1 eq. sulphurous acid. Thus, taking sulphite of soda, $\backslash text\{NaO,\; SO\}\_3\; +\; \backslash text\{S\}\; =\; \backslash text\{NaO,\; S\}\_2\backslash text\{O\}\_3$. Hyposulphite of soda thus formed, crystallizes readily in large transparent prisms. It will be seen that this acid contains sulphur and oxygen in the proportion of 1 eq. of each. But we adopt the formula $\backslash text\{S\}\_2\backslash text\{O\}\_3$ instead of $\backslash text\{SO\}$, because, in the neutral salts, 1 eq. of the base is neutralized by $\backslash text\{S\}\_2\backslash text\{O\}\_3\; =\; 48$, and not by $\backslash text\{SO\}\; =\; 24$. The acid may be viewed either as directly formed of $\backslash text\{S\}\_2$ and $\backslash text\{O\}\_3$, or as composed of sulphurous acid and sulphur, $\backslash text\{SO\}\_3\; +\; \backslash text\{S\}$. The latter may probably be true, as there are several acids apparently of analogous constitution.

The most remarkable character of this acid, or rather of its salts is their power of dissolving the compounds of silver which are insoluble in water and in strong acids, such as chloride, bromide, iodide, &c. of silver. The solution thus obtained has a most intensely sweet taste, followed by a metallic aftertaste. On account of this solvent power, hyposulphite of soda is much used in photography, to fix the image by removing the compound of silver unacted on by light, which if left would be acted on and destroy the pictures.

This acid is more permanent. It is formed when sulphurous acid is made to act on peroxide of manganese, a portion of the acid being oxidized to sulphuric acid, which with another portion of sulphurous acid forms the new acid, for $SO\_2\; +\; SO\_2\; =\; S\_2O\_3$. The acid thus formed combines with the peroxide of manganese. The action is as follows: $-2\; SO\_2\; +\; MnO\_2\; =\; MnO\_2\; \backslash cdot\; S\_2O\_3$. From the salt of manganese, others may be obtained, and from the hyposulphate of baryta, the base being removed by sulphuric acid, the acid is obtained. It is sour and tolerably permanent, unless strongly heated, but of little interest hitherto.

There are still three acids formed of sulphur and oxygen. They all contain, like the last-named acid, 5 eqs. of oxygen, with regularly increasing quantities of sulphur, and their names and formulae are—

1. 5. Monosulphuretted hyposulphuric acid, $S\_3O\_4\; =\; 88$
2. 6. Bisulphuretted hyposulphuric acid, $S\_4O\_4\; =\; 104$
3. 7. Tersulphuretted hyposulphuric acid, $S\_5O\_4\; =\; 120$

They all form crystallizable salts with baryta, and may be obtained from these by means of sulphuric acid, which precipitates the baryta. They are sour, and easily decomposed, yielding sulphuric acid, sulphurous acid, and sulphur. They very much resemble one another, and are of no practical interest, so that we shall not describe their preparation. It is remarkable that the third of them is polymeric with hyposulphurous acid, but the two acids are quite distinct; for not only can the acid $S\_3O\_4$ be obtained in solution, while the acid $S\_2O\_3$ is not known in a separate state, but they neutralize totally different weights of the same bases, or equal weights of the base require different weights of the two acids, in the proportion of 120 to 48.

These elements form two compounds, one of which is of great importance to the chemist. It is—

This compound, called also sulphuretted hydrogen, is a gas which sometimes occurs in volcanic districts, and is frequently found dissolved in mineral waters. It is formed during the putrefaction of organic matters containing sulphur, such as albumen in eggs, &c.

It is procured by the action of acids on sulphurets of iron; thus, with sulphuric acid, $FeS\; +\; H\_2SO\_4\; =\; FeSO\_4\; +\; HS$; and with hydrochloric acid, $FeS\; +\; HCl\; =\; FeCl\; +\; HS$. It is also produced by the action of hydrochloric acid on tersulphuret of antimony, $Sb\_2S\_3\; +\; 3HCl\; =\; SbCl\_3\; +\; 3HS$. It may be collected over warm water or brine; cold water absorbs it.

It is transparent, inflammable, burning with a pale blue flame, and depositing sulphur. It has a most offensive smell, that in fact of putrid eggs, which yield it, and this smell it communicates with its sulphureous taste to water, as in the water of Harrowgate. It also blackens the salts of lead and of many other metals, forming sulphurets of the metals. The action with a salt of lead is as follows:—if we suppose the lead to be present as oxide, the acid may be left out of view; $PbO\; +\; HS\; =\; HO\; +\; PbS$. It is the same whether

the lead be present as chloride, $PbCl$, or as sulphate, $PbO$, $SO\_3$, or $Pb$, $SO\_3$, or as carbonate, $PbO$, $CO\_3$. Chemistry.

It is this action of hydrosulphuric acid on metals and their salts which renders it so valuable as a test; for as the sulphurets of the heavy metals are insoluble in water, and usually of dark colours, it enables us to detect a very small quantity of those metals on the solutions of which it acts, and even to distinguish many of them by the colour of the precipitate. The salts of lead, copper, mercury, silver, gold, bismuth, and some others, form dark brown or black sulphurets; protosalts of tin give a chestnut brown; antimony, an orange-brown; persalts of tin, a dirty-yellow; arsenic and cadmium, a bright yellow, with this gas.

The solution of the gas reddens vegetable blues, but as in the case of carbonic acid, on standing or on boiling the blue colour returns.

The gas is liquefied by a powerful pressure. It is highly poisonous when inhaled, and has often caused fatal accidents in graves, when, a neighbouring coffin being perforated by the pickaxe, the gas rushes out and fills the grave, striking down the gravedigger like lightning. The same thing happens in the large cloacae of great cities. It is said that a horse whose head is in pure air, while his body is inclosed in an atmosphere containing only $\backslash frac\{1\}\{50\}$th of its volume of the gas, is soon killed by the gas absorbed through the skin. Taken internally, it is not only safe, but a useful remedy.

This compound is formed when lime and sulphur are boiled with water, till a deep yellow solution is formed. This is then poured into hydrochloric acid of moderate strength. The liquid becomes milky, and on standing deposits the bisulphuret as a heavy oily liquid, which may be withdrawn by a pipette. It undergoes spontaneous decomposition, which is hastened by the presence of metallic oxides. In these characters, and in its composition, it is analogous to the deutoxide of hydrogen. It has a pungent and offensive smell. When sealed up in one end of a bent tube, and left to itself, it is gradually resolved into sulphuric and hydrosulphuric acid, and the latter is liquefied at last by its own pressure.

There is said to be a compound of sulphur and nitrogen, $NS\_3$, a yellowish white powder. But this is doubtful.

When dry chlorine is passed over sulphur they readily unite and form two compounds, both liquid, volatile, and pungent. One is $S\_2Cl$, subchloride or dichloride of sulphur, a dense yellow liquid of specific gravity 1.687, boiling at $280^\backslash circ$. The other is $S\_3Cl$, protochloride of sulphur, a red liquid, of specific gravity 1.620, boiling at $150^\backslash circ$. Both decompose water, forming hydrochloric, sulphuric, and sulphurous acids.

When sulphur and iodine are heated together, they melt and form a black crystalline mass on cooling. It cannot be distilled, heat decomposing it, and its precise composition is unknown. Nothing is known of the compounds of bromine and sulphur, nor of those of fluorine with sulphur.

When the vapour of sulphur is made to pass over charcoal at a red-heat, a compound is formed which passes over and is collected in water where it sinks as a heavy liquid to the bottom. To purify it from a little sulphur that distils along with it, it is distilled again at a gentle heat. Its composition is $CS\_2$, bisulphuret of carbon.

It is a very mobile, transparent, colourless liquid, refracting light strongly, of a very peculiar odour, like that of decaying cabbage or horse-radish, very inflammable. Its specific gravity is 1.293, and it boils at $118^\backslash circ$.

Chemistry. Bisulphuret of carbon is a powerful solvent for many substances insoluble in water or alcohol; such as phosphorus, sulphur (which crystallizes beautifully from the solution), resins; and even such bodies as caoutchouc and gutta percha are softened if not dissolved by it. It is now manufactured on the large scale, and is used in making varnishes and certain preparations of caoutchouc.

10. Selenium.

Symbol Se. Equivalent = 39.5.

This element is so closely analogous to sulphur, that a very brief account of it will suffice. It occurs in nature chiefly as seleniuret of lead, a rare mineral. Some kinds of iron pyrites contain a little seleniuret of iron, or at all events a compound of selenium; and when sulphur made from such pyrites is converted into sulphuric acid, there is found in the leaden chambers a deposit consisting of sulphur, several metals, such as copper, lead, and arsenic, and selenium. From this deposit, or from the native seleniuret of lead, selenium is prepared. It is first dissolved in the form of selenious acid, and this is deoxidized by sulphurous acid, when the selenium separates as a deep red powder. At 392° it melts, forming a brown liquid, which boils at about 1290°, and yields a deep yellow gas. On cooling, the liquid first becomes viscous and tenacious, and finally consolidates to a nearly black solid mass, which in thin layers is translucent and red, and has a metallic lustre. When sublimed, the vapour condenses on a cold surface into a powder of a fine red, similar to the precipitated selenium.

In all its chemical relations selenium resembles sulphur, only that it is denser, its specific gravity being 4280 in the mass and 4800 in the powder; and less volatile. Its affinities are also less powerful.

Selenium appears to form three compounds with oxygen, namely, the oxide of selenium, a gas, the composition of which is not known, selenious acid $\backslash text\{SeO\}\_2$, and selenic acid $\backslash text\{SeO\}\_3$.

Heated in air, selenium burns with a pure blue flame, producing a very penetrating and peculiar odour, compared to that of putrid horse-radish. This belongs to the oxide, which has not been obtained in a state of purity. A very minute trace of selenium may be detected before the blow-pipe by this character. The gas appears to be poisonous.

Selenious acid, $\backslash text\{SeO\}\_2$, is the chief product of the combustion of selenium, and if the operation is conducted in oxygen in a proper apparatus, the acid collects in the cold part of it in colourless crystals, very soluble in water. This acid is also formed when selenium is oxidized by nitric or by nitro-hydrochloric acids. Selenious acid is deprived of its oxygen by many substances, such as iron, zinc, or sulphurous acid, when the selenium is deposited as a cinnabar red powder. The vapour of the acid is yellow.

Selenic acid, $\backslash text\{SeO\}\_3$, or, in the state of hydrate, $\backslash text\{HO\}\_2\backslash text\{SeO\}\_3$, = $\backslash text\{H\}\_2\backslash text\{SeO\}\_4$, is entirely analogous to sulphuric acid in properties. It is obtained in combination with oxide of lead, by heating seleniuret of lead with nitrate of potash; or by first heating selenium with that salt, which forms selenate of potash, and then adding to the solution of the potash salt a salt of lead, when selenate of lead is precipitated. This salt, $\backslash text\{PbO\}\_2\backslash text\{SeO\}\_3$, is decomposed by hydrosulphuric acid as follows: $\backslash text\{PbO\}\_2\backslash text\{SeO\}\_3\; +\; \backslash text\{HS\}\; =\; \backslash text\{PbS\}\; +\; \backslash text\{HO\}\_2\backslash text\{SeO\}\_3$; thus yielding the hydrated acid. The anhydrous selenic acid is unknown.

The salts of selenic acid are in form and properties so exactly similar to those of sulphuric acid, as only to be distinguishable from them by analysis.

With hydrogen, selenium forms a gaseous acid, hydroselenic acid, $\backslash text\{H\}\_2\backslash text\{Se\}$, of a most fetid odour, and still more poisonous than hydrosulphuric acid. It is obtained, like hydrosulphuric acid, by the action of hydrochloric acid on a compound of iron with selenium. $\backslash text\{HCl\}\; +\; \backslash text\{FeSe\}\; =\; \backslash text\{FeCl\}\_2\; +\; \backslash text\{H\}\_2\backslash text\{Se\}$. This gas is soluble in water, and acts on metallic

solutions in the same way as hydrosulphuric acid, only forming seleniurets instead of sulphurets. $\backslash text\{MO\}\; +\; \backslash text\{H\}\_2\backslash text\{Se\}\; =\; \backslash text\{HO\}\; +\; \backslash text\{M\}\_2\backslash text\{Se\}$, is a general equation representing the action of hydroselenic acid on metallic protoxides.

Like sulphur, selenium combines directly with metals, when heated with them.

From its great rarity, selenium is not applied, either itself or in its compounds, to any useful purpose. It is probable, however, that some of its compounds, from their energetic action on the system, may prove valuable remedies.

The metallic lustre of compact selenium has led some to class it with metals. But it is a nonconductor of heat and electricity, and so analogous to sulphur, that these elements must be considered together. A third substance belongs to the same group by its chemical relations, namely tellurium; but that element has the conducting power as well as the lustre of metals, and it is regarded as a metal.

11. Phosphorus.

Symbol P. Equivalent = 32.

This element is found chiefly in the animal kingdom, in bones, as phosphoric acid united to lime and a little magnesia. But all the phosphate of lime, or bone earth of animals, is derived from their food, and consequently from the soil. In fact, all fertile soils contain this phosphate in small but essential quantity. Without it, although all the other elements of plants were present, no plant could grow. Phosphate of lime is found in minute crystals scattered through all rocks, and there are occasionally beds containing large quantities of bone earth, derived from the bones of extinct animals; as the osteolite of the Rhine, and the so-called coprolite beds in the Suffolk Crag. Phosphoric acid also occurs sparingly, combined with the oxides of lead, copper, uranium, and iron.

Phosphorus is obtained from bones, by first burning them to destroy the animal matter. The bone earth is next bruised, and digested with sulphuric acid and water till all coarse grains have disappeared, and a uniform fine powder, sulphate of lime, is formed. The water now holds in solution acid phosphate of lime.

This is filtered from the sulphate, evaporated to the consistency of a syrup, mixed with charcoal, and exposed to a white heat in a retort. The charcoal deprives the phosphoric acid of oxygen, and while carbonic acid escapes, phosphorus distils over, and is collected under water. To purify it, it is melted under water and squeezed through leather. It is then, if necessary, redistilled by itself, and kept under water.

It is a solid, nearly colourless, translucent, of the consistency of wax, fusible a little above 100°, and taking fire in air at that temperature. It boils at 550°. The specific gravity of phosphorus is 1770, water = 1000. That of its vapour is 4326, air = 1000.

It is distinguished by its inflammability, which is such, that if exposed, dry, to air, in warm weather, it often takes fire spontaneously, and is therefore kept under water. When exposed to light, it becomes opaque externally, and the change spreads gradually inwards.

When heated, it burns with great splendour, whether in air or oxygen, especially in the latter; in burning, it forms phosphoric acid, which, if no moisture be present, condenses to a white powder like snow.

When phosphorus is heated for some time short of its boiling point, between 445° and 480°, it becomes dark

Fig. 27.

Chemistry. red, comparatively infusible, far less inflammable, and insoluble in sulphuret of carbon, which dissolves ordinary phosphorus. The red modification does not melt nor take fire, even at $480^\{\backslash circ\}$. At about $500^\{\backslash circ\}$ it passes into ordinary phosphorus. This is a most remarkable instance of allotropic modification of an element, which in these two states exhibits properties, both physical and chemical, more different than are those of many different elements. Yet the one form passes into the other, and both yield the same compounds.

Phosphorus and Oxygen.

The affinity of phosphorus for oxygen is very strong, so that even at ordinary temperatures it is slowly oxidized in the air, becoming luminous in the dark. The temperature also rises slightly, and in summer the oxidation is thereby so much accelerated that heat enough is evolved to melt and set fire to the phosphorus. This is a true case of spontaneous combustion, and illustrates the occurrence of that phenomenon in powdered charcoal, or in porous bodies moistened with oil. At any season it may be illustrated in phosphorus, by allowing a few drops of a solution of phosphorus in bisulphuret of carbon to dry up on blotting paper, on which it leaves a film of finely divided phosphorus. This very soon begins to oxidize, vapours rise from it luminous in the dark, it gradually becomes warm, and in a few minutes bursts into flame. It has recently been proved by Schönbein that phosphorus in air first causes the formation of ozone, which is an allotropic form of oxygen, and possibly also of a hydrate of this substance, having the composition $\backslash text\{HO\}\_3\; =\; \backslash text\{O\}\_2\; +\; \backslash text\{HO\}$. Both of these forms of ozone are much more powerful oxidizing agents than ordinary oxygen, and it appears that it is ozone which really oxidizes phosphorus, when exposed to air till the temperature rises so high as to cause combustion.

1. Phosphorous Acid, $\backslash text\{PO\}\_3\; =\; 56$.

This acid is formed when phosphorus undergoes slow oxidation. It is best obtained pure by the action of water on trichloride of phosphorus, $\backslash text\{P\; Cl\}\_3\; +\; 3\; \backslash text\{HO\}\; =\; 3\; \backslash text\{H\; Cl\}\; +\; \backslash text\{PO\}\_3$. It is very soluble and sour, and may be obtained in a mass of deliquescent crystals of the hydrated acid, $\backslash text\{PO\}\_3\; \backslash cdot\; 3\; \backslash text\{HO\}$. When heated, it yields phosphorette hydrogen gas, while phosphoric acid is left; thus, $4\; \backslash text\{PO\}\_3\; +\; 3\; \backslash text\{HO\}\; =\; 3\; \backslash text\{PO\}\_3\; +\; \backslash text\{PH\}\_2$. Phosphorous acid is not of much importance. It tends, by combining with 2 eqs. of oxygen, to form phosphoric acid.

2. Phosphoric Acid.

a. Anhydrous Phosphoric Acid, $\backslash text\{PO\}\_3\; =\; 72$.

This compound is formed when phosphorus is burned in dry air or oxygen, and appears as a snow-white substance,

Fig. 28.

which must be instantly sealed up hermetically, otherwise

it attracts moisture from the air, and deliquesces into the Chemistry. monobasic hydrated acid, $\backslash text\{PO\}\_3\; \backslash cdot\; \backslash text\{HO\}$.

It is doubtful whether the anhydrous acid be really an acid. It cannot be tasted without bringing it in contact with water on the tongue, when it instantly forms the hydrated or true acid. It is used in research on account of its tendency to abstract the elements of water from many organic substances, without charring them as sulphuric acid does.

b. Hydrated Phosphoric Acid.

1st, Monobasic, $\backslash text\{PO\}\_3\; \backslash cdot\; \backslash text\{HO\}$ or $\backslash text\{PO\}\_4$, $\backslash text\{H\}\; =\; 81$.

This acid is formed when the anhydrous acid acts on water. It is apt to pass into the dibasic and tribasic forms, by taking up more water, but is again obtained pure by heating to low redness, when the monobasic acid is left. It is very sour, coagulates albumen, and causes a white precipitate in nitrate of silver, the monobasic phosphate of silver, $\backslash text\{PO\}\_3\; \backslash cdot\; \backslash text\{AgO\}$ or $\backslash text\{PO\}\_4\; \backslash cdot\; \backslash text\{Ag\}$. It forms only one series of salts, containing one eq. of base or of metal, hence its name. Its solution passes, slowly in the cold, rapidly when heated, first into the dibasic, and then into the tribasic form.

2d, Dibasic Phosphoric Acid, $\backslash text\{PO\}\_3\; \backslash cdot\; 2\; \backslash text\{HO\}$ or $\backslash text\{PO\}\_5$, $\backslash text\{H\}\_2\; =\; 90$.

This acid is obtained by heating the solution of the tribasic or common phosphoric acid till only 2 eqs. of water are left, or by decomposing dibasic phosphate of lead or silver by hydrosulphuric acid. Its solution is not permanent, passing into the tribasic acid. But its salts are quite permanent. It does not coagulate albumen, nor precipitate nitrate of silver, unless ammonia or some base be added, when it forms a white dibasic phosphate of silver, quite different from the monobasic salt. It forms two series of salts, those with two eqs. of fixed base, such as dibasic phosphate of soda, $\backslash text\{PO\}\_3\; \backslash cdot\; 2\; \backslash text\{NaO\}$ or $\backslash text\{PO\}\_5\; \backslash cdot\; \backslash text\{Na\}\_2$, and those with 1 eq. of fixed base, and 1 eq. of basic water, as the acid dibasic phosphate of soda, $\backslash text\{PO\}\_3\; \backslash cdot\; \backslash text\{NaO\}\; \backslash cdot\; \backslash text\{HO\}$ or $\backslash text\{PO\}\_5\; \backslash cdot\; \backslash text\{NaH\}$.

3d, Tribasic Phosphoric Acid, $\backslash text\{PO\}\_3\; \backslash cdot\; 3\; \backslash text\{HO\}$ or $\backslash text\{PO\}\_6$, $\backslash text\{H\}\_3\; =\; 99$.

This is the common form of phosphoric acid, as the others, by taking up water, pass into it. It does not coagulate albumen, nor precipitate nitrate of silver, till some base is added, when it forms a lemon-yellow precipitate of tribasic phosphate of silver, $\backslash text\{PO\}\_3\; \backslash cdot\; 3\; \backslash text\{AgO\}$ or $\backslash text\{PO\}\_6\; \backslash cdot\; \backslash text\{Ag\}\_3$. It forms 3 series of salts, with 3 eqs. of fixed base, as the salt of silver just named; with 2 of fixed base and 1 of water, as in common phosphate of soda, $\backslash text\{PO\}\_3\; \backslash cdot\; 2\; \backslash text\{NaO\}\; \backslash cdot\; \backslash text\{HO\}$ or $\backslash text\{PO\}\_5\; \backslash cdot\; \backslash text\{Na\}\_2\; \backslash cdot\; \backslash text\{H\}$; and with 1 eq. of fixed base and 2 of water, as the acid phosphate of potash, $\backslash text\{PO\}\_3\; \backslash cdot\; \backslash text\{KO\}\; \backslash cdot\; 2\; \backslash text\{HO\}$ or $\backslash text\{PO\}\_6\; \backslash cdot\; \backslash text\{K\}\; \backslash cdot\; \backslash text\{H\}\_2$. This is the form of the acid existing in bones, and in the mineral kingdom.

It will be seen, that if we regard these acids as hydrates or compounds of water with dry acid, they all contain the same acid, and differ only in water. This is possible, for in sulphuric acid we have oil of vitriol, $\backslash text\{SO\}\_3\; \backslash cdot\; \backslash text\{HO\}$, the hydrate $\backslash text\{SO\}\_4\; \backslash cdot\; 2\; \backslash text\{HO\}$, and the hydrate $\backslash text\{SO\}\_3\; \backslash cdot\; 3\; \backslash text\{HO\}$, all of which undoubtedly contain the same acid, and yield the same salts with bases, namely, sulphates, of which there is but one class or series.

But the three phosphoric acids differ far more than is accounted for by mere differences in the proportion of water, as in their action on albumen, and on nitrate of silver, and above all, in forming salts entirely distinct, which are monobasic, dibasic, and tribasic, that is, contain 1, 2, or 3 eqs. of base, fixed or otherwise. Nothing of all this occurs in the different hydrates of sulphuric acid, the first of which is the true acid, $\backslash text\{H\}\_2\; \backslash text\{SO\}\_4$, as already explained, the other two compounds of this with water, $\backslash text\{H\}\_2\; \backslash text\{SO\}\_4\; \backslash cdot\; \backslash text\{HO\}$ and $\backslash text\{H\}\_2\; \backslash text\{SO\}\_4\; \backslash cdot\; 2\; \backslash text\{HO\}$. But this water in these two compounds is not basic, and is not replaceable by bases.

What is the cause of this difference between sulphuric

Chemistry. and phosphoric acid? It is not enough to say, that when the dry acid is combined with 1, 2, or 3 eqs. of water, it is disposed to take up 1, 2, or 3 eqs. of base. That is the fact to be explained, and not an explanation of it. Sulphuric acid exhibits no such tendency when combined with 2 or 3 eqs. of water, and we can hardly suppose the three phosphoric acids to contain the same acid.

Now, here the hydrogen theory of acids comes to our aid. According to it, the first hydrate is $\backslash text\{PO\}\_3\backslash text\{H\}$, like nitric acid, $\backslash text\{NO\}\_3\backslash text\{H\}$. It takes 1 eq. of base to form neutral salts, because it contains 1 eq. of replaceable hydrogen, as sulphuric and nitric acids do.

The second hydrate is $\backslash text\{PO\}\_2\backslash text\{H\}\_2$. Here the radical is different, and as there are 2 eqs. of replaceable hydrogen, it takes 2 eqs. of base to form neutral salts. And there are two series of these, according as half or the whole of the hydrogen is replaced by metals.

The third hydrate, in like manner, is $\backslash text\{PO\}\_3\backslash text\{H\}\_3$, with a different radical, and requiring 3 eqs. of base for the 3 eqs. of replaceable hydrogen, forming also 3 series of salts, according as the hydrogen is partially or entirely replaced by metals.

On this view, the three acids must be different, whereas, on the other, they ought to be the same, as in sulphuric acid. We see also why they form different salts, and why the first can form only one series, the second two, and the third three series of salts.

Phosphoric acid as such is not much used nor of much interest, but its salts, especially those of the tribasic modification, are of the utmost and most essential importance. The earth of bones is essentially a tribasic phosphate of lime; the principal salt in the blood, to which it owes its alkaline reaction, and its peculiar power of absorbing and of giving off carbonic acid, is a tribasic phosphate of soda, with 2 eqs. of fixed base and 1 eq. of basic water, $\backslash text\{PO\}\_3\backslash text\{Na\}\_2\backslash text\{HO\}$, or $\backslash text\{PO\}\_3\backslash text\{Na\}\_2\backslash text\{H\}$, and the chief salt in the juice of flesh, and in the gastric juice, that which gives the acid reaction to these fluids, is a tribasic phosphate of potash, with 1 eq. of fixed base and 2 eqs. of basic water, $\backslash text\{PO\}\_3\backslash text\{K\}\_2\backslash text\{HO\}$, or $\backslash text\{PO\}\_3\backslash text\{K\}\_2\backslash text\{H\}$. Since these peculiar phosphates have undoubtedly each its own peculiar function to perform in the animal economy, we see how important it is to study the most minute and apparently trifling peculiarities of such a compound as tribasic phosphoric acid; although these very researches, when first made, were regarded as scientific curiosities of no practical value. No one could have conjectured that it would ever be important to know that tribasic phosphoric acid with soda tends to form a salt with 2 eqs. of fixed base, of alkaline properties, though its composition is that of an acid salt, while the same acid with potash forms by preference a salt with 1 eq. of fixed base which is strongly acid, while yet the acid can form two other compounds with each alkali. Yet we now see that such apparently insignificant facts are closely connected with the due performance of the most essential vital functions. In hundreds of cases we can substitute potash for soda, and obtain the same results, but potash cannot replace the soda of the phosphate in the blood, nor soda the potash of that in the juice of flesh.

3. Hypophosphorous Acid, $\backslash text\{PO\}\; =\; 40$.

This acid is formed when phosphorus is boiled with bases, such as lime or potash. Phosphoric acid and phosphuretted hydrogen are formed at the same time, as will be shown under phosphuretted hydrogen, and the two acids both combine with the base. The phosphate of lime is insoluble, the hypophosphite soluble. The acid is little known, but has a tendency to absorb oxygen and pass into phosphoric acid.

Phosphorus and Hydrogen.

1. Phosphuretted Hydrogen Gas, $\backslash text\{PH\}\_3\; =\; 35$.

This compound is formed when phosphorus is boiled with

lime and water, or potash and water. The precise nature Chemistry. of the reaction is not ascertained, so as to enable us to represent it in an equation; but water is decomposed, and while its oxygen unites with one portion of phosphorus to form hypophosphorous acid, and apparently with another to form phosphoric acid, its hydrogen combines with a third portion, forming the new compound, which is given off in the form of a gas, not absorbed by water.

It is highly inflammable, and when prepared as above takes fire spontaneously in contact with air. But this property is owing to the presence of a minute quantity of another compound of the same elements, the liquid phosphuretted hydrogen; and when this is removed, the gas is no longer spontaneously inflammable, though still taking fire when slightly heated in air. The spontaneously inflammable gas loses that property by standing over water, when it deposits a little of a solid phosphuretted hydrogen, formed from the liquid one.

Each bubble of the spontaneously inflammable gas, as it rises through the water, takes fire, and forms a beautiful ring of white vapour (water and phosphoric acid), which expands as it ascends. Bubbles of the gas, allowed to enter a vessel of oxygen, produce each of them a slight explosion and a brilliant flash of light, but care must be taken that only one bubble at a time enters the oxygen, otherwise dangerous explosions may occur. The gas may also be prepared by the action of phosphuret of calcium on water.

When phosphuret of calcium is acted on by dilute hydrochloric acid, or when phosphorus is boiled with an alcoholic solution of potash, the pure gas is formed, not spontaneously inflammable. All substances which destroy the liquid phosphuretted hydrogen, such as alcohol, ether, and volatile oils, deprive the spontaneously combustible gas of that property. And the addition of a minute trace of the liquid compound to the non-spontaneously inflammable gas renders it at once spontaneously inflammable.

2. Liquid Phosphuretted Hydrogen, $\backslash text\{PH\}\_2\; =\; 34$.

When the spontaneously inflammable gas is passed through a U-shaped tube surrounded by a freezing mixture, it deposits water which freezes, and a small quantity of the liquid compound, $\backslash text\{PH\}\_2$. The gas has now lost its spontaneous inflammability. The liquid instantly takes fire in contact with air or oxygen. When kept the liquid is resolved into the gas $\backslash text\{PH\}\_3$, and a solid compound $\backslash text\{P\}\_2\backslash text\{H\}$, thus, $5\backslash text\{PH\}\_2\; =\; 3\backslash text\{PH\}\_3\; +\; \backslash text\{P\}\_2\backslash text\{H\}$.

3. Solid Phosphuretted Hydrogen, $\backslash text\{P\}\_2\backslash text\{H\}\; =\; 65$.

The formation of this compound from the liquid one has just been explained. It is deposited from the spontaneously inflammable gas when kept over water, as an orange-coloured film.

It should be mentioned that the gas, $\backslash text\{PH\}\_3$, has a composition analogous to that of ammonia, $\backslash text\{NH\}\_3$, and that it has also some analogy in properties. Thus it seems to be a weak base, and with hydriodic acid it forms a saline compound crystallizing in cubes, like the hydriodate of ammonia. The phosphuretted hydrogen also, like ammonia, admits of the replacement of its hydrogen by such radicals as methyl and ethyl, forming volatile bases, analogous to the volatile organic bases derived from ammonia.

With nitrogen phosphorus forms a compound, $\backslash text\{PN\}\_3$, a white solid, which resists a red heat and the action of the strongest acids.

Phosphorus and Chlorine.

1. Trichloride of Phosphorus, $\backslash text\{PCl\}\_3\; =\; 138.5$.

When chlorine comes in contact with phosphorus, the latter takes fire, and they combine. If the phosphorus be in excess, we obtain a colourless liquid, $\backslash text\{PCl\}\_3$. It is pungent, fuming, of specific gravity 1.450. When mixed with

Chemistry. water it sinks to the bottom, and there is rapidly dissolved, heat being developed. The action is as follows: $\backslash text\{PCl\}\_3\; +\; 3\backslash text\{HO\}\; =\; 3\backslash text\{HCl\}\; +\; \backslash text\{PO\}\_3$. The hydrochloric acid dissolves in the water along with the phosphorus acid, but may be expelled by a gentle heat, and hydrated phosphorus acid is left.

2. Perchloride of Phosphorus. $\backslash text\{PCl\}\_5\; =\; 209.5$.

When the chlorine is in excess, or when chlorine is passed through the last compound, there is formed the solid perchloride, $\backslash text\{PCl\}\_5$. This also decomposes water, as follows: $\backslash text\{PCl\}\_5\; +\; 5\backslash text\{HO\}\; =\; 5\backslash text\{HCl\}\; +\; \backslash text\{PO\}\_5$, yielding, therefore, hydrochloric and phosphoric acids.

Perchloride of phosphorus has of late been employed as an agent of research in organic chemistry. By its means, certain anhydrous organic acids, previously unknown, have been obtained, and much light thrown on the constitution of organic acids.

Phosphorus combines instantly with iodine, heat and light being evolved. The compounds are of a dark red or brown colour, and solid. They decompose water exactly as the chlorides do. It is probable that there are two iodides, $\backslash text\{PI\}\_3$ and $\backslash text\{PI\}\_5$, corresponding to the chlorides, but the former alone has been analysed, and another iodide, $\backslash text\{PI\}\_5$, has also been obtained. These two compounds may be had in orange and in dark red crystals by dissolving phosphorus and iodine in bisulphuret of carbon, and applying artificial cold.

Bromine acts very violently on phosphorus, and seems to form bromides corresponding to the chlorides.

No compounds are known of phosphorus with fluorine or carbon.

With sulphur, phosphorus readily combines in several proportions. The compounds are not only highly inflammable, but liable to explode with great violence when slightly warmed, and sometimes even spontaneously. Hence they are very dangerous, and must be very cautiously experimented with. Berzelius, who examined them, narrowly escaped from some frightful explosions, occurring quite unexpectedly. Some of them are liquid, others solid; and it appears that the different allotropic states of both the elements are seen also in these compounds, several of which are isomeric. As these compounds are not fully understood, and are for the present of no practical importance, we shall not enter into details in regard to them.

12. Boron.

Symbol B. Equivalent = 10.9.

This element is found in nature combined with oxygen, as boracic acid, which occurs free, dissolved in the vapours of certain volcanic districts in Tuscany. The hot vapours are received in reservoirs of water, in which the acid dissolves, and the heat of the vapours is employed to evaporate the water, till at last the acid crystallizes nearly pure. In Thibet boracic acid occurs in the soil near certain lakes, combined with soda, forming the crude borax of commerce, which is purified from the peculiar fatty matter it contains by a secret process known only to the Dutch. Borax is now largely manufactured in England, from the boracic acid of Tuscany, and of the volcanic islands of the Mediterranean.

Boron is obtained by heating dry boracic acid with potassium, $4\backslash text\{BO\}\_3\; +\; \backslash text\{K\}\_2\; =\; 3(\backslash text\{KO\},\; \backslash text\{BO\}\_3)\; +\; \backslash text\{B\}$. Water dissolves the borate of potash, and leaves the boron as a dark brown infusible powder, which, when heated to redness in oxygen, burns and reproduces boracic acid.

Boracic Acid, $\backslash text\{BO\}\_3\; =\; 34.9$.

Obtained as above described, and purified by repeated crystallization, or from borax (bicarbonate of soda) by the addition of sulphuric acid to a hot saturated solution, forms white

Chemistry. scaley crystals, composed of the anhydrous acid and water, $\backslash text\{BO\}\_3\; \backslash cdot\; 3\backslash text\{HO\}$. When heated the crystals melt, lose their water, and at a red heat leave the anhydrous acid perfectly fluid, which on cooling forms a transparent glass. This soon becomes opaque by attracting water from the air. The acid is very soluble in hot, sparingly soluble in cold water. It is a weak acid at ordinary temperatures, but at a red heat it expels all less fixed acids. It gives to all its compounds a peculiar tendency to melt when heated, and hence borax is much used as a flux in metallurgical operations on the small scale. Boracic acid, and the compounds of boron in general, colour flame green, and by this character the presence of boracic acid has been detected in various minerals in which it is present in small quantity, as in tourmaline and schorl, &c. Boracite and datholite contain boracic acid in larger proportion. There is no other compound of boron and oxygen.

With chlorine boron forms a gaseous compound, the trichloride, $\backslash text\{BCl\}\_3$, corresponding to boracic acid, which decomposes water, yielding boracic and hydrochloric acids, $\backslash text\{BCl\}\_3\; +\; 3\backslash text\{HO\}\; =\; 3\backslash text\{HCl\}\; +\; \backslash text\{BO\}\_3$.

With fluorine it forms a similar gas, trifluoride of boron, $\backslash text\{BF\}\_3$, which fumes strongly in moist air. Its action on water is as follows: $3\backslash text\{BF\}\_3\; +\; 3\backslash text\{HO\}\; =\; (3\backslash text\{HF\},\; 2\backslash text\{BF\}\_3)\; +\; \backslash text\{BO\}\_3$. The compound $3\backslash text\{HF\},\; 2\backslash text\{BF\}\_3$ is called hydrofluoroboric acid.

With nitrogen boron is said to form a white solid compound, BN, which is very stable. When fused with hydrate of potash it is converted into boracic acid and ammonia, the latter being expelled, while the former combines with the potash. $\backslash text\{BN\}\; +\; 3\backslash text\{KO\},\; \backslash text\{HO\}\; =\; \backslash text\{KO\},\; \backslash text\{BO\}\_3\; +\; 2\backslash text\{KO\}\; +\; \backslash text\{NH\}\_3$.

The chief uses of boracic acid are as a flux, both in itself and in the form of borax; and the latter substance is also used in medicine. Goldsmiths use borax to clean the surface of gold, silver, and other metals which are to be soldered together. Borax is sprinkled on the metal, and melted by the blowpipe, when it dissolves any oxide or other impurities, leaving a bright metallic surface.

13. Silicon or Silicium.

Symbol Si. Equivalent = 21.3.

Next to oxygen, this is perhaps the most abundant element. Its only oxide, silicic acid or silica, constitutes, whether free or combined, by far the greater part of all rocks and soils, excepting only the different forms of limestone, marble, and chalk, gypsum, and rock salt.

Silicon is obtained by the action of potassium on a compound of fluorine, silicon, and potassium; and appears, like boron, in the form of a dark brown powder, which, strongly heated in oxygen, burns with a brilliant light, and is converted into silicic acid. It has recently been stated that, by means of a galvanic current, silicon has been deposited on the surface of metals, and exhibits a bright metallic lustre. This requires confirmation.

Silicon and Oxygen. $\backslash text\{SiO\}\_2\; =\; 45.3$.

This, the most abundant of all minerals, occurs pure in the form of rock crystal and quartz; which is either crystallized in six-sided prisms, terminated by six-sided pyramids, or compact and massive. Many kinds of sandstone are also nearly pure silicic acid, or silica, as it is often called. With very small quantities of oxide of iron and other metals, it forms agate, calcedony, jasper, carnelian, bloodstone, and many other ornamental stones. In amethyst there is only a trace of manganese. In opal there is hardly any impurity; and flint is also very nearly pure silicic acid. The deposits and rocks known by the names of mountain meal, polishing slate, kieselguhr, &c., are also pure silicic acid, in the form of the exuviae or shells of the diatomaceæ, microscopic organisms which abound in almost all natural waters.

Felspar, which is an ingredient of almost all rocks, is a compound of silicic acid, alumina, and potash; and there are few other minerals occurring in rocks which are not also

Chemistry. silicates. This is the case with mica, hornblende, talc, serpentine, hypersthene, &c.; while porphyry, slate rock, grauwacke, and many others, such as basalt, trap rocks, and lavas, are chiefly modifications of felspar. The simple minerals found crystallized in nodules and veins in all rocks, such as zeolites and the like, are also in most cases silicates.

Silicic acid exists dissolved in sea-water and in all natural waters, although in small proportion, as is proved by the existence and rapid development of the siliceous shelled diatomaceae in all such waters. Flint has probably been originally in the form of these minute organisms, for it often contains them unaltered. Some springs, especially thermal springs, contain much dissolved silica, as, for example, the Geysers of Iceland.

Pure silicic acid is easily obtained by simply pulverizing rock crystal or white quartz. From impure quartz, or siliceous sand or minerals, it is obtained by fusion with three parts of potash, when a glass is formed, soluble in water. The addition of acids to the strong solution causes the silica to separate as a bulky jelly; if added to the very diluted solution, the silica remains dissolved; but on evaporation to a certain point gelatinizes. The jelly is dried up to a powder, and then the silica becomes absolutely insoluble in water and acids, except hydrofluoric acid. All soluble matter being washed away with the aid of acid, the silica remains as a powder, gray and translucent while moist, snow-white and opaque when dry. However finely divided, it is always gritty to the teeth. Although thus expelled from its salts at ordinary temperatures by almost all acids, at a red heat it expels in its turn all that are volatile at that temperature. With the alkalis it forms glass, soluble when the alkali is in excess, insoluble or ordinary glass when the silicic acid predominates.

The uses of silica are numerous. It is an essential constituent of plants, more especially of the gramineae, cerealia, and rush, cane, or bamboo tribes. It is employed in the manufacture of glass, and in the form of sand, for building mortar. It is also used along with lime, with which it forms a fusible silicate, as a flux in smelting metals, especially iron. Many forms of it are valued as ornamental stones, of which the opal is the most precious, from its rarity and beauty.

With chlorine, silicon forms a volatile fuming liquid, terchloride of silicon, $\backslash text\{SiCl\}\_4$, which decomposes water in the same way as terchloride of boron, $\backslash text\{SiCl\}\_4\; +\; 3\backslash text\{HO\}\; =\; 3\backslash text\{HCl\}\; +\; \backslash text\{SiO\}\_2$.

With fluorine it forms a gaseous terfluoride, $\backslash text\{SiF\}\_4$. This acts on water like the terfluoride of boron, producing a hydrofluosilicic acid. $3\backslash text\{SiF\}\_4\; +\; 3\backslash text\{HO\}\; =\; (3\backslash text\{HF\},\; 2\backslash text\{SiF\}\_2)\; +\; \backslash text\{SiO\}\_2$. The hydrofluosilicic acid ($3\backslash text\{HF\},\; 2\backslash text\{SiF\}\_2$) forms with potash and soda insoluble double salts, as, for example, in this reaction: $3\backslash text\{HF\},\; 2\backslash text\{SiF\}\_2\; +\; 3\backslash text\{KO\}\; =\; 3\backslash text\{KF\},\; 2\backslash text\{SiF\}\_2\; +\; 3\backslash text\{H\}\_2\backslash text\{O\}$. The new salt, $3\backslash text\{KF\},\; 2\backslash text\{SiF\}\_2$, may be regarded as a compound of fluoride of potassium with terfluoride of silicon.

It is in consequence of the great tendency of silicic acid to form the terfluoride with hydrofluoric acid, that the latter acid corrodes glass and porcelain. But the terfluoride of silicon may be prepared in glass vessels, by heating a mixture of fluor-spar, fine sand, and sulphuric acid. The sand is dissolved and the glass escapes. The terfluoride being made to pass through water decomposes it as above explained, forming hydrofluosilicic acid and silicic acid. The latter separates as a jelly, which would soon block up the tube, if we did not protect it by causing the end of it to dip just under the surface of mercury, by which means no water reaches the tube. The liquid is strained off from the jelly, and used as a test for potash and baryta. The jelly when dry forms a fine light bulky powder of silicic acid. If the liquid and the jelly be evaporated together, the terfluoride is reproduced, and the whole disappears.

METALS.

This numerous class of elements is characterized by two properties, both of which are present in every metal. These

are, the metallic lustre, and the power of conducting heat and electricity. Metals exhibit also, in general, a strong attraction for the more negative non-metallic elements, being themselves, as a class, positive. It is particularly towards oxygen, chlorine, bromine, iodine, fluorine, sulphur, and selenium, that metals show this attraction; which, however, varies remarkably in degree, from potassium and its congeners, which can with difficulty be kept in the uncombined metallic state, on the one hand, to the noble metals on the other, which frequently cannot be made directly to combine with oxygen.

Before briefly describing the more important metals, for the majority of them need only be enumerated, we shall prefix some general remarks on the physical and chemical properties of the metals, which may thus be conveniently compressed into a very small space. The most important physical properties of metals are their density, fusibility, volatility, malleability, ductility, tenacity, hardness, and colour.

Metals vary much in density; while potassium and sodium are lighter than water, and lithium and calcium not much heavier, many common metals are very heavy; as iron 7 times, silver 10, lead 11, mercury 13, gold 19, platinum 21, and iridium as much as 26 times heavier than water.

Their fusibility is equally various, for mercury melts at $71^\backslash circ$ below the freezing point of water, potassium somewhere about $+100^\backslash circ$, tin at $440^\backslash circ$, lead below a red heat, copper and silver at a full red heat, gold and iron at a white heat, platinum requires a heat stronger than that of any furnace, and iridium has not yet been melted.

So also with regard to volatility, mercury boils at about $600^\backslash circ$, potassium and sodium at a red heat, arsenic and tellurium even lower, zinc and cadmium at a strong red heat, antimony perhaps at a white heat, while the remaining metals have not yet been seen in the form of vapour or gas.

Of all metals, gold is the most malleable, and may be beaten into leaves of astonishing thinness; platinum and silver come next, then palladium, copper, nickel, tin, lead, cadmium, zinc, and iron. Most of the others are brittle, or have not been tried, from the difficulty of obtaining them.

Ductility is not proportional to malleability. The most ductile metal is platinum, which yields wire so fine as to be almost invisible to the naked eye; then come gold, silver, iron, copper, tin, zinc, cadmium, and lead. The difference between malleability and ductility is strongly seen in iron, which yields very fine wire, but cannot be beaten into thin leaves.

The tenacity of metals is measured by the weight required to break a rod of equal thickness and length of different metals. Iron is the most tenacious, a fine iron wire requiring a comparatively heavy weight to break it. The other ductile metals are all more or less tenacious.

Of the common metals, gold, platinum, and lead are the softest; tin, zinc, cadmium, and silver somewhat harder; copper and iron the hardest. But the hardest of all metals is iridium, which is so very hard that it cannot be wrought by any tools.

The colour of metals is usually either white, with a tinge of some other colour in many cases, or gray. One metal, gold, is yellow, and one, copper, is red. Titanium was supposed to be of the colour of copper, but the cubic crystals formerly supposed to be titanium are now known to contain other elements, cyanogen and nitrogen, in addition to the metal. Silver is pure white; tin and sodium are yellowish-white; zinc and potassium bluish-white; bismuth reddish-white; antimony, arsenic, and iron, gray. Metals often have a different colour in the compact state and in that of powder. Gold in powder is either brown or nearly black, according to the fineness of the powder. Platinum, gray in the compact and even in the spongy state, is jet black in powder.

The chemical characters of metals are determined by their attractions for oxygen, chlorine and its congeners, and

Chemistry, sulphur. A few general remarks on the relations of these substances to metals will greatly facilitate the subsequent description of the individual metals.

The attraction of metals for oxygen, as has been already stated, varies exceedingly. Some, such as potassium and sodium, rapidly attract oxygen from the air, and are thus oxidized. Others, such as iron and copper, attract oxygen very slowly at ordinary temperatures, and only when moist, but are readily oxidized at a red heat, as is seen on the anvil of the blacksmith, where the scales which form on red-hot iron are an oxide of that metal. Others again, as silver, gold, and platinum, not only do not attract oxygen, even at a red heat, but if already combined with it, lose it when heated to redness. Mercury at a certain temperature is oxidized, but the oxide is decomposed at a temperature very little higher.

Some metals decompose water, seizing its oxygen and liberating its hydrogen, at ordinary temperatures, as potassium and sodium. Others only do so at a red heat, as iron and zinc.

There are various methods of oxidizing metals indirectly. Nitric acid oxidizes and dissolves many metals, indeed most of them. It is particularly used for oxidizing copper, mercury, silver, and antimony. Many metals, such as zinc and iron, are oxidized by dissolving them in hydrochloric or sulphuric acid, and then adding an alkali. On the common view, metals when dissolved in sulphuric acid, are first oxidized at the expense of water, and the alkali only separates the ready formed oxide by taking the acid from it. But this cannot apply to the solution of metals in hydrochloric acid, which forms chlorides; and yet alkalies added to chlorides precipitate oxides, just as with the sulphates. This is an additional argument in favour of the opinion that sulphuric acid is a compound of hydrogen, and that in its salts this hydrogen is replaced by metals.

The action of an alkali, potash, on the chloride of a metal, zinc, is as follows, $KO\; +\; ZnCl\; =\; KCl\; +\; ZnO$, and its action on sulphate of zinc on the modern view, takes the same form. $KO\; +\; ZnSO\_4\; =\; K\_2SO\_4\; +\; ZnO$. We have already explained the action of metals on hydrochloric and sulphuric acids, and shown that the same view may be taken of both, the phenomena being precisely the same, namely, that the metal is dissolved and hydrogen liberated.

A few metals which cannot be dissolved either by hydrochloric, sulphuric, or nitric acids, are dissolved and converted into chlorides by a mixture of nitric and hydrochloric acids, which is called nitro-hydrochloric acid, or aqua regia. It yields abundance of chlorine, which being in the nascent state combines with the metal. The action of the two acids is as follows; $HCl\; +\; NO\_3,\; HO\; =\; NO\_2\; +\; Cl\; +\; 2HO$. According to some, the nitrous acid and chlorine combine to form a new acid, chloronitric acid, $NO\_2Cl$, and this is the true solvent. At all events, the result is a chloride of the metal, from which, by means of an alkali, the oxide may be formed.

Metals, having so strong an attraction for oxygen, are usually found combined with it, and the oxides of metals are the most important of their compounds. Protoxides, or oxides of the formula $MO$, are bases, generally powerful ones; sesquioxides $M\_2O\_3$ are weaker bases; deutoxides $MO\_2$ are neutral, or even weak acids; teroxides, $MO\_3$, are generally strong acids; as are also oxides of the formulae $MO\_4$ and $MO\_5$. The character, therefore, of the oxide depends on the amount of oxygen in it.

The deoxidation of metallic oxides, or their reduction to the metallic state is an operation of great practical importance, most metals being obtained by this means from their ores. The methods of reduction are various.

Some oxides are reduced by heat alone, as those of silver, gold, platinum, mercury.

Most oxides may be reduced by the combined action of

heat and hydrogen, or heat and carbon; the oxygen being carried off as water in the first case, and as carbonic acid gas in the second. Hydrogen is much employed in analytical reductions on the small scale, but carbon is alone used in smelting. The reduced metal is melted by the heat, and falls to the bottom of the crucible or furnace. To prevent it from being again oxidized, a flux is employed, that is, a fusible earthy or saline mixture, which covers the surface of the metal with a fluid mass, and protects it from the air.

Many oxides may be reduced from their solutions by means of other metals having a stronger attraction for oxygen. Thus, salts of silver are reduced by mercury, those of mercury by copper, lead, or tin; copper is reduced by zinc or iron, lead by zinc, tin by zinc. This method is often used in the laboratory. Silver, lead, and tin crystallize beautifully when thus reduced.

Many metallic oxides are also reduced by various deoxidizing agents, both from their solutions, and in the dry way with the aid of heat. Protosulphate of iron reduces gold from its solutions, as do oxalic and formic acids. Silver is reduced by formic acid, by aldehyde, and by oil of cloves. Cyanide of potassium, which combines the deoxidizing agency of carbon with that of potassium, is a most powerful reducing agent at a red heat.

Lastly, several metals are reduced, and that in a compact metallic-looking mass, although from solutions at the ordinary temperature, by the galvanic current. This constitutes the electrotyping, which is chiefly applied to copper, silver, and gold. The metals thus obtained are as dense, as hard, and as malleable, as if they had been melted, rolled, and hammered.

Metals are made to combine with chlorine in various ways. Most of them if in a state of fine division combine directly with chlorine gas, generally taking fire in it. But this method is not convenient.

The chlorides of metals may be obtained by acting on the metals or on their oxides with hydrochloric acid, as already explained. As all chlorides, except two, chloride of silver and protochloride of mercury, are soluble in water, this method is much used. The insoluble chlorides are obtained by adding hydrochloric acid or a soluble chloride to any solution of the metal. Thus, hydrochloric acid or chloride of sodium, added to nitrate of silver, precipitate the chloride of silver, $HCl\; +\; AgO,\; NO\_3\; =\; HO,\; NO\_3\; +\; AgCl$, or $NaCl\; +\; AgO,\; NO\_3\; =\; NaO,\; NO\_3\; +\; AgCl$.

The volatile chlorides of some metals, such as aluminum, titanium, &c., which decompose water, are obtained by mixing the oxide of the metal with charcoal heated red-hot, and passing chlorine over the mixture. Aided by the attraction of chlorine for the metal, the carbon deoxidizes the oxide, which otherwise it could not do, and the chloride is deposited in the cold part of the apparatus.

When a chloride is to be reduced to the metallic state, hydrogen, aided by heat, may be used on the small scale, but carbon is of no avail.

Chlorides may also be reduced by other metals, and by various mixtures; also by the galvanic current. Chloride of silver is reduced by the action of iron or zinc; by heating to redness with lime, or by boiling with potash and sugar.

All that has been said of the formation and decomposition of chlorides applies also to bromides and iodides, and with the exception that we cannot employ fluorine itself, which is unknown, to fluorides likewise.

With sulphur, most metals combine, when heated along with it. The insoluble sulphurets may also be formed, in solutions of the metal, by the addition of hydrosulphuric acid, or of a soluble sulphuret, which act on the oxides, chlorides, &c., in the same way.

In some cases the sulphuret of a metal is obtained by

Chemistry. deoxidizing the sulphate. Thus, in the case of barium or potassium, the sulphurets of which are soluble, the sulphate of the oxide is heated with charcoal or hydrogen.

The sulphurets are reduced in various ways. Some are heated with a mixture of charcoal and carbonate of potash (black flux). The potassium takes the sulphur, while the carbon takes the oxygen of the potash, and the metal is reduced. Some are heated in hydrogen gas, when hydrosulphuric acid is given off, and the metal is left. Some sulphurets are reduced by being heated with other metals, as when sulphuret of mercury is reduced by heating it with iron filings. But on the large scale the usual method is to roast the ore (sulphurets being the chief ores of many metals, as lead, antimony, bismuth, copper, &c.), so as to oxidize both metal and sulphur, and dissipate a great part or the whole of the latter as sulphurous acid. The oxidized residue is then heated with charcoal as usual.

Metals also combine together, especially when heat is applied. It is remarkable that two metals heated together generally melt far more easily than they do separately. This is because the compound metal is always more fusible than the less fusible element, and often more so than the more fusible of the two. Compounds of two or more metals are called alloys, as brass, composed of copper and zinc; bronze, of copper and tin; pewter, of lead and tin; and fusible metal, of lead, tin, and bismuth. Where mercury is one of the elements, the alloy is called an amalgam.

The physical properties of alloys are those of simple metals, and there is nothing in their appearance to indicate their compound nature. In many cases, the addition of a very small proportion of one metal very greatly modifies certain properties of the other, such as fusibility, hardness, tenacity, and the like. A mere trace of arsenic renders gold brittle, and one part of zinc, tin, antimony, and other metals, added to 100, 200, or even 500 parts of iron, much increases both its fusibility and hardness. The subject of alloys is as yet only imperfectly investigated, and many valuable alloys remain to be discovered. It has been ascertained, however, that, while many metals may be fused together in any proportions, the properties for which alloys are valued are best developed when the metals are in atomic proportions, or multiples and submultiples of these.

In briefly noticing the more important metals, we shall divide them into groups, according to their natural analogies. It will be found that these groups are characterized by marked differences in the attraction for oxygen, and also in the nature of the oxides formed, while, in these and many other points, the metals of each group closely resemble each other.

GROUP I.

Metals of the Alkalies Proper.

This group consists of three metals, potassium, sodium, and lithium, which resemble each other as closely as do chlorine, bromine, and iodine. This resemblance is indeed so close that it is often difficult to distinguish between their compounds. They have all so strong an attraction for oxygen that they cannot be preserved in the metallic state unless they are protected from the contact of air, water, and other oxidized substances. They are all oxidized by exposure to air, and all decompose water at ordinary temperatures. Their protoxides are the alkalies, potash, soda, and lithia.

14. Potassium.

Symbol K (Kalium). Equivalent = 39.2.

This metal occurs, in the form of salts of its oxide, potash, and in that of its chloride, in the ashes of plants, especially

land-plants. Chloride of potassium also occurs in those of Chemistry. sea-plants. Potash also occurs, combined with alumina and silicic acid, in felspar, which, as already mentioned, is one

Fig. 25.

of the most abundant minerals. The metal is best obtained from the carbonate of potash, $\backslash text\{KO,\; CO\}\_3$, by exposing it to a white heat, mixed with charcoal, in a bottle of malleable iron. The carbon is oxidized at the expense of the potash, forming carbonic oxide gas which escapes, while the metal is volatilized and condensed in a receiver filled with naphtha, a liquid containing no oxygen. The process is not very productive, because the carbonic oxide forms, with part of the metal, a dark gray pulverulent compound, which is carried forward by the current of gas, and is apt to choke up the tube. This compound is dangerous, as it takes fire and explodes in contact with water. The metal is purified by melting it under naphtha, and pressing it through leather. If necessary it may be distilled in a small iron retort, and collected in naphtha.

Potassium has a highly metallic lustre, and a bluish-white colour. It is somewhat lighter than water, its density being 8650. It melts at $150^\backslash circ$, and if heated takes fire, burning to oxide or potash, $\backslash text\{KO\}$. It combines with equal energy with chlorine, bromine, iodine, sulphur, &c. Its attraction for oxygen is such that it decomposes all oxidized substances when heated with them, and many at the ordinary temperature. When thrown on the surface of water, it instantly melts, takes fire, and floats on the water, burning with a pink flame. Here the first effect is to deprive the water of oxygen, liberating hydrogen, which is set fire to by the heat evolved. The flame therefore is that of hydrogen and potassium mixed. But if the metal, inclosed in paper, or held by a pair of forceps, be plunged under the surface, no flame appears. Hydrogen gas is abundantly disengaged, and the metal even becomes red-hot under water. The oxide formed is dissolved. Exposed to air, potassium rapidly tarnishes and attracts, first oxygen, then carbonic acid and water, so that, in a short time, it is transformed into a strong solution of carbonate of potash. When heated with the oxides, chlorides, fluorides, &c., of such bodies as boron, silicon, magnesium, aluminum, &c., it deprives these substances of oxygen, and liberates the boron, silicon, and metals they contain.

Protoxide of potassium, or potash, $\backslash text\{KO\}\; =\; 47.2$. Hydrate of potash or caustic potash, $\backslash text\{KO,\; HO\}\; =\; 56.2$.

The anhydrous protoxide or dry potash can only be obtained by heating the metal in dry oxygen gas. When it has once been dissolved, or when obtained from a solution,

Chemistry. it can only be got in the form of the hydrate, which is the true alkali.

Hydrate of potash is obtained by the action of hydrate of lime on a boiling solution of carbonate of potash, when the lime takes the carbonic acid, forming an insoluble carbonate, and the free potash dissolves. The clear solution, boiled rapidly down in a clean iron or silver vessel, till it flows like oil, forms, on cooling, a hard solid mass of hydrate of potash or caustic potash, $KO,\; HO$. This 1 eq. of water cannot be expelled by heat.

Caustic potash is very soluble and deliquescent. It also attracts carbonic acid from the air, and is converted into carbonate, if not kept in tightly-stopped vessels. It is very caustic, and has a burning alkaline taste. It seems to act as a caustic from its attraction for water. It neutralizes all acids, forming salts, which are called the salts of potash. Many of them are useful, as the carbonate, nitrate, sulphate, bitartrate, oxalate, &c. The solutions of potash and of its salts, as well as of all other soluble compounds of potassium, are recognised by giving, with excess of tartaric acid, a crystalline precipitate of the bitartrate, with bichloride of platinum a yellow crystalline precipitate of the double chloride of platinum and potassium, and with perchlorate of potash a crystalline precipitate of the perchlorate.

Caustic potash is much used both in surgery and in chemistry. It precipitates the insoluble oxides of most metals. In the arts it is employed in making soap, and in a variety of other ways. The carbonate is used in the manufacture of glass, and the nitrate in that of gunpowder.

Potassium forms, in certain circumstances, not well understood, a peroxide, $KO\_2$, which is a yellow powder of no particular interest.

Chloride of potassium, $KCl$, is produced when hydrochloric acid acts on potash. $HCl\; +\; KO,\; HO\; =\; KCl\; +\; 2HO$. It crystallizes in cubes, has a bitterish saline taste, and much resembles chloride of sodium or sea-salt. This salt is found in kelp, the ashes of sea-weed, and is used in making alum.

Iodide of potassium, $KI$, is formed along with iodate of potash, $KO,\; IO\_3$, when iodine is dissolved in solution of potash. $I\_2\; +\; 6KO\; =\; 5KI\; +\; KO,\; IO\_3$. The mixture, when heated to redness, gives off the oxygen of the iodate, and leaves the pure iodide. $5KI\; +\; KO,\; HO\; =\; 6KI\; +\; O\_2$. This salt is much used in medicine and also as a reagent in chemistry. It crystallizes in cubes like the chloride, and is soluble in alcohol.

The bromide and fluoride are very like the chloride and iodide, crystallizing also in cubes.

The sulphuret of potassium, $KS$, is formed by heating the elements together, or by heating sulphate of potash in a current of hydrogen gas. $KO,\; SO\_4\; +\; H\_2\; =\; 4HO\; +\; KS$. It is a white or yellowish powder, soluble in water. There are other sulphurets with more sulphur, especially the pentasulphuret, $KS\_5$, which forms a solution of a deep yellow colour.

15. Sodium.

Symbol $Na$ (Natrium). Equivalent = 23.

This metal is found, oxidized, as carbonate, in the ashes of sea-plants, or kelp, barilla, and varec. But it occurs chiefly as chloride of sodium constituting sea-salt and rock-salt, which are most abundant mineral compounds. Chloride of sodium is also present in small proportion in all waters, even in rain-water, and in large amount in salt springs, which are very common in many districts. It is probably also present in all soils, and in most rocks, and is a constant ingredient of the ashes of plants.

Sodium is obtained from the carbonate exactly as potassium is; only, as the metal forms no combination with carbonic oxide, the process is more productive. Like potas-

sium, it must be collected and preserved under naphtha. Chemistry. It has a yellowish-white colour and bright lustre, is rather heavier than potassium, its specific gravity being 9700, and also less fusible and less volatile. It decomposes water, but does not burn on its surface, although if a few drops of water be sprinkled on a bit of sodium, the hydrogen will then take fire and set fire also to the metal, giving an intensely yellow flame. In all other respects sodium resembles potassium; but this character of giving a strong and pure yellow colour to flame is found in all the salts of sodium, and at once distinguishes them from those of potassium, which colour flame of a faint lilac, not easily observed, since the presence of a trace of sodium or of many other metals overpowers it. Potassium itself burns with a lively pink flame, but in its salts the effect is much less marked, whereas in those of sodium the effect on flame is as strong as in the metal itself.

Peroxide of sodium or soda, like potash, is best known in the form of the hydrate, or caustic soda, $NaO,\; HO$, which is entirely similar to caustic potash, and is prepared from the carbonate exactly in the same way. Hydrate of soda, like that of potash, is deliquescent, and also attracts carbonic acid from the air, but the carbonate of soda differs from that of potash, which is anhydrous and very deliquescent, and can hardly be made to crystallize, whereas carbonate of soda forms very large crystals, containing at least half their weight of water of crystallization and efflorescing, that is, losing water and becoming opaque and powdery in the air.

Soda is best distinguished from potash by its action on flame, and by the characters of its salts. We have seen that potash forms nearly insoluble salts with perchloric acid, bichloride of platinum, and excess of tartaric acid, to which may be added carbazotic or nitropicric acid. With all of these, and indeed with acids in general, soda forms soluble salts. There are only two salts of soda insoluble, or nearly so, namely, the silicofluoride of sodium, formed by hydrofluoric acid, when added to soda or its salts, and the antimonate of soda. But potash also forms an insoluble silicofluoride; and hence the only test which can be used to distinguish soda from potash by forming a precipitate is antimonate of potash, which of course does not act on the salts of potash. Unfortunately the solution of this test does not keep well; so that chemists generally make use of the action of sodium and its salts on the flame of alcohol, or convert the soda into certain salts which differ from the corresponding salts of potash, as has been mentioned in regard to the carbonate. The sulphate of soda is also efflorescent, and crystallizes in four-sided prisms, while that of potash is anhydrous, and forms six-sided prisms and pyramids. The nitrate of soda forms rhombic crystals, while that of potash yields six-sided prisms.

Soda is used for much the same purposes as potash; and as pure carbonate of soda is now made at a cheap rate from sea-salt, while the commercial carbonate of potash is not only dearer, but very impure, soda is generally preferred. Its chief uses are in making soap (the soaps of soda being hard, while those of potash are soft), and glass. It is also used in bleaching and calico-printing, the cloth being boiled with soda in various processes.

There is a peroxide of sodium, which seems to be a deut-oxide, $NaO\_2$, but is little known.

Chloride of sodium, $NaCl$, sea-salt or rock-salt, is the most important compound of sodium. It is the type of all neutral salts, which indeed, as a class, are named from it. It has a purely saline taste, and strong antiseptic properties, fitting it for use as a condiment to food (for which purpose it is indispensable, since blood cannot be formed without salts of sodium), and for preserving meat. Chloride of potassium cannot be used for either of these purposes.

Sea-salt crystallizes in cubes, which are generally hollow.

Chemistry. Rock-salt is often found in large transparent masses, which cleave readily in the boundary planes of the cube, and may thus be shaped into perfect cubic crystals. It is almost equally soluble in hot and in cold water.

In the arts, salt is used for the production of chlorine, chloride of lime or bleaching powder, and hydrochloric acid. It is also used as a manure.

The bromide, iodide, fluoride, and sulphuret of sodium, are quite analogous to those of potassium.

16. Lithium.

Symbol Li. Equivalent = 6.5.

This metal, the third of the alkaline group, is rare, occurring only in small proportion, seldom more than 3 or 4 per cent. in a few rare minerals, such as spodumene, petalite, lithion-mica, and lepidolite. The metal is little known, but is analogous to sodium and potassium, being heavier than either, and having apparently stronger affinities.

The hydrated oxide of lithium, $\backslash text\{LiO\}\; \backslash cdot\; \backslash text\{HO\}$, analogous to caustic potash and soda, is less soluble and less caustic, but like them attracts moisture and carbonic acid from the air. The carbonate is sparingly soluble. The sulphate, nitrate, and chloride are similar to those of potassium, except that the two last-named salts are deliquescent. Lithium and all its salts are easily recognised before the blow-pipe by the property of giving to flame a blood-red colour. It differs from potassium and sodium also in forming a sparingly soluble carbonate, and a nearly insoluble phosphate.

Lithia has not been applied to any useful purpose, being too rare. It occurs in small proportion in various waters, and possibly contributes to their action on the system. It is worth while to mention that the minerals which contain lithia, especially lithion-mica, and lepidolite, which is a kind of mica, have been found hitherto always associated with certain minerals, namely, topaz, albite, or soda-felspar, and tin ore. The occurrence, therefore, of lithia, especially along with topaz and albite, may be regarded as an indication that tin ore is not distant. The above-named minerals are found together in all the tin districts of Europe, in Cornwall, Saxony, and Sweden, and we have seen the same combination in two districts, one in Scotland, the other in Ireland; in both of which tin ore was also found, but as yet not in available quantity.

The group just described is very remarkable from the great analogy which pervades it, and from its complete parallelism with the group of chlorine, bromine, and iodine. In both, the gradation of properties is perfect, and the atomic weight of the middle elements, sodium and bromine, is, in both cases, the mean between those of the extremes. The most probable explanation of these relations is, that all these bodies are really compound, and contain in each group some common element, the variation in the amount of which causes the change of properties. In other words, these would be regarded as groups of homologous compounds, and they closely resemble such groups. But in the meantime, as we cannot prove them to be compounds, they remain elements to us.

GROUP II.

Metals of the Alkaline Earths.

The next group contains four metals, those of the alkaline earths, in which the gradation of properties seen in the first group is repeated or rather continued. In this group the protoxides are more and more sparingly soluble till the last, which is insoluble; the carbonates are insoluble in water, and the sulphates, with one exception, are either sparingly soluble or insoluble. By these characters they are readily distinguished from the metals of the first group.

17. Barium.

Symbol Ba. Equivalent = 68.5.

Chemistry. This metal, which is little known, has been obtained hitherto only by means of a powerful galvanic battery. It is much heavier than the preceding metals, its density being above that of vitriol.

The protoxide of barium or baryta is found in nature, combined with carbonic acid and sulphuric acid, forming the minerals wetherite and heavy spar. It occurs also in a few other minerals, chiefly silicates. To obtain the pure anhydrous oxide, $\backslash text\{BaO\}$, the carbonate is heated to whiteness, mixed with a little charcoal. By this means the carbonic acid is partly expelled as such, partly as carbonic oxide, and baryta is left. Or the nitrate of baryta is cautiously heated in an earthen crucible, when it melts, and gives off nitrous acid and oxygen, baryta being left. It forms a gray porous earthy mass, which, like quicklime, produces intense heat when brought in contact with water, with which it forms a fine white powder, hydrate of baryta, $\backslash text\{BaO\}\; \backslash cdot\; \backslash text\{HO\}$. This is dissolved in considerable quantity by hot water, and the hot saturated solution deposits, on cooling, fine tabular crystals of another hydrate, $\backslash text\{BaO\}\; \backslash cdot\; 10\; \backslash text\{HO\}$. The hydrate is much less soluble in cold water than in hot, but still the solution has a strong styptic, almost caustic, alkaline taste, and attracts carbonic acid from the atmosphere, forming the insoluble carbonate. The nitrate of baryta, chloride of barium, and other salts of this base, are easily obtained from the carbonate by the action of the proper acids. But as the carbonate is much more rare than the sulphate, and as the sulphate is quite insoluble in water and acids, it must be decomposed, so as to allow of its being converted into other salts. This is effected by mixing it in fine powder with $\backslash frac\{1\}\{4\}$th of its weight of charcoal, and heating the mixture to a strong red heat for two hours in a covered crucible, leaving a small aperture for the escape of gas. The action is as follows: $\backslash text\{BaO\}\; \backslash cdot\; \backslash text\{SO\}\_3\; +\; \backslash text\{C\}\; =\; 4\; \backslash text\{CO\}\; +\; \backslash text\{BaS\}$. The products are carbonic oxide gas and sulphuret of barium. The latter compound is dissolved by boiling water, which leaves any impurities as well as any excess of charcoal undissolved. The solution, treated with nitric acid, yields the nitrate; with hydrochloric acid, the chloride; with carbonate of potash, soda, or ammonia, the carbonate; and if boiled with oxide of copper, it yields the hydrate of baryta. The formation of the nitrate is as follows: $\backslash text\{BaS\}\; +\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{NO\}\_2\; =\; \backslash text\{HS\}\; +\; \backslash text\{BaO\}\; \backslash cdot\; \backslash text\{NO\}\_2$. That of the chloride is $\backslash text\{BaS\}\; +\; \backslash text\{HCl\}\; =\; \backslash text\{HS\}\; +\; \backslash text\{BaCl\}$. That of the carbonate is $\backslash text\{BaS\}\; +\; \backslash text\{KO\}\; \backslash cdot\; \backslash text\{CO\}\_2\; =\; \backslash text\{KS\}\; +\; \backslash text\{BaO\}\; \backslash cdot\; \backslash text\{CO\}\_2$, and that of the oxide is $\backslash text\{BaS\}\; +\; \backslash text\{CuO\}\; =\; \backslash text\{CuS\}\; +\; \backslash text\{BaO\}$. The oxide thus formed instantly combines with water to form the crystallized hydrate.

Baryta is characterized by the extreme insolubility of its sulphate in water and acids, and its salts are used as tests for sulphuric acid; which again is used as a test for baryta. Hydrofluoric acid produces, in the salts of baryta, an insoluble crystalline precipitate. Baryta differs also from the three preceding metals in forming an insoluble carbonate, and in its own sparing solubility.

Chloride of barium, prepared as above stated, crystallizes in tabular crystals, $\backslash text\{BaCl\}\; \backslash cdot\; 2\; \backslash text\{HO\}$. It is much used as a test for sulphuric acid, and as a means of determining the quantity of that acid in analysis. It is also used in medicine.

The sulphuret of barium, $\backslash text\{BaS\}$, is soluble in water, as has just been mentioned. It is much used as a means of obtaining the other salts of baryta from the insoluble sulphate.

Baryta, and all its salts, except the sulphate, which, being insoluble in all menstrua, is inert, are very poisonous.

18. Strontium.

Symbol Sr. Equivalent = 43.8.

This metal is hardly known, but is very analogous to barium. The protoxide, strontia, is found as carbonate or

Chemistry. strontianite, and sulphate or celestine. The anhydrous oxide, $\backslash text\{SrO\}$; the hydrates, $\backslash text\{SrO\},\; \backslash text\{HO\}$, and $\backslash text\{SrO\},\; 10\; \backslash text\{HO\}$; the carbonates, $\backslash text\{SrO\},\; \backslash text\{CO\}\_2$; the nitrate, $\backslash text\{SrO\},\; \backslash text\{NO\}\_3$; the chloride, $\backslash text\{SrCl\}$; the sulphate, $\backslash text\{SrO\},\; \backslash text\{SO\}\_4$, and the sulphuret, $\backslash text\{SrS\}$, are all prepared precisely as in the case of barium, and are as similar to the corresponding compounds of that metal as the salts of sodium are to those of potassium. The chief differences are, that the sulphate is not absolutely insoluble, and the hydrate rather less soluble than the corresponding compounds of baryta. The chloride is deliquescent, and strontia and all its salts give to flame a fine crimson colour. The nitrate is used, indeed, in making red fire for signals and for the theatres. There is no other useful application of strontia or its salts.

This metal also is little known. But the protoxide, $\backslash text\{CaO\}$, is the important substance quicklime. It is found, like baryta and strontia, as carbonate in marble, limestone, chalk, and calcareous spar, and as sulphate in gypsum, alabaster, and selenite. The shells of shell-fish consist chiefly of carbonate of lime, and the rocks above mentioned have been in many cases derived from the accumulation of shells and of their debris, as is proved by the frequent occurrence of chalk, limestone, and marble, entirely composed of shells.

Pure lime, $\backslash text\{CaO\}$, is obtained by heating pure marble, &c., to redness in a current of air, when the carbonic acid is expelled and quicklime is left. Quicklime is still more sparingly soluble than strontia. It combines with water with great energy, giving out much heat, and producing hydrate of lime or slaked lime, $\backslash text\{CaO\},\; \backslash text\{HO\}$. Hydrate of lime is very sparingly dissolved by water, forming a solution called lime-water, which has an alkaline styptic taste, neutralizes acids, and attracts carbonic acid from the air, forming the insoluble carbonate. Lime-water is a much weaker solution than strontia-water, and strontia-water than baryta-water, so that there is a regular gradation of solubility in the group.

Lime is much employed for making mortar and in agriculture. The uses of marble and limestone for building, and of chalk for various purposes in medicine, and for the preparation of effervescing drinks, are well known. The presence of lime in any solution is detected by adding first ammonia to neutralize any acid, and then oxalate of ammonia, which produces a precipitate of the insoluble oxalate of lime. Lime and its salts are also used to detect oxalic acid.

Chloride of calcium, $\backslash text\{CaCl\}$, is formed when hydrochloric acid acts on carbonate of lime. It is very soluble, crystallizes with difficulty, and is very deliquescent. It is much used by chemists from its strong attraction for water, to deprive other substances, such as gases, ethers, and the like, of moisture; also to collect water in analysis, so that its weight may be ascertained.

This metal is obtained by the action of potassium on the chloride, $\backslash text\{MgCl\}\; +\; \backslash text\{K\}\; =\; \backslash text\{KCl\}\; +\; \backslash text\{Mg\}$. It is silvery white, of a brilliant lustre, and malleable. It may be kept in dry air or under water. When heated in oxygen it burns with much light, producing the protoxide or magnesia, $\backslash text\{MgO\}$.

This oxide magnesia, $\backslash text\{MgO\}$, is best obtained, like lime, by heating the carbonate. Hence it was called calcined magnesia, the carbonate being then called magnesia. Magnesia is insoluble in water, but has an earthy taste. It is quite white, while the three preceding alkaline earths are more or less gray. Hydrate of magnesia, $\backslash text\{MgO\},\; \backslash text\{HO\}$, is formed by precipitating the soluble salts of magnesia by caustic potash, soda, or ammonia. It is white, and re-

sembles the anhydrous oxide, into which it is converted by Chemistry. a low red heat, water being expelled.

Magnesia is a strong base, and neutralizes all acids. It is distinguished from the three preceding oxides by its insolubility and by forming a soluble sulphate. It agrees with them in forming an insoluble carbonate. It is found in nature as carbonate in some localities, but chiefly in the form of the double carbonate of lime and magnesia, dolomite, or magnesian limestone. The sulphate occurs in some springs, as in Epsom and Cheltenham waters. Hence its name of Epsom salt.

Magnesia and its carbonate are much used in medicine as antacids. The sulphate is an excellent laxative.

Magnesia has a remarkable tendency to form double salts, as carbonate of lime and magnesia; sulphate of magnesia and potash (ammonia and soda may be substituted for the potash); and phosphate of ammonia and magnesia. The latter being quite insoluble, especially where an excess of ammonia is present, magnesia is detected by converting it into this double phosphate, which is done by adding first carbonate of ammonia and then phosphate of soda.

The chloride of magnesium, $\backslash text\{MgCl\}$, is obtained by dissolving magnesia in hydrochloric acid, evaporating to dryness, and igniting after the addition of sal-ammoniac, the vapours of which protect the chloride from the action of the air, which would otherwise oxidize the metal, expelling the chlorine. The fused mass of chloride must be kept in well-stopped vessels, as it is very deliquescent. It is from this salt that magnesium is obtained, and we mention it here because, as the metal is not rapidly oxidized in the air like those which precede, and appears to be malleable, it is possible that it may be in time applied to useful purposes.

It will be observed, that while barium, strontium, and calcium form a triad, parallel to that of potassium, sodium, and lithium, and with the same gradation in properties, such as solubility, force of attraction, and atomic weight, magnesium, with some points of analogy to these, yet in other points stands by itself. It is the first metal which, although having a strong attraction for oxygen, can yet be kept unchanged in air and water, the first also whose oxide (a protoxide) is insoluble in water. But it is placed with the three preceding metals, because it differs still more from those which follow. Strictly speaking, it does not belong to the same group with the three which precede it, and which form so well-marked a triad; and we shall see that its analogies are rather with zinc, a metal belonging to a different part of the series. It is, in fact, isomorphous with zinc.

The next group is that of the metals of the earths proper, which are five in number. Only one of these, however, is of much importance, the others being rare, and two of them exceedingly so. They form sesquioxides, which are bases, but not powerful, being expelled from their combinations by protoxides in general. These oxides are insoluble in water, and have an earthy aspect. The metals are little known, but aluminum is said to have been recently obtained on a larger scale than formerly, and to admit of useful applications.

This metal is obtained from the chloride by the action of potassium, and has hitherto been described as a dark-gray nearly black powder, infusible, but taking metallic lustre under the burnisher. Recently it is said to have been deposited on the surface of other metals, by means of the galvanic current, as copper is in the electrotyping, in a perfectly compact metallic state, with a bright silvery lustre and co-

Chemistry. lour, and permanent in the air. When heated in oxygen, it burns with a brilliant light to sesquioxide. It is this sesquioxide which is so important from its abundance in nature.

Sesquioxide of Aluminum—Alumina, $Al\_2O\_3\; =\; 51.4$.

This earth is, next to silica, the most abundant solid constituent of the earth's crust. It forms a large proportion of all felspar, and felspar is a constituent of most rocks. In decayed or disintegrated felspar, which constitutes clay, alumina preponderates; and in some clays, such as pipe-clay and porcelain clay, it is nearly pure. Alumina also occurs crystallized in the sapphire and ruby, which contain only a mere trace of colouring matter, and in corundum.

Pure alumina is obtained from alum, which is a double sulphate of alumina and potash, by adding carbonate of potash, which produces a bulky gelatinous precipitate of hydrate of alumina. This is well washed with hot water. Since alumina does not combine with carbonic acid, the carbonic acid of the carbonates escapes as gas. The washed hydrate is not yet pure, retaining some potash. It is dissolved in hydrochloric acid, and reprecipitated by ammonia, again washed, dried, and ignited, when the bulky hydrate, becoming anhydrous, shrinks to a small bulk. Another method, which does not require the long and tedious washing of the hydrate, is to precipitate alum by chloride of barium, which throws down the sulphuric acid as sulphate of baryta, while chloride of potassium and hydrochlorate of alumina remain dissolved. The solution is dried up and the residue ignited, when the hydrochloric acid is expelled, and there is left a mixture of anhydrous alumina and chloride of potassium. The chloride is removed by hot water, and the alumina, which in this state is easily washed, is then dried and ignited, when it appears as a dense earthy white powder. That obtained by igniting the hydrate is translucent, and forms hard lumps.

Alumina is insoluble in water, but forms with it a plastic mass, which can be moulded into any shape, and when ignited retains the form given to it, and becomes hard, and much contracted in volume from the loss of water. When freshly precipitated the hydrate is very soluble in acids, but after ignition it dissolves in them very slowly. Its solutions have a sweetish astringent taste, and are styptic. Alumina is recognised by its being precipitated by caustic and carbonated potash, soda, and ammonia; and by its redissolving easily in an excess of caustic potash or soda, but not in ammonia. It may also be recognised by forming, with sulphuric acid and potash, a solution which, on evaporation, readily yields octahedral crystals of alum.

Alumina is useful as the chief constituent of all plastic clays, such as pipe-clay, porcelain-clay, brick-clay, and fire-clay. In these it is combined with a little silica. In the form of alum and acetate of alumina, it is much employed in dyeing and calico-printing. Alum is also used in medicine.

Alumina is the first sesquioxide we have come to, and is the type of such oxides.

Chloride of aluminum is a volatile fuming liquid, formed when chlorine gas is passed through a red-hot tube containing alumina intimately mixed with charcoal. The charcoal, aided by heat alone, cannot deoxidize alumina; but when the attraction of chlorine for the metal is added, the carbon is oxidized, and passes off as carbonic acid or carbonic oxide. $Al\_2O\_3\; +\; C\; +\; Cl\_2\; =\; 3CO\; +\; Al\_2Cl\_3$. The chloride is a sesquichloride. It must be carefully kept out of contact with water, which it instantly decomposes, forming hydrochlorate of alumina: $Al\_2Cl\_3\; +\; 3HO\; =\; (Al\_2O\_3,\; 3HCl)$. This action on water is the reason why the chloride fumes in the air. Potassium decomposes it, setting free the metal as a dark-gray powder.

The other metals of this group are—22. Glucinium, $G\; =\; 26.5$, which is found as sesquioxide in the beryl and the emerald, and is named from the sweet taste of its salts. The

oxide glucina, $G\_2O\_3$, resembles alumina, but it differs from Chemistry. it in being soluble in carbonate of ammonia. 23. Yttrium, $Y\; =\; 32.2$, the sesquioxide of which, or yttria, is found in two or three very rare minerals, such as gadolinite, and yttrotan-talite, which occur at Ytterby, in Sweden; hence the name. Its salts are also sweetish, but it is of little importance. 24. Thorium, $Th\; =\; 59.6$, found only in one very rare mineral, thorite, and hitherto only seen by one or two chemists; and 25. Zirconium, $Zr\; =\; 22.4$, the sesquioxide of which, zir-conia, is found in the zircon or hyacinth. It also is some-what analogous to alumina, but is not of sufficient impor-tance to justify us in dwelling on it more fully.

Zirconium, in some of its properties, connects this group with the next.

GROUP IV.

This group consists of metals, the oxides of which are so little known, that we cannot say with certainty that any one of them is known in a perfectly pure state. The oxides of these metals, in fact, occur generally together, and being very similar to one another, their separation is a matter of very great difficulty. We shall therefore only mention their names, for their atomic weights are for the most part still doubtful. They are, 26. Cerium; 27. Lan-tanium; 28. Didymium; 29. Erbium; and 30. Terbium. It is in this group that the oxides begin to be coloured. Peroxide of cerium is brown, and the salts of several of them have some degree of colour. It is now believed that the colour observed in some of the salts of yttria really de-pends on the presence of one or more of these metals. Zir-conium has also a certain degree of analogy to tin.

GROUP V.

We shall now proceed to a group of metals of far greater importance, the first of which is iron. The metals of this group are seven in number, namely, iron and manganese, zinc and cadmium, cobalt and nickel, and tin. It will be seen that six of them are enumerated in pairs, and in fact the two metals of each pair have not only a striking analogy in many points, but also occur associated, one being seldom found without the other. In this group the attraction for oxygen is still very powerful, but the metals do not decom-pose water rapidly at ordinary temperatures, although they do so at a red heat. Their protoxides are powerful bases, and their sesquioxides, where they exist, are weak bases. When they form teroxides, these are strong acids. The compounds of most of them are coloured.

31. Iron.

Symbol Fe (Ferrum). Equivalent = 28.

This, the most valuable of all metals, is the first we have come to that is found uncombined, or in the metallic state; it occurs in that state in meteoric iron, and perhaps also in masses of terrestrial origin. There are, in different parts of the world, as in Siberia, in South America, and on the west coast of Africa, large masses of iron, declared by the native tradition to have fallen from heaven. That on the coast of Africa is almost a hill in size, the others are smaller, but yet of many tons weight, and many still smaller masses of the same kind exist. There is one of several tons weight in the court of the Government Building at Aix la Chapelle, which was found just below the surface of one of the streets, and of the fall of which no record is left. It probably fell before the city was founded. All the known masses of meteoric iron agree in character, being very hard, but malleable, and containing small quantities of nickel, cobalt, chromium, arsenic, and sulphur, with fre-quently a little silica. Such masses have been often seen to fall, and have been picked up while yet hot. When they fall at night, they are seen as luminous meteors, which finally explode and burst into fragments, and these, if large, bury themselves in the ground by the force of their fall. This

Chemistry. form of iron, however, is rare, although very interesting, from its peculiar composition, always containing the same impurities, and from the uncertainty of its origin. At one time these meteorites were supposed to come from the moon; but the prevalent opinion now is, that they are portions of planetary matter, either undergoing condensation, or else the fragments of some heavenly body that has burst or exploded, and continuing to revolve round the sun. They frequently enter our atmosphere, when they cross the earth's orbit, and then appear to take fire in it, and to be attracted to the surface. Multitudes of them pass through without falling, their centrifugal force being sufficient to resist the gravitation of the earth.

Iron is usually found oxidized; either as sesquioxide $Fe\_2O\_3$, called red hematite, or black hematite, a hydrate of the same oxide; or carbonate of protoxide, $FeO,\; CO\_2$, which is called clay iron ore; or magnetic oxide, or black oxide of iron, or loadstone, of which there are two kinds, $Fe\_3O\_4$ and $Fe\_4O\_5$. It also occurs as bisulphuret of iron, $FeS\_2$, called iron pyrites, and another sulphuret, called magnetic pyrites. The metal is obtained chiefly from hematite, magnetic oxide, and carbonate. These ores are heated with charcoal or coke, and a flux composed of sand and limestone. The reduced metal combines with a small quantity of carbon, forming a fusible compound, pig or cast-iron. This is subsequently melted in a puddling furnace, and exposed to a current of air, to burn off the carbon. As it becomes purer, it becomes also less fusible, till at last at a white heat the purified metal is only in a pasty, not a fluid state. At this stage it is hammered and rolled to expel the last traces of carbon, and then constitutes pure or malleable iron, such as is used for horse-shoes, horse-shoe nails, gun-barrels, and rails, as well as wire; for it is only when pure that it can be drawn out into fine wire.

Iron has a specific gravity of about 7800 (water = 1000). It is rather soft, especially when hot, and malleable. It is fusible only in the most intense heat of a wind furnace. Heated in a current of air, or in oxygen, to whiteness, it burns and is rapidly oxidized. At a red heat it is still oxidized on the surface; and the oxide, which is black or magnetic oxide, scales off under the hammer, and is called smithy ashes. At ordinary temperatures, if exposed to air and moisture, it is slowly oxidized, forming the brown sesquioxide, and is said to rust.

The uses of iron, in the pure metallic state, are too well known to require enumeration. The chief objection to it is that it cannot be cast nor even hammered at ordinary temperatures, so that it must be heated red-hot in order to be manufactured. Its value depends on its tenacity, a property in which it surpasses all other metals.

Iron and Oxygen.

Iron forms several oxides. Three appear to be direct compounds of iron and oxygen, and two to be composed of the others united together.

1. Protoxide of iron, $FeO$, is obtained by heating the carbonate or sesquioxide to a low red heat in hydrogen gas. If heated too strongly, it is reduced to the metallic state. When pure, it is of a grass-green colour, but it is apt to absorb oxygen from the air and to become sesquioxide, so that it is little known. It is a powerful base, and its salts are either bluish-green or colourless, the latter in a few cases only. These salts have an inky taste, and give a deep blue precipitate of Prussian blue with the ferricyanide or red prussiate of potassium. Caustic alkalies precipitate a white hydrate, which instantly attracts oxygen, and passes through green to the brown sesquioxide. Carbonated alkalies give a white bulky carbonate, which also becomes green, and finally brown, being converted into sesquioxide, and losing its carbonic acid. Hydrosulphuric acid or sulphuretted hydrogen does not precipitate these salts; sul-

phuret of ammonium causes a black precipitate of sulphuret of iron. Infusion of galls and meconic and sulphocyanic acids have no action on pure solutions of the protoxide. That oxide and its salts are characterized by their strong tendency to pass into sesquioxide, by absorbing oxygen; and they deoxidize many substances in consequence. The carbonate occurs in many mineral springs, dissolved by excess of carbonic acid.

2. The sesquioxide or peroxide of iron $Fe\_2O\_3$, is far more permanent, not having any tendency to absorb oxygen under ordinary circumstances. It is found pure in red hematite, hydrated in black hematite. To prepare it, the sulphate of the protoxide, or green vitriol, is boiled with nitric acid, some sulphuric acid being added, when it becomes sulphate of the peroxide or persulphite. The addition of an alkali throws down a bulky brown hydrate of the sesquioxide, which, being washed, dried, and ignited, leaves the sesquioxide. It is a weak base, and is expelled from its salts by all protoxides, even by protoxide of iron. Its formation from the protosulphate or green vitriol is as follows:—$2(FeO,\; SO\_4)\; +\; NO\_2\; +\; SO\_2\; =\; NO\; +\; (Fe\_2O\_3,\; 3SO\_2)$. Like alumina and other sesquioxides, it requires 3 eqs. of acid to form a neutral salt. On account of the permanence of this oxide, when we wish to test for iron, it is first converted into sesquioxide by boiling with nitric acid. Its characters are well marked. Alkalies, caustic and carbonated, throw down a brown precipitate of sesquioxide, which does not combine with carbonic acid. Ferrocyanide of potassium, or yellow prussiate, produces the deep blue precipitate of Prussian blue; the red prussiate only changes the colour of the solution to a dirty green. Infusion of galls produces ink, and sulphocyanic and meconic acids produce a deep blood-red colour. The salts of sesquioxide of iron are generally brown, and have a strong styptic inky taste.

Both oxides and their salts are used in medicine, as tonics and astringents. The salts of the peroxide are much employed in dyeing and calico-printing, as they strike either black or various shades of purple, lilac, and gray, with cochineal, logwood, and other red dyes, with which alumina gives crimson or pink colours.

The blood of animals contains iron in considerable quantity, which is found in the ashes of the blood as sesquioxide. It is present chiefly in the red globules, and indeed in the colouring matter of the globules. But it is not known whether their colour depends on the presence of iron, nor in what form of combination the iron exists. There can be no doubt that it has an important function to perform, probably connected with the absorption of oxygen in respiration; and as iron is thus essential to animal life, so it is found as a never-failing constituent of those plants on which animals feed, and that, in the form of sesquioxide, at least, it takes that form in the ashes.

3. Ferric acid, $Fe\_2O\_3$, is formed when the sesquioxide is ignited along with potash, or with nitrate of potash. The oxide takes up oxygen, and the acid thus formed combines with the potash, forming a salt which is very soluble in water, to which it gives a deep purple colour. It does not keep, however; for on standing, oxygen is evolved, and sesquioxide is deposited. It may be preserved somewhat longer, and even obtained in dark crystalline grains, if the solution contain a large excess of potash. Ferrate of potash, $KO,\; FeO\_3$, is rapidly deoxidized by contact with organic matter, such as the skin, or paper.

4. Magnetic oxide or black oxide of iron, exists in two forms, both of which may be produced artificially by precipitating a mixture of protoxide and sesquioxide in due proportion. The two magnetic oxides are $Fe\_2O\_4$ and $Fe\_3O\_4$. The former may be regarded as formed of 1 eq. of protoxide and 1 eq. sesquioxide, $FeO\; +\; Fe\_2O\_3\; =\; Fe\_3O\_4$. The latter contains the elements of 2 eqs. protoxide, and 1 eq. sesquioxide, $2FeO\; +\; Fe\_2O\_3\; =\; Fe\_4O\_5$. The native loadstone

Chemistry. or natural magnet, or magnetic iron ore, is a mixture or compound of the two. To prepare the oxide $\backslash text\{Fe\}\_2\backslash text\{O\}\_3$ artificially, sulphate of the protoxide is divided into two equal parts, and one of these converted into sulphate of sesquioxide by boiling with nitric acid. The two portions are then mixed, and the addition of an alkali, aided by a boiling heat, throws down a black precipitate, which, when dry, is strongly magnetic, and does not attract oxygen from the air. As the two halves of the sulphate contain equal quantities of iron, the oxide thus prepared must be that which is $\backslash text\{Fe\}\_2\backslash text\{O\}\_3\; =\; 2\backslash text\{FeO\}\; +\; \backslash text\{Fe\}\_2\backslash text\{O\}\_3$. The oxide $\backslash text\{Fe\}\_2\backslash text\{O\}\_3$ is prepared exactly in the same way, only two-thirds of the sulphate are converted into sesquioxide, which gives the oxide $\backslash text\{Fe\}\_2\backslash text\{O\}\_3\; =\; \backslash text\{FeO\}\; +\; \backslash text\{Fe\}\_2\backslash text\{O\}\_3$. This also is black, permanent, and magnetic.

With chlorine, iron forms two chlorides. The protochloride, $\backslash text\{FeCl\}$, is formed when iron is dissolved in hydrochloric acid. It is soluble, and has the green colour and character of a salt of protoxide. The sesquichloride, $\backslash text\{Fe\}\_2\backslash text\{Cl\}\_3$, is formed when the sesquioxide is dissolved in hydrochloric acid, and evaporated to dryness. It forms an orange yellow mass, which is volatile at a high temperature. It is much used in medicine, in the form of an alcoholic tincture, called tincture of muriate of iron. It occurs among the volatile products of volcanic action.

The protoiodide of iron, $\backslash text\{FeI\}$, is formed, when iron and iodine, the former in excess, are placed in contact under water. Heat is developed, and a green solution of the iodide is formed. When boiled down, it leaves a dark green deliquescent mass. The iodide is decomposed by the oxygen of the air, which sets free iodine, and converts the iron into sesquioxide. The iodide is much used in medicine, and is preserved by drying it up with the addition of sugar, which covers the particles, and protects them from the air. Sugar is used in the same way to preserve the precipitated carbonate of protoxide of iron from oxidation.

Sulphur and iron unite in several proportions. The protosulphuret, $\backslash text\{FeS\}$, is formed when iron filings and sulphur are heated together, when much heat and light are developed. It is also formed when a stick of sulphur touches a white-hot rod or bar of iron. The bar is instantly perforated, and the melted sulphure drops down. It is used in the preparation of hydrosulphuric acid. The bisulphuret $\backslash text\{Fe\}\_2\backslash text\{S\}\_3$, called iron pyrites or firestone, because some varieties of it absorb oxygen from the air and become hot, is a very abundant mineral. It forms cubic, octahedral, or dodecahedral crystals, of a yellowish colour and metallic lustre. When heated, it yields half its sulphur as a sublimate, but as the mineral generally contains arsenic, sulphur thus obtained is only used when better cannot be procured. Magnetic pyrites, $\backslash text\{Fe\}\_3\backslash text\{S\}\_4$, is characterized by its magnetic properties.

With carbon, iron forms two important compounds, which, however, contain very little carbon. These are cast-iron, which contains from 2 to 4 or 5 per cent., and steel, which contains only 1 or 2 per cent. of carbon. The former is hard, fusible, and brittle, but may be cast into any shape. The latter is sufficiently fusible to be cast if required, hard and elastic, as well as tough, which properties fit it for springs and cutting instruments. Both are made by simply heating iron in contact with charcoal.

No definite compound of iron with phosphorus is known; but a very small amount of phosphorus destroys all the good qualities of iron.

32. Manganese.

Symbol Mn. Equivalent = 27.6.

This metal appears to accompany iron; for while the ores of iron almost always contain traces of manganese, the ores of manganese contain traces of iron or even larger quantities.

The metal itself is little known, but may be got by heating to intense whiteness oxide of manganese with charcoal. It is fusible with extreme difficulty, and so brittle as to be of no use in the metallic state. In appearance it resembles iron. Its density is 8000.

The only important compounds of manganese are its oxides, especially the deutoxide or peroxide. This occurs in considerable quantity in veins and nodules, in the form of black prismatic crystals, or as a crystalline mass with a black streak. One variety of it, called wad, is earthy in aspect. It also occurs hydrated, when it is more brown, and has a brown streak. The hydrate is less valuable than the pure oxide. Peroxide of manganese is much used in the manufacture of bleaching powder, and also by chemists in preparing oxygen and chlorine. Its formula is $\backslash text\{MnO\}\_2$, and it is a neutral or indifferent oxide.

Peroxide of manganese, $\backslash text\{MnO\}\_2$, is obtained by heating the carbonate, $\backslash text\{MnO\}\_2\; \backslash cdot\; \backslash text\{CO\}\_2$, in a current of hydrogen; it is of a grass-green colour, and unless it be very compact, attracts oxygen from the air, and passes into peroxide. It is a powerful base, and forms salts which are colourless, or have a slight pink tinge. Alkalies produce in them a white precipitate of hydrated protoxide, rapidly passing into the brown hydrated peroxide by absorption of oxygen. Carbonated alkalies cause a white precipitate of carbonate of protoxide, which in drying becomes very slightly coloured, but is not altered in composition. Sulphuret of ammonium causes a flesh-coloured precipitate of hydrated sulphuret, while bleaching solution, and ozonized air, precipitate hydrated peroxide. This action of bleaching liquor is employed in calico-printing, to give a fine bronze brown.

Sesquioxide of manganese, $\backslash text\{Mn\}\_2\backslash text\{O\}\_3$, is left when the peroxide is heated to redness in close vessels. It is of a pale brown, and in its relations analogous to sesquioxides of aluminum and iron. It gives to glass an amethyst colour, and is believed to be the cause of the colour of the amethyst.

Red oxide of manganese, $\backslash text\{Mn\}\_2\backslash text\{O\}\_3$, is left when any oxide is ignited in open vessels. It seems to be a compound of protoxide and sesquioxide, analogous to magnetic oxide of iron.

Manganic acid, $\backslash text\{MnO\}\_3$, is formed in the same way as ferric acid; and its solution and that of its salts have a fine deep emerald green colour. Manganate of potash, $\backslash text\{K\}\_2\backslash text\{MnO\}\_4$, is isomorphous with sulphate of potash. Its fine green solution cannot be preserved, but, like that of the ferrate, loses oxygen, while peroxide is deposited. But if diluted with hot water, the colour changes from pure green to bottle-green, olive-green, bluish-green, bluish-purple, and finally to a splendid reddish-purple, which belongs to the salt of another acid, permanganic acid.

Permanganic acid, $\backslash text\{Mn\}\_2\backslash text\{O\}\_7$, is obtained in combination with potash, by adding hot water to the solution of the manganate. When the change is complete, a brown precipitate of hydrated peroxide settles, and the clear red solution, evaporated to a small bulk, yields beautiful bronze-coloured crystals of the permanganate, which have a metallic lustre; from the intensity of their purple colour, one small crystal will give to a quart of water a fine deep purple tint. This salt is much more permanent than the green manganate, but is rapidly deoxidized by all organic substances. In fact, it is a most powerful oxidizing agent, and produces peculiar results, from the nature of its composition, and the presence of an alkali. It is used in organic research. Its formula is $\backslash text\{K\}\_2\backslash text\{Mn\}\_2\backslash text\{O\}\_7$, and its formation from the green manganate is as follows: $3(\backslash text\{K\}\_2\backslash text\{MnO\}\_4)\; =\; \backslash text\{Mn\}\_2\backslash text\{O\}\_7\; +\; 2\backslash text\{KO\}\; +\; (\backslash text\{K\}\_2\backslash text\{MnO\}\_4)\_2$, so that the liquid becomes alkaline from free potash, and peroxide is separated. The action of this salt on sugar will serve as an example of its action on organic substances, by which acids are generally formed which combine with the potash. We shall suppose the sugar to be dry grape sugar, $\backslash text\{C\}\_\{12\}\backslash text\{H\}\_\{22\}\backslash text\{O\}\_\{11\}$.

Chemistry. $C\_\{12\}H\_\{22\}O\_\{12\}\; +\; 6(KO,\; Mn\_2O\_7)\; =\; 12MnO\_2\; +\; 12HO\; +\; 6(KO,\; C\_2O\_4)$

One eq. of sugar, with 6 eqs. of the salt, yield 12 eqs. of peroxide, 12 of water, and 6 of oxalate of potash. This proves that no part of the carbon in sugar can be in the form of carbonic acid, as some have supposed. For we get all the carbon as oxalic acid; and as that acid contains less oxygen than carbonic acid, it is impossible that it can have been formed from carbonic acid by means of a powerful oxidizing agency.

There are two chlorides of manganese, the protochloride $MnCl$, and the perchloride $Mn\_2Cl\_7$, corresponding to permanganic acid. The protochloride, from which all the salts of manganese are obtained, is obtained pure from the solution made in preparing chlorine by the action of hydrochloric acid on the peroxide. This solution contains protochloride of manganese, and sesquichloride of iron. One part of it is precipitated by carbonate of soda, and the washed precipitate, consisting of carbonate of protoxide of manganese with sesquioxide of iron, is added to the remaining solution and boiled. The protoxide in the precipitate expels the iron from the solution, so that, if the proportion of the solution precipitated has been rightly calculated, the whole manganese is now found in the liquid, and all the iron in the precipitate. It is best to manage it so that a little manganese remains in the precipitate, for in that case no iron can remain in the liquid, provided the iron has been in the state of sesquichloride or sesquioxide. This very beautiful process, in which one part of an impure solution is made to purify the whole, itself included, depends on the fact that protoxides, even in the form of insoluble carbonates, expel sesquioxides from their solutions. The solution must first be rendered neutral by evaporation to dryness, for free acid would prevent the result. A solution of protochloride of manganese is thus obtained quite free from iron, and containing only traces of copper, cobalt, and nickel. The first is separated by hydrosulphuric acid; the two others by a few drops of sulphuret of ammonium. From the pure chloride, which forms pink crystals, all the salts of manganese may easily be prepared.

The perchloride is volatile, and its vapour is of a greenish-yellow colour. It has not been much studied, but it is said to decompose the vapour of water, producing a purple cloud of minute particles of permanganic acid, along with hydrochloric acid.

Protosulphuret of manganese, $MnS$, is formed as a flesh-coloured hydrate, when sulphuret of ammonium is added to the salts of protoxide of manganese. This appearance is quite characteristic of the pure salts of manganese.

It is worthy of notice, that while 1 eq. of manganese is isomorphous with 1 eq. of sulphur in the manganate of potash, isomorphous with sulphate of potash, 2 eqs. of manganese are isomorphous with 1 eq. of chlorine in the permanganate, isomorphous with perchlorate. The four salts are as follows:—Sulphate of potash, $KO,\; SO\_4$. Perchlorate of potash, $KO,\; ClO\_4$. Manganate of potash, $KO,\; MnO\_4$. Permanganate of potash, $KO,\; Mn\_2O\_7$. This seems to indicate that our equivalent of chlorine ought to be halved, as it is on the Continent, when the formula of the perchlorate would be, $KO,\; Cl\_2O\_7$. Such a change, however, would imply the halving of the equivalents of bromine, iodine, hydrogen, and many metals; and although it may ultimately be adopted, at present it would cause confusion.

Iron and manganese form a group of two, with many points of analogy, and remarkable in this, that their atomic weights are almost if not exactly the same. We have now to examine two similar pairs.

The first of these includes zinc and cadmium. These metals are usually found, like the two last, together, and the analogy between them is very strong. Both are obtained from the chief ore of zinc, calamine, or silicate of oxide of zinc, and both are volatile at a red heat.

Symbol Zn. Equivalent = 32.6.

The ore is heated with charcoal in crucibles closed above, from which a tube leads downwards, and conveys the vapours to the condensing vessel where they are collected in water. The vapours which first come burn, if set fire to, with a peculiar brown flame, and deposit an impure oxide, containing nearly all the cadmium, which is more volatile than zinc.

When this brown blaze, as it is called, is over, the pure vapour of zinc is collected in water, where it forms metallic masses. Since the discovery of cadmium, the brown blaze is no longer burnt but collected separately in water if the cadmium be required; or the oxide may be used to obtain that metal. The above process is called distillation per descensum.

Zinc is a crystalline, easily-fusible metal, of a bluish-white colour, specific gravity about 7000. It melts at a low red heat, and if heated more strongly is converted into vapour, which takes fire and burns with a greenish-white flame, producing the oxide, a large part of which is carried upwards by the heated current, and deposited in light white flocks, called Lana Philosophorum. The oxide remaining in the crucible, when washed from any particles of metal, is pure, and is not in itself volatile. The metal is not malleable when cold, but is broken, although not very easily, by the hammer. At 300° it may be rolled into plates or sheets, which are used instead of sheet lead.

There is only one oxide of zinc, the protoxide, $ZnO$, which is prepared by burning the metal, or precipitated as a hydrate by the alkalies, an excess of which redissolves it. The best way is to precipitate it as carbonate, and to ignite that salt, when the oxide is left. While hot, it is yellow, but becomes white on cooling. It is distinguished from alumina in solutions by its ready solubility in excess of ammonia. Its salts are colourless and generally isomorphous with those of magnesia. The sulphate of zinc is so like that of magnesia, that its density alone distinguishes the two salts, unless we apply chemical tests. Solutions of zinc give, when quite neutral, a white precipitate of hydrated sulphuret with hydrosulphuric acid, but in acid solutions this test does not act. Sulphuret of ammonium produces, in neutral solutions, the same compound more abundantly. Ferrocyanide of potassium causes a white precipitate, and the action of caustic and carbonated alkalies has been already mentioned. The salts of zinc have a styptic metallic taste. The sulphate, acetate, and carbonate, as well as the oxide, are used in medicine. The sulphate is astringent, and in large doses emetic and poisonous. It is much used as a collyrium.

There is only one chloride, the protochloride, $ZnCl$, formed when zinc is dissolved in hydrochloric acid. It forms a crystalline deliquescent mass, which, in solution, is used to preserve anatomical preparations, and to prevent decay in wood, according to Sir W. Burnett's method.

The iodide, $ZnI$, is made by the action of an excess of zinc on iodine under water, when the compound dissolves. It is used in medicine.

The sulphuret, $ZnS$, or zinc blende, is a common mineral in Cornwall and other mining districts. It forms dense black crystals, and when converted into oxide by roasting, yields the metal when heated with charcoal. The white hydrated sulphuret, formed in solutions of zinc by sulphuret of ammonium, when heated, loses water, and becomes black.

Zinc is much used in the form of sheets and pipes, not having the poisonous properties of lead. It is also a constituent of brass and of German silver. One of its chief uses now is the formation of galvanic batteries, in which plates of zinc are placed alternately with those of copper, or of platinized silver. For the purposes of the electric telegraph and the electrolyte zinc is therefore most valuable.

Chemistry. Carbonate of zinc prepared by a peculiar process has lately been introduced under the name zinc white, as a substitute for the poisonous white lead.

34. Cadmium.

Symbol Cd. Equivalent = 56.

The preparation of this metal from the common ore of zinc, of which it constitutes from 1 or 2 to 10, 12, or even 15 per cent., has been described under zinc. Cadmium also occurs in very small quantity as sulphuret, or cadmium blende, in fine orange crystals, of an octahedral form, first observed in trap rock in the west of Scotland by Lord Greenock, and hence called, as a mineral, Greenockite. In this rare ore no zinc is present.

Cadmium has a darker gray colour than zinc, and is very fusible and malleable. Its specific gravity is 8.700. It is more volatile than zinc, and very combustible at a high temperature. Like zinc it forms but one oxide $\backslash text\{CdO\}$, chloride $\backslash text\{CdCl\}$, sulphuret $\backslash text\{CdS\}$, &c. Its salts, like those of zinc, are astringent, styptic, and emetic. Sulphate of cadmium is an excellent collyrium. Cadmium is reduced to the metallic state from its solutions by zinc, and this furnishes a beautiful method for its purification. The oxide of the brown blaze of zinc works contains much zinc and some cadmium. It is dissolved in hydrochloric acid or sulphuric acid, and the acid solution in a platinum vessel acted on by a piece of zinc. The reduced cadmium adheres to the platinum, and after being washed may be dissolved off by hydrochloric acid. Another character of its salts is, that their acid solutions give with hydrosulphuric acid a fine orange yellow precipitate of sulphuret of cadmium. In this point it agrees with arsenious acid; but there is no difficulty in distinguishing between a basic oxide, such as oxide of cadmium, which forms an insoluble carbonate and is itself insoluble in water, and a soluble acid like arsenious or arsenic acid.

Zinc and cadmium form a pair of closely analogous metals, and the atomic weight of cadmium is not far from double that of zinc. It is probable that both metals will be used to coat the surface of other metals as we coat copper with tin; for already lead has been successfully coated with both, and even iron has been coated with zinc, under the name of galvanized iron. The two metals in question have the advantage of not being poisonous, as lead and copper are.

The next pair is even more remarkable for the analogy between them. The metals are cobalt and nickel. Like the metals of the two preceding pairs they are found associated, the ores of the one always containing more or less of the other. They are both as infusible as iron and manganese, and have more analogy to them than to zinc and cadmium. Indeed, there is a curious relation between them; for while cobalt and nickel are found in metoric iron, and are like iron naturally magnetic, iron is always found in the ores of nickel and cobalt, often in large proportion. Again, there are traces of cobalt and nickel in the ores of manganese, except in a very few instances. Lastly, the atomic weight of all four metals is nearly the same, varying only from 27.6 to 29.6.

35. Cobalt.

Symbol Co. Equivalent = 29.5.

The metal is found chiefly as arseniuret, along with arseniurets of iron and nickel; also as sulphuret. It is best purified from arsenic by melting with sulphur and alkalies, when it is converted into sulphuret; or the arseniuret is dissolved in nitric acid, and the arsenic precipitated as sulphuret by hydrosulphuric acid, which does not precipitate cobalt, nickel, or iron in acid solutions; but this latter process is very tedious. When a solution is obtained free from arsenic it is boiled with nitric acid to convert the iron into

sesquioxide. Excess of alkali is then added, and the precipitate, consisting of protoxides of cobalt and nickel, and sesquioxide of iron, is acted on by oxalic acid, which forms with the two first insoluble salts, with the last a soluble one. The iron is thus entirely removed by washing, and a pale pink powder is left, a mixed oxalate of cobalt and nickel, the former predominating. When the ore of nickel (arseniuret) is treated in the same way, the mixed oxalate is pale green, the nickel predominating. This powder is dissolved in liquor ammonia, and the dark red solution exposed to the air in a loosely-covered jar. As the excess of ammonia is dissipated, a green salt is deposited, oxalate of nickel and ammonia, while the colour of the liquid becomes a pure wine red, and it contains now only a similar but soluble double oxalate of cobalt and ammonia. This is dried up and ignited, or boiled with caustic potash, and yields pure black sesquioxide of cobalt. If the metal is wanted, this oxide is dissolved in hydrochloric acid, precipitated by a fixed alkali, and reconverted into oxalate, which is now of a pure pink colour, and when ignited in a covered crucible with a small aperture for gas, leaves the pure metal either spongy, or if the heat be high enough, melted into a button. The action of heat on the oxalate is as follows, $\backslash text\{CoO\},\; \backslash text\{C\}\_2\backslash text\{O\}\_4\; =\; 2\backslash text\{CO\}\_2\; +\; \backslash text\{Co\}$. The oxalate of nickel and ammonia is ignited, and leaves oxide of nickel, from which the metal may be prepared exactly as cobalt is.

Cobalt is gray, brittle, very infusible—specific gravity 8.500. Its protoxide $\backslash text\{CoO\}$ is a powerful base, has an ash-gray colour, and forms salts, which are either pink, crimson, or blue. Its solutions are pink, and give with alkalies a lilac hydrate, changing to brown; a permanent lilac carbonate with carbonated alkalies, and a black sulphuret with sulphuret of ammonium. Salts of cobalt, ignited with alumina, give it a fine small-blue colour. In fact small or zaffre is the silicate of oxide of cobalt, named zaffre (or sapphire originally, Italian zaffir) from its colour. It is got by roasting the ores of cobalt with sand, and is much used to give the blue colour to chinaware, and also to be added in small quantity to paper to improve its colour. There are several oxides of cobalt, but the protoxide is the most important.

The protochloride, $\backslash text\{CoCl\}$, forms beautiful deep red crystals, which, when heated, melt to a deep rich blue liquid. Traces made on paper with a very dilute solution of this salt are invisible when dry, their pink colour being very pale. But on being warmed they turn blue and become visible, again disappearing on cooling, moisture being absorbed. This is one of the oldest and best known sympathetic inks, and has this advantage that the characters may be made to appear and disappear any number of times, provided the heat applied be not too strong.

36. Nickel.

Symbol Ni. Equivalent = 29.6.

The preparation of this metal has been given under cobalt. It is about as difficult to melt as iron, is of specific gravity 8.800, has a whiter colour, and is malleable and magnetic, so that needles for the compass may be made of it, and these do not rust as steel is apt to do. But the metal is principally used as the chief ingredient in German silver. The more nickel this alloy contains, up to a certain point, the more it resembles silver in colour and lustre, as well as in permanence. The other ingredients are zinc and copper.

Protoxide of nickel, $\backslash text\{NiO\}$, is a powerful base, its salts are all green, and their colour is complementary to that of the corresponding salts of cobalt. Its solutions are easily recognised by their colours, and by the pale apple-green hydrate and carbonate produced by caustic and carbonated alkalies. Ammonia in excess redissolves the precipitated hydrate, forming a sapphire-blue solution. Sulphuret of ammonium causes a black precipitate of the sulphuret $\backslash text\{NiS\}$. From salts of copper, which are redissolved by ammonia, with a deep

Chemistry. violet-blue, and precipitated black by sulphuret of ammonium, those of nickel are distinguished by not being precipitated, if free acid be present, by hydrosulphuric acid, and by the action of ferrocyanide of potassium which gives with copper a chestnut-brown precipitate. The colour of the solution of copper too is generally blue.

Nickel forms a peroxide which is not of much interest, nor are the other compounds of nickel very important. The astonishing analogy between nickel and cobalt in so many points, while they differ in the colour of their compounds, as well as the fact that they always occur associated, and that they seem to accompany iron, not only in the mineral kingdom, but also in meteoric iron, and that both are magnetic, seem to indicate some hidden relations of an interesting kind.

Symbol Sn (Stannum). Equivalent = 59.

This metal is the last of the present group, and stands here by itself, although it has many points of analogy with zirconium, and perhaps titanium, in its compounds.

Tin occurs chiefly as deutoxide or peroxide, sometimes called stannic acid, from its weak acid properties, in the form of rounded or water-worn crystals, called tin-stone, or stream tin, being found in the beds of streams in tin districts. It is easily reduced by heating with charcoal, the metal being very fusible. Tin also occurs as tin-blende or sulphuret, which is roasted and then heated with charcoal.

The metal has a yellowish-white colour and bright lustre, and melts at $440^\{\backslash circ\}$. Its specific gravity is 7290. It is a most valuable metal, being used to make pipes and sheets or leaves, also to coat iron plate, forming tinned iron, commonly called tin-plate, and to form with copper the various kinds of bronze and bell metal; with lead, pewter and solder.

When heated to whiteness in air, it burns into oxide, and may also be easily oxidized by nitric acid. It forms two oxides—the protoxide $\backslash text\{SnO\}$, a somewhat weak base, and the deutoxide $\backslash text\{SnO\}\_2$, which has rather weak acid properties. Peroxide of tin, or stannic acid, seems to exist in two forms; in one it dissolves easily in acids, in the other it is insoluble. The latter is supposed to have 5 times the equivalent of the former, and to be $\backslash text\{Sn\}\_5\backslash text\{O\}\_\{10\}$.

There are two chlorides corresponding to the two oxides, $\backslash text\{SnCl\}$ and $\backslash text\{SnCl\}\_2$, both of which are much used in dyeing and calico-printing to brighten the colours. It is only by means of tin that cochineal is made to yield the fine colour given to scarlet cloth. The protochloride of tin crystallizes very readily. The perchloride is a volatile fuming liquid, but is commonly prepared in aqueous solutions by dissolving tin in nitro-hydrochloric acid. The protochloride is formed when tin is heated with hydrochloric acid, hydrogen being disengaged.

Iodide of potassium strikes a deep red colour with salts of tin, from the formation of an iodide.

There are two sulphurets of tin. The protosulphuret $\backslash text\{SnS\}$, is dark brown or black. The bisulphuret has a dirty yellow colour, and some degree of metallic lustre; it used to be called aurum musivum. These sulphurets are precipitated from the solutions of tin by hydrosulphuric acid.

Tin is thrown down in beautiful metallic crystals, from its solutions, by zinc.

The next group of metals is characterized by a remarkable tendency to form acids with oxygen, and by the absence of basic protoxides. They are eleven in number, but only three of them are of practical importance, and one or two of the others are hardly known. Most of the acids of this class of metals are teroxides, and most of the metals form volatile chlorides.

Symbol Cr. Equivalent = 26.7.

This metal, which is in some points analogous to iron and manganese, connects this group with the last. It is chiefly found in two forms, as chromic acid, combined with oxide of lead, in the beautiful and scarce red lead ore of the Ural, and as sesquioxide, combined with sesquioxide of iron in the more abundant chrome iron ore. The metal is little known. It is very infusible, hard, and brittle. Specific gravity about 6000.

It forms two oxides—the sesquioxide or green oxide, $\backslash text\{Cr\}\_2\backslash text\{O\}\_3$, and the orange-red chromic acid, $\backslash text\{CrO\}\_3$. The former, like other sesquioxides, is a weak base. It is obtained from bichromate of potash, by heating it with carbonate of soda and sal-ammoniac, when the acid is deprived of half its oxygen by the hydrogen of the ammonia. Water dissolves the saline matter from the residue, and leaves the sesquioxide of a fine deep green. It is precipitated from its solutions as a bluish-green bulky hydrate. Like all sesquioxides, it has a great tendency to form double salts, and in fact may be substituted for alumina in alum without altering the form of the salt. Chrome alum, as it is called, is of a dark reddish-purple by transmitted light, black by reflected light, green in powder and in solution, of a mixed green and red by day-light, pure red by candle-light. Another beautiful double salt, the oxalate of chromium and potash, is azure-blue by transmitted sun-light, red by transmitted candle-light, black by reflected light. The remarkable action of these salts on light has been described by Brewster.

Chromic acid, $\backslash text\{CrO\}\_3$, is formed when chrome iron ore is heated with nitre. The sesquioxide, being oxidized, forms the acid, which unites with the potash of the nitre, forming a lemon-yellow neutral chromate, $\backslash text\{KO\}\_2\backslash text\{CrO\}\_3$. The strong solution of this salt, mixed with nitric acid, loses half its potash, and deposits beautiful orange red crystals of the bichromate of potash, $\backslash text\{KO\}\_2\backslash text\{CrO\}\_3$. Both these salts are much used in calico-printing. To obtain the acid pure, a cold saturated solution of bichromate is mixed with oil of vitriol; and the mixture, on cooling, for heat is developed, deposits beautiful red acicular crystals of the acid. These are drained on a fire-brick, excluding the air, and when dry, sealed up in a tube. If any sulphuric acid or bisulphate of potash adhere to them, they may be purified by dissolving them in a little hot water, adding some sulphuric acid, and recrystallizing.

Chromic acid is very soluble in water, even deliquescent. It oxidizes organic substances, such as alcohol or ether, with flame. One very convenient method of using it as an oxidizing agent is to heat the organic substance with bichromate of potash and sulphuric acid, when the action is more gentle. In all cases the acid is reduced to sesquioxide.

The salts of chromic acid are remarkable for their fine colours, which are yellow, orange, and red. The chromates of potash (neutral), baryta, zinc, and lead (neutral), are of various shades of yellow. The bichromate of potash is orange. The dichromate of lead and the chromate of mercury are orange-red, and that of silver is deep red. The chromates of lead are used as pigments, and also formed on the cloth in calico-printing. The sesquioxide is used to give the beautiful green now so often seen on fine porcelain.

The chromates of potash are poisonous, and the workmen who make or use them suffer from ulcers on the hands of a peculiar kind. The solutions of these salts are antiseptic in a high degree. They may also be used like nitre to give to cotton or cloth the property of burning as in moxas.

There are two chlorides of chromium, a sesquichloride, which is of a peach-blossom colour, but forms a green solution and a volatile chloride, either a terchloride $\backslash text\{CrCl\}\_3$ or $\backslash text\{Cr\}\_2\backslash text\{Cl\}\_3$. But the latter is not well known. There is an oxychloride which is a volatile liquid of a dark red colour, acting violently on combustible substances, which is $\backslash text\{CrO\}\_2$.

Chemistry. Cl, that is, chromic acid, in which 1 eq. of oxygen is replaced by chlorine, or else $2\backslash text\{CrO\}\_3\; +\; \backslash text\{CrCl\}\_3$, a compound of 2 eqs. of chromic acid with 1 eq. of terchloride. The proportions are the same, the latter being 3 times the former.

Symbol V. Equivalent = 68.6.

This very rare metal is found as vanadic acid $\backslash text\{VO\}\_3$, combined with oxide of lead, and also in small proportions in some of the ores of iron. It forms three oxides, a protoxide $\backslash text\{VO\}$, a deutoxide $\backslash text\{VO\}\_2$, and vanadic acid $\backslash text\{VO\}\_3$, which is analogous to chromic acid. The acid has a reddish orange colour, is sparingly soluble in water, and when fused forms on cooling very large crystals even from a small quantity. It is easily deoxidized and yields the deutoxide, which is a weak base, and forms blue salts. The protoxide is black, and is not known to form salts. Vanadium forms two chlorides, a bichloride $\backslash text\{VCl\}\_2$, and a volatile terchloride $\backslash text\{VCl\}\_3$. Vanadium is too rare to be applied to any useful purpose.

Symbol Mo. Equivalent = 46.

This metal occurs as molybdic acid, combined with oxide of lead, and also as tersulphuret of molybdenum. The metal is little known. Molybdic acid, $\backslash text\{MoO\}\_3$, corresponds to chromic acid. Its only use is as a test for phosphoric acid, with which, when heated in solution with it, it forms a peculiar yellow precipitate. It is used in the form of molybdate of ammonia.

Like chromium, molybdenum forms a volatile terchloride and an oxychloride. But we need not dwell on this metal here. There is an oxide, with less oxygen than the acid, which has a fine blue colour.

Symbol W (Wolfram). Equivalent = 95.

This metal is of no great interest. It is found as tungstic acid, combined with lime, or with oxides of iron and manganese, the former being the mineral tungsten, the latter wolfram.

Tungstic acid, $\backslash text\{WO\}\_3$, is a nearly insoluble yellow powder. Like molybdenum, it forms, with less oxygen than in the acid, a blue oxide like indigo. This appears to be a compound of the acid with another inferior oxide, oxide of tungsten, which is black. Tungsten, too, forms a volatile terchloride, and an oxychloride.

Symbol Ti. Equivalent = 25.

This metal also occurs as titanic acid in the mineral rutile. According to some, the formula of the acid is $\backslash text\{TiO\}\_3$, while others make the acid $\backslash text\{TiO\}\_2$. From its analogy to oxide of tin, it is probable that the latter formula is correct. Like peroxide of tin, it exists in two forms, one of which is brown and insoluble in acids; the other white and soluble in hydrochloric acid. The white variety is used to form an enamel for artificial teeth. It was at one time thought that the copper-coloured cubic crystals occasionally found in the slag of iron furnaces, were metallic titanium; but these have been shown to contain nitrogen and cyanogen besides. This metal, like the two preceding, forms a volatile chloride. There appear to be two oxides, the sesquioxide and the acid, and two chlorides, sesquichloride and bichloride, corresponding to these. The solutions of titanic acid in acids are decomposed by boiling, the titanic acid being deposited; and when the acid is fused with alkalis, the fused titanate is decomposed by water, leaving an acid salt with large excess of acid.

The composition and properties of the acid and of the

bichloride show a great analogy with tin; but these metals are easily distinguished, since the compounds of tin, heated on charcoal before the blowpipe, readily yield metallic tin.

Symbol Ta (Tantalum). Equivalent = 92.7

This metal occurs in the form of columbic acid, $\backslash text\{TaO\}\_3$, in a few very rare minerals. Its properties are but imperfectly known; and it has recently been found to be associated with two other analogous metals, namely, 44. Niobium, and 45. Pelopium, which are still less known, but appear also to form acids with oxygen.

Symbol Sb (Stibium). Equivalent = 129.

This metal occurs as sulphuret, and as double sulphuret of antimony and silver; also as bournonite, a triple sulphuret of lead, copper, and antimony. It is obtained from the sulphuret by heating it with iron filings, or with carbonate of soda, after roasting it. The metal is fusible at a moderate red heat, of a gray colour and very bright lustre; specific gravity 6800. At a white heat it is converted into vapour, and burns in air. It forms two distinct oxides, and these combine to form a third.

Teroxide of antimony, $\backslash text\{SbO\}\_3$, is a weak base. It is obtained by pouring the terchloride into water, when an oxychloride is formed. This is boiled with excess of carbonate of soda, and leaves the teroxide as a grayish-white powder. It has a great tendency to form double salts, such as tartar emetic, which is a tartrate of antimony and potash. Its solutions give with hydrosulphuric acid a characteristic brownish-orange precipitate of hydrated tersulphuret.

Antimonious acid, $\backslash text\{Sb\}\_2\backslash text\{O\}\_3$, is obtained by the action of nitric acid on the metal; it resembles the teroxide in appearance, but has weak acid properties. Both oxides are used in medicine.

There is another oxide, sometimes called quadroxide, but which appears to be $\backslash text\{Sb\}\_2\backslash text\{O\}\_4\; =\; \backslash text\{SbO\}\_3\; \backslash cdot\; \backslash text\{SbO\}\_2$, an antimonate of the teroxide. It is formed by heating the antimonious acid.

Antimony forms with hydrogen a gaseous compound, probably $\backslash text\{SbH\}\_3$, analogous to phosphuretted hydrogen. It is not yet known in a state of purity, but only mixed with hydrogen. It is formed when hydrogen is disengaged in a solution containing antimony. The mixed gas burns with a yellow flame, giving off fumes of oxide, and which deposit metallic spots of antimony on glass or porcelain held in the flame. In these characters, antimony agrees with arsenic, and, indeed, it may be said to form with phosphorus and arsenic a group of three, like some of those we have already seen.

There are two chlorides of antimony, the terchloride, $\backslash text\{SbCl\}\_3$, and the perchloride, $\backslash text\{SbCl\}\_5$, both volatile; the former semi-solid at ordinary temperatures, and hence called buter of antimony. Mixed with water, it deposits an oxychloride, $\backslash text\{SbCl\}\_2\; +\; 2\backslash text\{SbO\}\_3$, called powder of algaroth, which is used in preparing the oxide as well as tartar emetic. The terchloride is formed by the action of hydrochloric acid on the tersulphuret.

There are also two sulphurets, the tersulphuret $\backslash text\{Sb\}\_2\backslash text\{S\}\_3$, and persulphuret $\backslash text\{Sb\}\_2\backslash text\{S\}\_5$; the former is the ore of antimony, and is dissolved by hydrochloric acid, yielding the terchloride and hydrosulphuric acid gas, $\backslash text\{Sb\}\_2\backslash text\{S\}\_3\; +\; 3\backslash text\{HCl\}\; =\; \backslash text\{SbCl\}\_3\; +\; 3\backslash text\{HS\}$.

The compounds of antimony are much employed in medicine; and the metal is a chief ingredient in the alloy used for types, in which it is combined with lead and a little tin, and in some alloys with tin used for various purposes.

Symbol As. Equivalent = 75.

This metal is found chiefly combined with sulphur, and also

Chemistry. with metals, such as cobalt and nickel. It likewise occurs as arsenic acid, $As\_2O\_5$, combined with various oxides. When the minerals containing it are roasted, part of it is sublimed in the metallic state, and part as arsenious acid $As\_2O\_3$, which is consequently obtained in very large quantities from the ores of cobalt and nickel. To obtain the metal from this acid, it is simply heated with charcoal, when the metal sublimes and forms a bright metallic crust. Its density is 5800. Under the ordinary pressure, it is converted into vapour before reaching its melting point. Its vapour has a strong garlic odour, and is very poisonous. It burns when heated in air, forming arsenious acid, which also sublimes as a white crystalline powder.

Arsenious acid, $As\_2O\_3$, is volatile; its vapour has no smell, unless when heated on charcoal, when it is reduced to the metallic state, and it is the metallic vapour the smell of which is then observed. It is sparingly soluble in water, and its solution gives, with excess of lime-water, a white precipitate of arsenite of lime; with ammoniacal-sulphate of copper, a grass-green precipitate of arsenite of copper, or Scheele's green; with ammoniacal-nitrate of silver, a lemon-yellow precipitate of arsenite of silver; and with hydrosulphuric acid, a bright yellow sulphuret, $As\_2S\_3$. By these characters this substance when pure is easily recognised; and from its very poisonous nature, and the facility of procuring it, poisoning by its means is so frequent, that it has to be sought for oftener than any other poison.

The best antidote to it is the administration of a large quantity of moist hydrated sesquioxide of iron, which has the property of forming with it an insoluble and inert arsenite of iron. Calcined magnesia has a similar effect.

When arsenious acid has to be sought for in mixed fluids, the above tests are inapplicable, except the last, which may be used to separate the arsenic from the mixed fluid, after adding an acid and filtering. The impure sulphuret thus obtained is decomposed by heating it with a mixture of charcoal and carbonate of potash, or with cyanide of potassium, and the metal sublimed. It is recognised by its volatility and the odour of its vapour, but it may also be oxidized and the tests applied. Even when the acid is pure, we ought always to reduce at least a part of it to metal, as the most characteristic property. Only antimony at all resembles it in this particular, but we shall see presently that these two metals are easily distinguished.

Where the quantity of arsenic present is very small, the method of Marsh, in which it is converted into arseniuret hydrogen, must be employed as the most delicate and the most certain. We shall describe this presently, under the head of arseniuret hydrogen.

Arsenic acid, $As\_2O\_5$, corresponds to phosphoric acid as arsenious acid does to phosphorous acid. It is formed by heating arsenious acid with nitric or nitro-hydrochloric acid. It is very soluble, very acid, and very poisonous. Its salts are exactly similar to and isomorphous with those of phosphoric acid.

When hydrogen is disengaged in a solution containing arsenious acid, there is formed arseniuret hydrogen, $AsH\_2$, mixed with hydrogen. The gas is obtained purer, but still mixed with some hydrogen, by the action of hydrochloric acid on an alloy of tin and arsenic. It has a most offensive alliaceous smell, and burns when heated in air, with a pale flame, producing arsenious acid and water. It is extremely poisonous, and several chemists have lost their lives from incautiously inhaling a little of it.

When this gas, even mixed with a large excess of hydrogen, is passed through a tube, part of which is red-hot, it is entirely decomposed, and a ring of metallic arsenic is deposited just beyond the hot part of the tube. On this property is founded Marsh's process for detecting arsenic.

The suspected liquid is added to diluted sulphuric acid, and zinc is introduced. The gas is then passed through a

narrow tube, and a slow but constant current of it is kept Chemistry. up. As soon as all the air is expelled from the tube, a part of it is heated to redness, and if there be but a trace of

arsenic present, a ring soon appears. Or the gas may be burned at the end of the tube without heating it in the tube, and a cold piece of glass or porcelain held in the flame, which, if arsenic be present, will deposit metallic stains of that metal. The former method is the best, as the whole arsenic may in time be collected in one ring. Under similar circumstances, antimony also forms a ring of metal, but the arsenical ring is sublimed by a moderate heat, and if air be present is changed into arsenious acid, easily recognised by its octahedral crystals, its volatility, and by other tests. The antimony is with difficulty volatilized, and if oxidized, the oxide remains, and is not crystalline. Nitric acid dissolves the arsenic, but not the antimony.

This method of detecting arsenic is so delicate, that by its means arsenic is found where it was not before suspected. We often find it in the commercial sulphuric acid, hydrochloric acid, and zinc, the very substances we use in the process. Hence, before introducing the suspected fluid, we must always try the acid and zinc first for a time. If no ring appear, and if, on adding the suspected liquid, the ring be at once formed, we may safely conclude that the arsenic comes from that liquid. But without this precaution, we dare not make any such statement.

There is only one chloride of arsenic, a tetrachloride, $AsCl\_4$, which is volatile.

There are three sulphurets of arsenic. Realgar, which is reddish-brown, $As\_2S\_3$; orpiment, which is yellow, $As\_2S\_2$; and the persulphuret, which is pale yellow, $As\_2S\_4$. The two former are found in nature.

The reader has no doubt remarked the analogy between the chemical relations of antimony and arsenic and those of phosphorus. Indeed, no two elements are so analogous in this respect as arsenic and phosphorus, save that one is metallic, the other not. There is a resemblance, however, even in physical properties for the odour of phosphorus closely resembles that of arsenic. These three elements, in spite of one being non-metallic, form a group distinguished by forming oxides with 3 and 5 eqs. of oxygen, and gaseous compounds of a fetid odour with 3 eqs. of hydrogen.

48. Tellurium.

Symbol Te. Equivalent = 64.2.

This metal forms one of another remarkable group of three, the two others being non-metallic, namely, sulphur and selenium. This case and the preceding, show that there is no essential and absolute division between metals and non-metallic bodies.

Tellurium is found, but very rarely, combined with gold, silver, lead, and bismuth, in the Transylvanian mines. In appearance it resembles antimony, and its specific gravity is 6260. With oxygen it forms two acids, tellurous acid, $TeO\_2$, and telluric acid, $TeO\_4$, corresponding to sulphurous and sulphuric acids.

Chemistry. With hydrogen, it forms a gaseous compound, $\backslash text\{H\}\_2\backslash text\{Te\}$, corresponding to hydrosulphuric acid, and resembling it in its properties and its action on metallic solutions.

Tellurium is volatilized by an intense white heat. It is chiefly interesting from its analogy to sulphur and selenium; but its rarity prevents it from being applied to useful purposes, although it seems likely to have a powerful action on the animal system.

GROUP VII.

This group has no peculiar character, from which it can be properly named, but is intermediate between group 5 and the next or last group, that, namely, of the noble metals. The number of metals in it is five, namely, bismuth, uranium, copper, lead, and mercury. Their attraction for oxygen is less than that of the metals of group 5, but still considerable. The oxides of one of them, mercury, are reduced by heat alone, thus connecting these metals with the noble metals. The protoxides, and in some cases higher oxides, are basic.

49. Bismuth.

Symbol $\backslash text\{Bi\}$. Equivalent = 71? or 106?

This metal is found in the metallic state, and as sulphuret, also combined with tellurium. It is obtained entirely from the native metal, which, being very fusible, is separated from its gangue or matrix by fusion. It contains a little sulphuret, also sulphurets of other metals and arseniurets, but is easily purified by heating with $\backslash frac\{1\}\{4\}$th of its weight of nitre. The sulphur, arsenic, and other metals are oxidized and combine with the potash, as does also a little bismuth; the rest forms a pure button of metal below the saline mass. Bismuth is very crystalline, and may be obtained in fine crystals by melting a mass of it, and when it is half solidified, breaking the crust and pouring out the liquid part. The hollow mass is found lined with fine hollow rhombohedral crystals, not far removed from the cube in their angles. Its colour is reddish-white; its specific gravity 9900. It melts at about $507^\backslash circ$, and expands in solidifying. It is volatile at a very intense heat.

Bismuth forms two oxides: 1. A basic oxide, which is thought by some to be a protoxide, $\backslash text\{Bi\}\_2\backslash text\{O\}\_3$, in which case the equivalent of the metal is 71; but which others regard as a sesquioxide, $\backslash text\{Bi\}\_2\backslash text\{O\}\_3$, which makes the equivalent of the metal 106. It is a weak base, and its salts are decomposed by water, which dissolves very acid salts, leaving insoluble subsalts. This is particularly the case with the nitrate, which, diluted with water, deposits a fine white crystalline powder of sub-nitrate, used as a white paint for the face. It is, however, very liable to be blackened by hydrosulphuric acid and other analogous compounds of sulphur. The salts of bismuth are recognised by the action of water and that of hydrosulphuric acid.

2. A peroxide, which has weak acid properties, and is also called bismuthic acid. It has a light red colour, and has not been much studied. If the basic oxide be $\backslash text\{Bi\}\_2\backslash text\{O\}\_3$, the acid is $\backslash text\{Bi\}\_2\backslash text\{O\}\_5$; but if the former be $\backslash text\{Bi\}\_2\backslash text\{O\}\_3$, the latter is probably $\backslash text\{Bi\}\_2\backslash text\{O\}\_7$. There seems to be an intermediate oxide, a compound of the acid with the base.

Chloride of bismuth is volatile, and, taking the larger equivalent, its formula is $\backslash text\{Bi\}\_2\backslash text\{Cl\}\_3$. It dissolves in hydrochloric acid, but the solution is decomposed by water, which throws down an oxychloride, $\backslash text\{Bi\}\_2\backslash text\{Cl\}\_5\; +\; 2(\backslash text\{Bi\}\_2\backslash text\{O\}\_3,\; 3\backslash text\{HO\})$. This powder is pearl white, used like the subnitrate as a cosmetic.

Sulphuret of bismuth, $\backslash text\{Bi\}\_2\backslash text\{S\}\_3$, occurs in nature, and is isomorphous with sulphuret of antimony.

Bismuth forms with lead and tin alloys of remarkable fusibility. That with 1 part of lead, 1 of tin, and 2 of bismuth, melts at $201^\backslash circ$; that with 5 parts of lead, 3 of tin, and 8 of bismuth, melts at $209^\backslash circ$. These alloys are used for

taking casts for electrotyping, and for various other purposes. Chemistry.

50. Uranium.

Symbol $\backslash text\{U\}$. Equivalent = 217.2.

This metal is found as an oxide, combined with oxides of iron, lead, copper, and other metals, along with silica, in the mineral pitchblende; and also as a phosphate of uranium and lime, in uranite. The pitchblende is acted on by nitric acid, and the solution purified from lead, copper, and arsenic, by hydrosulphuric acid. The nitrate of uranium is then crystallized. This salt, which forms beautiful yellow crystals with green reflection, when heated leaves an oxide. This, mixed with charcoal, and heated in a current of chlorine, gives a volatile protochloride, $\backslash text\{UCl\}$, in dark green crystals. These, heated with potassium, yield the metal as a black powder, parts of which, by the heat evolved, are brought into a compact state, and exhibit a silvery lustre. Heated in air, it burns vividly, producing the oxide. It is permanent at ordinary temperatures, and appears to be in some degree malleable. It only decomposes water at a high temperature.

Uranium forms two oxides, both basic, a protoxide, $\backslash text\{UO\}$, and a sesquioxide, $\backslash text\{U\}\_2\backslash text\{O\}\_3$. The former is dark brown, and forms green salts; the latter is yellow, and forms beautiful yellow salts. From the circumstance that the salts of the sesquioxide differ in the amount of acid from those of all other sesquioxides—the sulphate, for example, being $\backslash text\{U\}\_2\backslash text\{O\}\_3,\; \backslash text\{SO\}\_3$, and not like that of alumina, $\backslash text\{Al\}\_2\backslash text\{O\}\_3,\; 3\backslash text\{SO\}\_3$—it is supposed that the sesquioxide is really a protoxide, not of uranium, but of uranyl, $\backslash text\{U\}\_2\backslash text\{O\}\_3$. On this supposition, the salts of the sesquioxide are no longer anomalous. These salts give yellow precipitates with the alkalies, which are compounds of the sesquioxide with the alkalies, for that oxide has both weak acid and weak basic properties. Ferrocyanide of potassium produces in the salts of sesquioxide of uranium a fine rich brown precipitate.

The chloride, $\backslash text\{UCl\}$, has been already mentioned. There is an oxychloride $\backslash text\{U\}\_2\backslash text\{O\}\_3\backslash text\{Cl\}$, which may be regarded as the protochloride of uranyl. When heated with potassium it leaves not the metal but the protoxide, $\backslash text\{UO\}$, or uranyl $\backslash text\{U\}\_2\backslash text\{O\}\_3$, which was long supposed to be the metal, and appears, in fact, to play the part of a metal.

Oxide of uranium is now much used to give to glass the beautiful yellow colour, with green reflection, so much admired in the Bohemian glass.

51. Copper.

Symbol $\backslash text\{Cu\}$. Equivalent = 31.7.

This valuable metal occasionally occurs native, but its chief ores are the mixed sulphurets of iron and copper or copper pyrites, the carbonate or blue copper ore, and the compound of carbonate and hydrate called green malachite. The latter are easily reduced by heating with charcoal; the former is roasted so as to oxidize it and dissipate the sulphur, and the roasted ore is then heated with charcoal. This process is repeated, and at last pure copper is obtained. Another ore of copper, very easily worked, is the red oxide or suboxide. Lastly, in mines where sulphuret of copper is present, it is oxidized by the air into sulphate of protoxide, which is dissolved by the water filtering through the rocks. This water, where the sulphate is observed, is collected in reservoirs, and the copper precipitated by fragments of iron. The copper thus obtained is very pure, and has only to be melted.

Copper has a bright red colour and high lustre, of specific gravity from 8780 to 8960. It melts at a strong red heat, and at a white heat gives vapours which burn with a green flame. It is very malleable and ductile, and has much tenacity.

Copper is rapidly tarnished and covered with a green rust

Chemistry. or verdigris, in moist air. At a red heat it is rapidly converted into the black protoxide.

Protoxide of copper, $\backslash text\{CuO\}$, is black, and forms with acids blue or green salts, with water a blue hydrate. It is a pretty strong base. Its solutions are known by their colour, by the action of a rod of iron which is quickly covered with precipitated copper, and by the action of ammonia which first causes a pale blue precipitate of subsalt or hydrate, and when added in excess, redissolves this, forming a deep violet-blue solution. Potash and soda throw down the blue hydrate, which, when boiled, loses water and becomes black. Hydro-sulphuric acid gives a black precipitate of sulphuret, and ferrocyanide of potassium gives, in acid solutions, a chestnut-brown precipitate. The last is a most delicate and characteristic test, and by its means traces of copper have been found in most soils, as well as in plants, and in animal and vegetable food. Copper vessels are rapidly corroded by cold vegetable acids, but the same acids may be boiled in them without corroding the metal. As the salts of copper are very poisonous, this must be carefully attended to. Preserves of acid fruits, such as red currants, &c., are constantly boiled in vessels of copper, but must not be allowed to cool in them. Copper is sometimes improperly added to pickles to improve their green colour. Its presence is easily detected in the acid liquor by the ferrocyanide, or by introducing the blade of a knife. Several of the salts of copper are used in medicine. Sulphate of copper, or blue vitriol, is an escharotic externally, and an emetic in small doses given internally. The violet-coloured ammoniac-sulphate of copper is used in epilepsy, and also as a test for arsenious acid.

Suboxide of copper, $\backslash text\{Cu\}\_2\backslash text\{O\}$, is red and crystallizes in octahedrons. It is found in nature, and may be formed by heating the protoxide with copper filings, or by adding to a solution of sulphate of copper sugar and potash, till the precipitate at first formed is redissolved. On boiling, the suboxide is deposited as a fine deep red crystalline powder. It forms colourless salts, and a colourless solution in ammonia, but the latter solution absorbs oxygen from the air, and becomes violet-blue. The oxide gives to glass, when melted with it, a fine red colour.

Copper seems to form two oxides with more oxygen than the protoxide, a deutoxide and an acid. But these are very little known.

There are two chlorides of copper; the subchloride or dichloride $\backslash text\{Cu\}\_2\backslash text\{Cl\}$, and the protochloride $\backslash text\{CuCl\}$. The former is white and sparingly soluble. The latter is green and very soluble. It is formed by dissolving the protoxide in hydrochloric acid.

There are also two sulphurets corresponding to the two oxides and chlorides. The disulphuret, $\backslash text\{Cu\}\_2\backslash text\{S\}$, is formed by melting together copper and sulphur. It is fusible and crystalline. The protosulphuret, $\backslash text\{CuS\}$, is the black precipitate produced in salts of copper by hydrosulphuric acid. $\backslash text\{HS\}\; +\; \backslash text\{CuO\},\; \backslash text\{SO\}\_3\; =\; \backslash text\{HO\},\; \backslash text\{SO\}\_3\; +\; \backslash text\{CuS\}$. It rapidly attracts oxygen from the air, and is converted into sulphate. Copper pyrites consists of the disulphuret, combined with sesquisulphuret of iron, and mixed moreover with iron pyrites, or bisulphate of iron. From this mineral the greater part of the copper of commerce is obtained.

Copper forms many valuable alloys. With zinc or zinc and a little tin or lead, it forms brass; with tin it gives bronze, bell metal, gong metal, and gun metal, according to the proportions. In the bronze used for medals, a trace of zinc is added.

52. Lead.

Symbol Pb (Plumbum). Equivalent = 103.7.

This valuable metal occurs in various forms, but the only ores which are wrought are galena or sulphuret of lead, $\backslash text\{PbS\}$, and carbonate of lead, $\backslash text\{PbO\},\; \backslash text\{CO\}\_3$. The latter is simply

heated with charcoal. The former is roasted, when sulphurous acid is given off, and oxide of lead, $\backslash text\{PbO\}$, and sulphate of lead, $\backslash text\{PbO\},\; \backslash text\{SO\}\_3$, are formed. A part of the ore is left unchanged; and on increasing the heat, a most remarkable reaction takes place between the unaltered ore and the oxide and sulphate. The reaction with the oxide is this: $\backslash text\{PbS\}\; +\; 2\backslash text\{PbO\}\; =\; \backslash text\{SO\}\_2\; +\; \backslash text\{Pb\}\_2$. With the sulphate it is this: $\backslash text\{PbS\}\; +\; \backslash text\{PbO\},\; \backslash text\{SO\}\_3\; =\; 2\backslash text\{SO\}\_2\; +\; \backslash text\{Pb\}\_2$. In both cases, metallic lead and sulphurous acid are the only products. When the ore is less pure, it is first roasted and then heated with iron, which removes the sulphur. The lead thus obtained frequently contains silver in sufficient quantity to repay its extraction.

Lead is a metal of a dark-gray colour and high lustre, quickly tarnished in air, but only superficially at ordinary temperatures. It is very soft, malleable, and ductile, but has little tenacity. Its specific gravity is 11.44. It is easily melted, and when hot is rapidly oxidized, forming protoxide. The uses of lead are well known.

With oxygen lead forms three compounds, a suboxide $\backslash text\{Pb\}\_2\backslash text\{O\}$, a protoxide $\backslash text\{PbO\}$, and a deutoxide having feeble acid properties, peroxide of lead or plumbic acid.

The protoxide, $\backslash text\{PbO\}$, is formed when lead is heated rather beyond its melting point. It is a yellow powder called massicot. This, heated more strongly, melts, and on cooling forms a crystalline brownish-yellow mass, called litharge. Heated for a long time in air, massicot takes up oxygen and forms the red oxide or minium, a compound of plumbic acid with the protoxide. The protoxide is a strong base, isomorphous with baryta and lime. Its salts have a sweet taste, and are poisonous, more especially the carbonate. They are recognised by the following tests. Alkalies throw down a white hydrate, soluble in excess; carbonated alkalies a white carbonate; hydrosulphuric acid a black sulphuret; sulphuric acid and sulphates a white insoluble sulphate; and iodide of potassium a bright yellow crystalline iodide. The most important of its salts are the carbonate or white lead, the nitrate, the acetates, which are used in calico-printing, the latter also extensively in medicine, and the chromate, valued for its fine yellow colour.

Plumbic acid or peroxide of lead is obtained by acting on red lead with nitric acid. Red lead is $\backslash text\{Pb\}\_2\backslash text\{O\}\_4\; =\; 2\backslash text\{PbO\},\; \backslash text\{PbO\}\_2$ and also $\backslash text\{Pb\}\_4\backslash text\{O\}\_8\; =\; 3\backslash text\{PbO\},\; \backslash text\{PbO\}\_2$. The acid dissolves out the protoxide, and leaves the plumbic acid as a purple-coloured dense powder. It combines with bases, and even forms crystallizable salts with some of them.

With chlorine, lead forms but one compound, a protochloride, $\backslash text\{PbCl\}$. It is formed as a sparingly soluble white crystalline powder when hydrochloric acid or soluble chlorides are added to the salts of lead. The chloride, melted with the oxide, forms several oxychlorides, which are used as yellow pigments, under the names of mineral yellow, Turner's yellow, &c.

There is but one iodide, $\backslash text\{PbI\}$, formed when iodide of potassium is added to a salt of lead. It is a bright yellow sparingly soluble powder, generally formed of minute scales, which dissolves in boiling water, forming a colourless solution. On cooling, large and beautiful scales of a golden lustre are deposited. They are regular hexagons.

There is also but one sulphuret, $\backslash text\{PbS\}$, formed as a black powder when hydrosulphuric acid or a soluble sulphuret acts on salts of lead. It is the same substance with galena, the ore of lead, which forms cubic crystals of a metallic lustre, but very brittle.

Lead forms with antimony the alloy used for type metal; with tin it forms pewter and various kinds of solder.

53. Mercury.

Symbol Hg (Hydrargyrum). Equivalent = 200.

This metal is found native, and in the form of bisul-

3 R

Chemistry. phuret or cinnabar. The latter substance, when heated in iron bottles with iron filings, yields metallic mercury. The metal is liquid at all temperatures from $-39^\{\backslash circ\}$ to about $600^\{\backslash circ\}$, which is its boiling point. Below $-39^\{\backslash circ\}$ it is solid. It has a bluish-white colour and bright lustre. Its specific gravity is about 13.6. When heated to about its boiling point, in air, it absorbs oxygen, and forms the red oxide or deutoxide, which is again decomposed by a temperature somewhat higher.

Mercury forms two oxides, a protoxide and a deutoxide. The protoxide, $\backslash text\{HgO\}$, is obtained by the action of potash on calomel, which is the protochloride, $\backslash text\{HgCl\}\; +\; \backslash text\{KOH\}\; =\; \backslash text\{KCl\}\; +\; \backslash text\{HgO\}$. It has an ash-gray colour, and by the action of light is resolved slowly into deutoxide and metal, $2\backslash text\{HgO\}\; =\; \backslash text\{HgO\}\_2\; +\; \backslash text\{Hg\}$. It is a base, and its salts give a dark gray or black precipitate with alkalies, a white one of calomel with hydrochloric acid or soluble chlorides, and a dirty greenish-yellow protoiodide with iodide of potassium. They are reduced to the metallic state by copper and by other metals; also by protochloride of tin, which first forms calomel and then reduces it.

The peroxide or deutoxide, $\backslash text\{HgO\}\_2$, is also a base. It is obtained by heating the nitrate of either oxide till the acid is entirely expelled, taking care not to decompose the oxides. It is a very dark red, nearly black while hot, and light red when cold. The same oxide is formed slowly when mercury is kept at its boiling point in a vessel with a long neck, in which the vapours are condensed and fall back. Prepared in this way it is of a darker red, and is called oxidum hydrargyri rubrum per se, while that from the nitrate is called oxidum hydrargyri rubrum per acidum nitricum. The latter generally contains some subnitrate, and is more active as an escharotic. It is much used also in the form of ointment. Its salts give a yellow precipitate of hydrated peroxide with potash; a white with ammonia, which is a compound containing amide; and a fine scarlet periodide with iodide of potassium. Both the salts of protoxide and those of peroxide give a black precipitate with hydrosulphuric acid, which in the latter case is the bisulphuret, in the former a mixture of bisulphuret with the metal. The salts of the peroxide are reduced by the same reagents as those of the protoxide; by copper and other metals, by protochloride of tin, and by formic acid.

Mercury forms two chlorides: calomel, which is the protochloride, $\backslash text\{HgCl\}$, a white, insoluble, heavy powder, formed by the action of hydrochloric acid or chloride of sodium on salts of the protoxide; and corrosive sublimate or bichloride, $\backslash text\{HgCl\}\_2$, which is formed when hydrochloric acid acts on the peroxide, $2\backslash text\{HgO\}\; +\; \backslash text\{HCl\}\; =\; 2\backslash text\{HO\}\; +\; \backslash text\{HgCl\}\_2$. It is a crystalline heavy substance, soluble in water, and very poisonous. The chlorides are also formed by subliming a mixture of sulphate of peroxide with common salt, which gives a sublimate of bichloride, and, if metallic mercury be added, calomel. Both are volatile, and calomel, whichever way prepared, always contains at first some bichloride. To render it fit for medical use, this must be removed by repeated boiling with water, as long as potash causes a yellow precipitate in the filtered liquid. Both chlorides are much used in medicine, calomel being the milder, and corrosive sublimate the more active of the two.

There are two bromides and two iodides, corresponding to the two chlorides. The iodides are formed by the action of iodide of potassium on the salts of the two oxides. The protoiodide, $\backslash text\{HgI\}$, is of a dirty yellowish green, insoluble, and is resolved by light into periodide and metal, $2\backslash text\{HgI\}\; =\; \backslash text\{HgI\}\_2\; +\; \backslash text\{Hg\}$. The periodide is of a beautiful scarlet, and insoluble, or nearly so. A moderate heat renders it lemon-yellow, and the yellow form is instantly reconverted into the red by friction, or by the touch of a hard point. In the latter case, the part touched becomes red, and the red colour spreads from it through the mass. The change of colour is

owing to a change of position in the molecules, or a change of crystalline form. Both iodides are used in medicine, and the periodide also as a pigment. Chemistry.

Mercury rubbed with 1 eq. sulphur, forms a black powder called ethiops mineral, which is a mixture of the metal with the bisulphuret. With 2 eqs. of sulphur it forms the bisulphuret, which is also produced by the action of hydrosulphuric acid on salts of the peroxide. It is black when thus prepared; but if sublimed becomes dark red, and when finely powdered this acquires the colour of vermilion, which is a form of the bisulphuret.

The compounds of mercury with other metals are called amalgams. They are sometimes liquid, oftener semisolid. Amalgam of silver occurs native in crystals. Amalgam of tin is used for silvering the backs of mirrors, and an amalgam of tin and zinc for rubbing the cushion of electrical machines.

GROUP VIII.

The Noble Metals.

The metals of this group are so called, because, from their small attraction for oxygen, they do not tarnish in the air, nor even in the fire. Hence their use for ornamental purposes, and for coinage. They are eight in number, namely, silver, gold, platinum, and five rarer metals associated with platinum.

54. Silver.

Symbol Ag (Argentum). Equivalent = 108.

This beautiful metal is found native; also combined with sulphur, arsenic, and antimony, with tellurium, and with chlorine. It occurs also frequently in small but available quantity in galena or lead ore. Native silver, when disseminated in the rock, is extracted by amalgamation with mercury.

Silver has a pure white colour and brilliant lustre. Its specific gravity is about 10.500. It melts at a strong red or white heat, and is quite unaltered in the fire. By the intense heat of the galvanic battery it is volatilized. It is very malleable and ductile, and has considerable tenacity. It is rather soft, and therefore copper is added to form standard silver, which is hard, and wears better. Spanish dollars consist of pure silver, with only a trace of gold, which is present more or less in all silver.

Silver appears to form three oxides; a suboxide, $\backslash text\{Ag\}\_2\backslash text\{O\}$, little known; the protoxide, $\backslash text\{AgO\}$, a strong base, and a deutoxide or peroxide, $\backslash text\{AgO\}\_2$. The protoxide alone is of interest. It is obtained by adding potash to nitrate of silver, as a brown hydrate, which, when gently heated, loses water, and leaves the protoxide as an olive-coloured powder. Its salts are recognised by the brown precipitate produced by potash, which like nearly all compounds of silver dissolves in ammonia, by the curdy white chloride produced by hydrochloric acid or soluble chlorides, and by the black sulphuret thrown down by hydrosulphuric acid. The metal is reduced by copper, zinc, and iron, and even by mercury.

There is only one chloride, $\backslash text\{AgCl\}$, which appears as a curdy white substance, insoluble in water and acids, soluble in ammonia, when hydrochloric acid or chloride of sodium is added to the salts of silver. It is fusible, and on cooling forms a semitransparent mass, called horn silver. It is blackened by light, a character common to all the compounds of silver, especially the chloride, iodide, bromide, &c.; on this property is founded the daguerreotype. The chloride, moistened with diluted acid, is reduced by zinc or iron; or it may be boiled with potash and sugar, or ignited to whiteness with lime and a little charcoal, when a button of silver is obtained.

The bromide and iodide resemble the chloride, but the iodide is yellowish and sparingly soluble in ammonia.

The sulphuret is black. It is found in nature both pure

Chemistry. and combined with arseniuret and antimoniuuret of silver, also in galena with sulphuret of lead.

The chief uses of silver are for coinage and for plate. Nitrate of silver is used as a caustic and also internally, and the chloride, iodide, and bromide, are the substances used in photography in the form of a film on the surface of silver, paper, or other substances.

The compounds of silver possess the remarkable property of dissolving in hyposulphite of soda, even when insoluble in water and acids, and this property is made use of in photography to fix the images by removing the unaltered chloride, &c. of silver.

55. Gold.

Symbol Au (Aurum). Equivalent = 99.6.

This metal is found native, as in the gold fields of California, Australia, and others. It is very widely distributed, but often in quantity too small to pay for its extraction. The sand of the Rhine and of most rivers contains traces of it. It is also found combined with tellurium, and alloyed with silver and palladium. Many kinds of pyrites contain gold enough to pay for the working, and it has also been found in galena.

Gold is distinguished by its yellow colour, bright lustre, and great density. Its specific gravity is 19,500. It is the most malleable of all metals, and also highly ductile. It melts in a white heat.

Gold does not combine directly with oxygen, and is dissolved by no single acid, but only by a mixture of nitric and hydrochloric acids, called aqua regia, or nitro-hydrochloric acid. This forms a chloride from which alkalies precipitate the oxide, which is a teroxide, $Au\_2O\_3$, as a brownish-yellow powder, which loses its oxygen when exposed to a moderate heat. It has weak acid properties, and is sometimes called auric acid. There is a suboxide of a violet colour, little known.

The terchloride of gold, $AuCl\_3$, is by far the most important compound. It is formed, as above stated, by means of aqua regia. It is reduced to the metallic state by protosulphate of iron, by formic acid and formiates, by oxalic acid, and by various metals. With the chlorides of the positive metals terchloride of gold forms crystallizable double chlorides.

The compounds of gold with sulphur are little known. Hydrosulphuric appears to form, in solution of the terchloride, a bisulphuret, which is black.

56. Platinum.

Symbol Pt. Equivalent = 98.7.

This metal is also found native, but combined or mixed with five other metals, namely iridium, rhodium, palladium, ruthenium, and osmium, all of which occur in very small quantity in the native platinum which is found in grains. Aqua regia dissolves the whole, except the black scales of an alloy of iridium and osmium, and a few particles of iridium. The solution is mixed with sal-ammoniac, and the bichloride of platinum which has been formed combines with that salt, chloride of ammonium, to form a nearly insoluble double chloride, $PtCl\_2\; \backslash cdot\; NH\_4Cl$, which is deposited as a bright yellow powder. If much iridium be present, the salt is coloured reddish from the presence of the corresponding double chloride of iridium. The washed salt is dried and ignited, when it leaves spongy platinum. By repeating the process the iridium is got rid of in the mother liquid. The spongy metal is made into a paste with water in a wooden mortar, and this paste gradually but powerfully compressed till it assumes a metallic aspect. It is then heated red-hot and hammered gently, and this is repeated till it becomes of full density. It has a grayish-white colour, a high lustre, and a specific gravity of 21,500.

It forms two oxides, a protoxide and a deutoxide, which

are feeble bases, and little studied. Its most important compound is the bichloride, $PtCl\_2$. This, with the chlorides of the positive metals, forms crystallizable and permanent double chlorides. Those of potassium and ammonium are nearly insoluble, and the latter, as above stated, yields pure spongy platinum when heated. That of sodium is soluble even in alcohol, and forms fine large crystals. In consequence of this property bichloride of platinum is used to absorb the ammonia formed from the nitrogen of organic compounds, and thus to determine the quantity of that nitrogen.

When the bichloride, which is an orange-coloured soluble salt, is heated to a certain point, it loses half its chlorine, and leaves a protochloride, as a green powder, insoluble in water, but soluble in hydrochloric acid. This chloride also forms double chlorides with the chlorides of the positive metals. With ammonia it gives rise to a remarkable series of bases containing platinum, nitrogen, hydrogen, and in some cases chlorine. These resemble organic bases.

The other metals found along with platinum are extracted from the mother liquid after the precipitation of the platinum by sal-ammoniac, by a very tedious and complex series of operations. Iridium and osmium are also obtained from an alloy of these metals left undissolved by the acid. They all resemble platinum, iridium the most, then rhodium, ruthenium, palladium, and osmium. Their chlorides are the chief compounds, except in the case of osmium, which forms with oxygen a remarkable crystallized and volatile acid, osmic acid, $O\_4O\_2$. These chlorides form double chlorides with potassium, ammonium, sodium, &c., analogous to those of platinum. The salts of rhodium are rose-coloured, those of iridium of various colours, those of palladium brown, and those of ruthenium reddish. All these metals are very infusible, and unaltered in the fire, like platinum. But they are so scarce, except palladium, which is malleable, and is used by dentists instead of gold, that they can hardly be applied to useful purposes. Iridium, or rather an alloy of it with a little platinum, as found in the ore, in small grains, is used for the points of gold pens, and from its extreme hardness does not perceptibly wear. Iridium is also the heaviest of metals, its specific gravity being 26,000. We shall not enter into further detail in regard to these very scarce metals.

Having concluded this brief notice of the elementary bodies, and of their chief binary compounds, the reader is now acquainted with the fundamental facts of chemistry. There is, however, an important class of compounds which we have only noticed incidentally, namely the salts, and although we cannot describe them in detail, we may offer some general remarks which will facilitate the study of them.

One division of saline compounds has been described in the place they naturally occupy, namely, those salts called haloid salts, formed of a metal united to such bodies as chlorine, bromine, &c. These, in fact, are true typical salts, for the very substance from which the name of the class is taken, common or sea salt, is one of them, being the chloride of sodium. In one sense, every body having similar properties may be called a salt; every body which is crystalline, more or less soluble, sapid, and neutral. Accordingly at an early period the term salt was extended to such bodies in general, and even after the true nature of common salt was known (for it had long been viewed as a compound of hydrochloric acid and soda) those bodies which really consisted of an acid and a base, both generally oxidized, such as sulphate of soda, nitrate of potash, and the like, were still regarded as salts, and even considered as the only true salts, on the theory that a true salt was a neutral compound of an acid and a base. This theory, in the end, actually excluded common salt from the class of salts; and therefore two kinds of salts were admitted; oxygen salts, of which sulphate of soda, $NaO\; \backslash cdot\; SO\_3$, is a type; and haloid salts, of which common salt, $NaCl$, is the

Chemistry. type. We have already shown how, by the progress of chemical discovery, we have been enabled to reduce to one series the two classes of acids, namely, oxygen acids, such as sulphuric acid, $\backslash text\{HO\}\_2\backslash text\{SO\}\_4$, and hydrogen acids, such as hydrochloric acid, $\backslash text\{HCl\}$. This is done by regarding the hydrogen in the former as being combined, not first with oxygen to form water, and the water with the dry acid, but as united with a radical, $\backslash text\{SO\}\_4$, differing from chlorine only in being certainly compound. Although this particular radical, $\backslash text\{SO\}\_4$, be not yet known in a separate form, many such compound radicals are known, and play exactly the part of chlorine. Of these cyanogen, $\backslash text\{C\}\_2\backslash text\{N\}=\backslash text\{Cy\}$, is the type. On this view, therefore, while hydrochloric acid can only be $\backslash text\{HCl\}$, sulphuric acid, represented as $\backslash text\{H\}\_2\backslash text\{SO\}\_4$, or more simply, making $\backslash text\{SO\}\_4=\backslash text\{Su\}$, by $\backslash text\{HSu\}$, becomes as perfectly analogous to it in formula as it is in properties.

Now, if we adopt this view of acids, or at least of all such acids as cannot exist in an active state without hydrogen, formerly supposed to be present as water, we must apply the same view to the salts of those acids. For when hydrochloric acid acts on a base, such as soda, two phenomena are constantly observed—the formation of a neutral salt and the separation of water. This is perfectly obvious from the equation ($\backslash text\{M\}$ representing any metal), $\backslash text\{HCl\}+\backslash text\{MO\}=\backslash text\{MCl\}+\backslash text\{HO\}$. But when sulphuric acid acts on the same bases the phenomena are absolutely the same; a neutral salt is formed, and water separated. And this we express by the equation, $\backslash text\{H\}\_2\backslash text\{SO\}\_4+\backslash text\{MO\}=\backslash text\{M\}\_2\backslash text\{SO\}\_4+\backslash text\{HO\}$.

Here, then, we see the first great principle to be admitted concerning salts. They represent hydrogen acids, in which the hydrogen is replaced by a metal, and therefore acids (that is, hydrogen acids, which includes what used to be called hydrated acids) and salts form in reality but one series, the general formula of which is $\backslash text\{XR\}$; $\backslash text\{R\}$ being any negative acid radical or salt radical, for the terms are synonymous, and $\backslash text\{X\}$ standing for the positive element, which may be either hydrogen or a metal. So that acids, in this view, are salts of hydrogen, and salts are salts of metals. Hydrogen, indeed, is possibly or even probably the gas of a very volatile metal, whose salts are peculiar in this, that they possess those characters which we call acid. The analogy between neutral salts and acids, nay, even between neutral salts, acids, and bases, or alkalies, did not escape the first scientific chemists, although they knew nothing of the true constitution of any of them. In all works published towards the end of the last century, acids, alkalies, and neutral salts are all included under the head of saline substances. This view, founded on acute observation, was for a time displaced by the limited notion of a salt being a compound of an acid and an alkali, which, as we have seen, actually excluded common salt from the class of salts, as soon as it was shown to contain neither oxygen nor hydrogen, and consequently not to be, as was supposed, a compound of hydrochloric acid with soda. But the progress of science has shown that as oxygen can no longer be called the acidifying principle, since hydrogen is better entitled to the name, so all acids, bases, and salts really belong to one series, that represented by $\backslash text\{XR\}$. Water may be taken as the type of all such compounds, and while its oxygen may be replaced by chlorine, bromine, iodine, fluorine, sulphur, selenium, cyanogen, the radicals, $\backslash text\{SO\}\_4$ (in sulphuric acid), $\backslash text\{NO\}\_3$ (in nitric acid), &c. &c., its hydrogen may be replaced by any metal, or by such compound radicals as can play the part of metals, as cyanogen and $\backslash text\{SO\}\_4$ play that of chlorine. In organic chemistry there are, as we shall see, many such radicals, of which ethyle, $\backslash text\{C\}\_2\backslash text\{H\}\_5=\backslash text\{Ae\}$, is a good example.

Such being the nature of salts in general, it is easy to see how neutral salts are formed when acids and bases act on each other. They were formerly supposed to combine directly, but we now know that if the acid be a hydrogen

acid, or a hydrated acid, and the base an oxide, this is not the case, but that water is always formed, as in the general equation, $\backslash text\{HR\}+\backslash text\{MO\}=\backslash text\{MR\}+\backslash text\{HO\}$. We have just stated how this applies to sulphuric acid, $\backslash text\{HO\}\_2\backslash text\{SO\}\_4$, or rather, $\backslash text\{H\}\_2\backslash text\{SO\}\_4$. But when anhydrous sulphuric acid acts on an anhydrous base, the reaction is different; no hydrogen being present, no water can be formed, but yet the same neutral salt is formed, as, to take the case of sulphuric acid and soda, both anhydrous, in the equation, $\backslash text\{NaO\}+\backslash text\{SO\}\_3=\backslash text\{Na\}\_2\backslash text\{SO\}\_4$. Here, instead of simply uniting together, the acid and base react on each other; the oxygen of the base is transferred to the acid, producing the radical, $\backslash text\{SO\}\_4$, and with this the metal unites.

The salts of ammonia come into the same category as those of metallic protoxides. For, as has been already explained, in every salt of ammonia, $\backslash text\{NH\}\_3$, with an oxygen acid, there is found, besides the ammonia and anhydrous acid, invariably 1 eq. of water, $\backslash text\{HO\}$, or rather its elements. The salt, $\backslash text\{NH\}\_4\backslash text\{SO\}\_4$, if it exist at all, is not sulphate of ammonia. That salt contains the elements, $\backslash text\{NH\}\_3$, $\backslash text\{HO\}$, $\backslash text\{SO\}\_4$, which, on the old view, are arranged as $\backslash text\{NH\}\_4\backslash text\{O\}$, $\backslash text\{SO\}\_4$, sulphate of oxide of ammonium, and on the new as $\backslash text\{NH\}\_4$, $\backslash text\{SO\}\_4$, or, making ammonium, $\backslash text\{NH\}\_4=\backslash text\{Am\}$, it is written $\backslash text\{Am\}\_2\backslash text\{SO\}\_4$, perfectly corresponding to $\backslash text\{K\}\_2\backslash text\{SO\}\_4$, sulphate of potash, which it resembles very closely in properties. When ammonia, $\backslash text\{NH\}\_3$, acts on hydrogen acids, such as hydrochloric acid, is was formerly supposed to combine directly with the acid, and the resulting salt, sal-ammoniac or hydrochlorate of ammonia, was written $\backslash text\{NH\}\_4\backslash text\{HCl\}$. It is now believed that here also ammonium is formed by the hydrogen of the acid combining with the ammonia, and that the compound metal thus formed combines with the residual chlorine, just as potassium would do; so that the result is $\backslash text\{NH\}\_4+\backslash text\{HCl\}=\backslash text\{NH\}\_4\backslash text\{Cl\}$, $\backslash text\{Cl\}=\backslash text\{AmCl\}$, and the salt is now called, accordingly, chloride of ammonium, being perfectly analogous in all respects to chloride of potassium or sodium. In other words, as cyanogen or $\backslash text\{SO\}\_4$ may replace the oxygen of water or the chlorine of sea-salt, so ethyle, $\backslash text\{C\}\_2\backslash text\{H\}\_5=\backslash text\{Ae\}$, and ammonium, $\backslash text\{NH\}\_4=\backslash text\{Am\}$, can replace the hydrogen in water and the metal in salt, being, in fact, compound metals.

On the views above explained, neutral salts become exceedingly simple, and the law of their formation very easily remembered, while water, acids, and bases all fall into the same category.

Besides neutral salts, there are acid or supersalts, and alkaline or subsalts. The former, in the case of such acids as sulphuric acid, may be regarded as compounds of the acid with a neutral salt. Thus acid sulphate, or bisulphate of potash is $\backslash text\{KO\}\_2\backslash text\{HO\}\_2\backslash text\{SO\}\_4=\backslash text\{K\}\_2\backslash text\{SO\}\_4+\backslash text\{H\}\_2\backslash text\{SO\}\_4$. The latter are compounds of the neutral salt with an additional quantity of the base, as, for example, subnitrate or basic nitrate of lead, which is $2\backslash text\{PbO\}\_2\backslash text\{HO\}\_2\backslash text\{NO\}\_3=\backslash text\{PbO\}\_2\backslash text\{NO\}\_3+\backslash text\{PbO\}\_2\backslash text\{HO\}\_2$.

There is, however, another kind of acid salts, namely, those of polybasic acids. Of such acids, phosphoric acid is an excellent type, in two of its three modifications, the dibasic and tribasic phosphoric acids. Polybasic acids, especially dibasic ones, are very frequent in organic chemistry. Of the organic dibasic acids, oxalic acid is the type.

A dibasic acid is one which, to form a neutral salt, requires 2 eqs. of base. Dibasic phosphoric acid is $\backslash text\{PO\}\_3\backslash text\{H\}\_2$, or rather $\backslash text\{H\}\_2\backslash text\{PO\}\_3$, the 2 eqs. of hydrogen being replaceable by metals. The neutral dibasic phosphate of silver is $\backslash text\{Ag\}\_2\backslash text\{PO\}\_3$. But if only 1 eq. of hydrogen be replaced by a metal, we have the acid only half neutralized, or an acid salt, as the acid dibasic phosphate of silver, which would be $\backslash text\{Ag\}\backslash \{\backslash text\{H\}\backslash \}\backslash text\{PO\}\_3$, or that of potash, which is $\backslash text\{K\}\backslash \{\backslash text\{H\}\backslash \}\backslash text\{PO\}\_3$.

The same is true of tribasic acids, which form three series of salts with the same base, according as 1, 2, or 3 eqs. of

Chemistry. hydrogen are replaced by the metal. Tribasic phosphoric acid is $\backslash text\{PO\}\_3$, $3\backslash text\{HO\}\; =\; \backslash text\{H\}\_3\backslash text\{PO\}\_3$; and it forms with soda, three salts, namely, the acid salt $\backslash text\{Na\}^+\; \backslash text\{H\}\_2\backslash text\{PO\}\_3^-$, the acid salt $\backslash text\{Na\}^+\; \backslash text\{HPO\}\_3^-$, and the neutral salt $\backslash text\{Na\}\_2\backslash text\{PO\}\_3$. The two latter are alkaline in reaction on test paper, although acid and neutral in composition; but, in fact, all three, as well as the two salts of the dibasic acid, are, strictly speaking, neutral salts in composition, since they all contain the same number, whether two or three, according to the acid, of basic equivalent, whether metal or hydrogen. Accordingly, all the three salts of soda with the tribasic acid, form, with salts of silver, the same yellow precipitate of neutral tribasic phosphate of silver, $\backslash text\{Ag\}\_3\backslash text\{PO\}\_3$. Oxalic acid is $\backslash text\{C\}\_4\backslash text\{O\}\_4$, $2\backslash text\{HO\}\; =\; \backslash text\{H\}\_2\backslash text\{C\}\_4\backslash text\{O\}\_4$, and it forms a neutral oxalate of potash, $\backslash text\{K\}\_2\backslash text\{C\}\_4\backslash text\{O\}\_4$; an acid oxalate of potash called binoxalate, $\backslash text\{K\}^+\; \backslash text\{H\}^-\; \backslash text\{C\}\_4\backslash text\{O\}\_4$, and a double acid oxalate of potash, called quadroxalate, $\backslash text\{K\}^+\; \backslash text\{H\}^+\; \backslash text\{C\}\_4\backslash text\{O\}\_4\; +\; \backslash text\{H\}\_2\backslash text\{C\}\_4\backslash text\{O\}\_4$.

With regard to salts considered individually, they may be studied either under the head of all the salts of one acid radical with different bases, or all those of one base with different acid radicals. On the former plan they are arranged as sulphates, nitrates, phosphates, chlorides, bromides, &c.; on the other as the salts of potassium, sodium, ammonium, iron, lead, silver, &c. There are conveniences belonging to both systems, but our space will not permit us to enter into either. We can only point out that all the salts of one radical, all the sulphates, or all the chlorides exhibit the characters of that radical. Thus all the soluble sulphates form an insoluble precipitate of sulphate of baryta

when mixed with salts of that base. All the chlorides are in like manner precipitated by salts of silver. All the nitrates deflagrate with red-hot charcoal, &c. Nor are we confined to one character; for all the sulphates, for example, when ignited with charcoal, yield sulphurets, and the smell of hydrosulphuric acid is perceived when acids are added to the ignited mass. This is because they all contain sulphur.

Again, all the salts of one metal or base exhibit the chemical characters of that metal. Thus all the salts of sesquioxide of iron give Prussian blue with ferrocyanide of potassium. All the salts of silver give the insoluble chloride on the addition of hydrochloric acid or solution of salt.

Now, as we have in all cases given the characters by which acids and radicals, metals and bases, may be recognised, the chemical characters of any salt may be known by referring to those of its elements. We may merely state here that the most valuable and important salts, most of which have been noticed incidentally under the head of their acids or their bases, are the sulphates, especially those of lime, magnesia, baryta, soda, potash, alumina and potash, iron, zinc, copper, and ammonia; the nitrates of potash, soda, strontia, lead, mercury, and silver; the carbonates of lime, magnesia, potash, soda, ammonia, iron, zinc, lead, &c.; the phosphates of soda, potash, lime, of magnesia and ammonia; the chlorate of potash, the chromates of potash and of lead; and many salts of organic acids not yet described, as oxalates of lime and of ammonia, acetates of potash, soda, alumina, iron, zinc, and lead; and tartrates of potash, potash and soda, and potash and antimony. The haloid salts, such as chloride of sodium, iodide of potassium, fluoride of calcium, &c., have been described in their proper place.

ORGANIC CHEMISTRY.

We must now turn to the chemistry of organized bodies, animal and vegetable, and of the unorganized or structureless products of animal or vegetable life, both of which classes of compounds are called organic.

As formerly explained, the elements of organic bodies are the same as those which constitute the inorganic world, save that the relative proportions are different, and that few comparatively of the elements can enter into the composition of organic compounds. The chief mass of such compounds is formed of only four elements, carbon, hydrogen, nitrogen, and oxygen; frequently of carbon, hydrogen, and oxygen alone; sometimes of carbon and hydrogen only. In every case of an organized structure, however, or of any substance capable of being formed into such a structure or tissue, there are not only the four elements just mentioned, but also sulphur, and several mineral salts in small proportion, but equally essential with the rest. These salts are phosphates, chlorides, alkalies, probably introduced as carbonates, oxide of iron, and frequently iodides and fluorides. Where these saline matters are absent—that is, where the substance, on being burned, leaves no ashes—it is invariably destitute of organized structure, and frequently either crystallized or liquid, both these forms being incompatible with organization.

Organic substances then are characterized by the small number of their elements, and, since they are themselves exceedingly numerous, by the great variety of proportions in which the same elements are united. By far the greater number of organic compounds are only products of organic life, and not themselves organized; and these consist either of the four elements which may be called organic, carbon, hydrogen, nitrogen, and oxygen, or of carbon, hydrogen, and oxygen, without mineral matter or ashes. Such compounds are the vegetable acids, sugar, gum, oils and fats, the vegetable bases, colouring matters, resins, and the like, in plants, and in animals such compounds as fats and oils, bile, urea, animal acids, and bases, &c. To these must be

added the innumerable compounds derived from them by various chemical processes. The few organized tissues, and the substances of which these may be formed, are such bodies as bone, muscle, nerve, membranes, woody tissue, albumen, fibrin, casein, gelatine, chondrine, and the like.

Now, all such organic substances, organized or not, agree in containing a large amount of carbon; and as this is combined with the elements we have named, oxygen, hydrogen, nitrogen, and in some cases sulphur, the action of heat on them, with or without the access of air, is peculiarly characteristic, and enables us at once to recognise organic compounds. Heated in close vessels, they blacken, give off water, oily matter, and combustible gases, and leave behind a black mass, composed of carbon and ashes, if there be ashes present. Heated in the open air, they are oxidized, the whole of their hydrogen being given off as water, their carbon as carbonic acid, their nitrogen as ammonia or as nitrogen gas, while the ashes alone remain. In the first case, which is called the destructive distillation of organic matter, the oxygen present is chiefly taken up by hydrogen to form water, while the carbon enters into new combinations with hydrogen and oxygen, or with hydrogen alone, or with nitrogen; and as all the compounds thus formed are gaseous, the carbon is carried off, as far as the quantity of the other elements admits of, but a large part is always left as charcoal, for want of other elements to combine with it. All organic bodies, therefore, are charred by heating when air is excluded, as in a retort or in the bottom of a tube, and burn when heated with a full supply of air. These characters depend on the nature of the elements they contain, and above all, on the presence of so large a proportion of carbon.

Organic bodies differ from the inorganic in their comparatively complex formulae. Thus, in inorganic chemistry we constantly meet with such formulae as those of water, $\backslash text\{HO\}$, hydrochloric acid $\backslash text\{HCl\}$, potash $\backslash text\{KO\}$, sulphuric acid $\backslash text\{SO\}\_3$ or $\backslash text\{HO\}\_2\backslash text\{SO\}\_3$, and the like. But the simplest organic

Chemistry. compounds are more complex than this. Formic acid is $C\_1H\_2O\_2$, oxalic acid $C\_2H\_2O\_4$, acetic acid $C\_2H\_4O\_2$, and these are by far the simplest organic compounds. Benzoic acid is $C\_7H\_6O\_2$, urea is $C\_2H\_4N\_2O\_2$, sugar is $C\_\{12\}H\_\{22\}O\_\{11\}$, quinine is $C\_\{20\}H\_\{24\}NO\_4$; and while many organic compounds are equally or more complex, some of them, especially those which are capable of forming blood and tissues, contain hundreds of atoms in a single molecule. This peculiar complexity of constitution, while but a small number of elements are employed, is a very marked character of organic compounds, and one consequence of it is, that they are much more easily decomposed by heat or by chemical means than inorganic substances, while the products of their decomposition are singularly varied. In fact, when so many atoms are present, and the elements are capable of uniting two and two or three together, in an endless variety of proportions less complex than the compound acted on, the results may be modified almost ad infinitum by altering the circumstances, such as temperature, or the reagents employed, which may be oxygen, or different oxidizing agencies of various energy, or acids, or bases, or chlorine, or any two or more combined, or any of the various modes of deoxidation, or of removing hydrogen, and so on; when such are the conditions, it is easy to see that the results must be infinitely more varied than they can possibly be in the case of inorganic compounds, the composition of which is in general so little complicated. This capability of undergoing numerous decompositions or transformations under the influence of the known chemical agencies is another characteristic feature of organic compounds; and there are causes of change which are almost peculiar to organic substances, such as the action of what are called ferments, and the power we have of replacing one or more atoms of one of the elements of such compounds, generally the hydrogen, by its equivalent of other elements, or even of compound radicals, or groups acting the part of elements. We may say, then, that organic compounds are complex in their constitution, and in consequence peculiarly liable to transformations and decompositions of various kinds—a character which renders them eminently fitted for the functions they have to perform in plants and animals. And while this is true of organic compounds generally, it is especially true, as was formerly hinted, of those which contain nitrogen, from the fact that that element has nearly equal affinities for the three elements which accompany it, and a great tendency also to combine with two or with all three of them in various proportions. Those organic bodies, and they are numerous, which undergo spontaneous transformations, are always compounds containing nitrogen.

Another peculiarity of organic compounds is this, that all their chief elements are either gaseous in themselves, or have a strong tendency to form gaseous or volatile compounds; so that the ultimate result of the transformations of the most complex of them, aided by the atmosphere, is their entire conversion (the ashes alone excepted) into the least complex gaseous forms; and they are sent into the air in those forms, which are, carbonic acid, $CO\_2$; water, $H\_2O$; ammonia, $NH\_3$; hydrosulphuric acid gas, $HS$; and sulphuric acid, $HO,\; SO\_2$. The decay of a dead animal or of a dead tree produces these very substances; and the whole mass of both, excepting the ashes, is dissipated in these forms, chiefly by the action of the atmospheric oxygen, aided by their own tendency to change. In this way, not only is the accumulation of dead organic matter prevented, but its disappearance is, as we shall see, the regular and unfulfilling source of food to the new generation of plants, and to the animals which feed on them. There are no elements known to us which are capable of undergoing these successive changes, except those which we find in organic matter.

Such are the characters which distinguish organic from inorganic compounds; and it will be seen that, while in

both classes the same laws of combination prevail, the peculiarities of the former depend on the peculiar nature of their principal elements.

The number of natural organic compounds is so great, and that of the compounds derived from them artificially is so much greater, that we cannot pretend to describe them all. We can only state such general laws as are deducible from what is known, and indicate the groups or series into which organic substances naturally fall. We shall find that they have a peculiar tendency to form groups of analogous constitution and properties, and that these groups exhibit a repetition, on a larger scale, of what we have already seen among elementary bodies. There are, no doubt, many organic compounds which as yet appear to stand alone; and this renders a satisfactory classification for the present unattainable. But this is entirely due to the imperfection of our knowledge, and the rapid daily progress of organic chemistry is constantly opening up new views, which enable us to include in the known series more and more compounds every day. This progress also demonstrates that the natural groups of organic compounds are formed on the same principles as those of inorganic substances; that the types, so to speak, are the same, while the differences do not affect essential or fundamental points, and the general analogy is unmistakable.

COMPOUND ORGANIC RADICALS.

Organic chemistry has been defined as the chemistry of compound radicals, as opposed to inorganic chemistry—that of elementary or simple radicals. But since we have come to admit in the latter such compound radicals as $SO\_2$ in sulphuric acid and sulphates, $NO\_2$ in nitric acid and nitrates, and many others, this definition cannot be maintained. We might say, indeed, that organic chemistry was that of such compound radicals as contain carbon. It is probable that all organic compounds contain at least one such radical (except in the case of ammonia and its derivatives, amide and ammonium, if considered organic), and that this is the cause of their complexity.

A compound organic radical is a group which plays the part of an element, and enters as a whole into combination with either elements or other compound radicals. Only a few of the organic radicals admitted by chemists are known in the separate state, but some are known with certainty; and where this is not the case, yet the assumption of the existence of such a radical often simplifies, in an extraordinary degree, the understanding of a whole series of connected substances, and reduces them to a form as easily remembered as in the case of a metal or a negative radical, such as chlorine.

Among organic radicals we find the same diversity of character as among elementary bodies. Some are negative, resembling chlorine, &c., in their relations; others positive, resembling hydrogen or metals; some, again, are more analogous to combustible bodies like carbon and sulphur; and some cannot be referred exactly to any of these categories, but partake of more than one. These differences of negative and positive, however, are, as the reader knows, differences of degree; so that all radicals, compound as well as simple, may be arranged in a series, in which each is positive to the next on one side, and negative to the next on the other. Among the elements this is well known; of the organic radicals, too few are yet known to enable us to establish such a series, save in a very fragmentary manner.

Of strongly negative organic radicals, analogous to chlorine, cyanogen is the type, and the analogy is here truly surprising. Thus we have—

Chemistry. There are other negative radicals, chiefly derived from cyanogen, such as ferrocyanogen, $C\_2N\_2Fe\; =\; Cy\_2Fe\; =\; Cy$, sulphocyanogen, $C\_2N\_2S\_2\; =\; Cy\_2S\_2\; =\; Cy$, and various others, which exhibit a general resemblance to chlorine in their relations, forming acids with hydrogen and salts with metals.

Of such positive radicals as are analogous to hydrogen and metals, and can, indeed, replace them, the simplest and the type is methyle, $C\_2H\_2\; =\; Me$. Let us compare it with potassium. We have—

This list might be extended to a great length, and the analogy would hold good. It is easy to see how much the assumption of the radical methyle simplifies its compounds, the formulae of which, written at full length, no one could be expected to remember, many of them being very like each other. Thus the carbonate is $C\_3H\_3O\_3$; the cyanide, $C\_4H\_3N$; the hydrated oxide, $C\_2H\_4O\_2$; and the acetate, $C\_4H\_6O\_2$.

There are very many radicals of this class, and all more complex than methyle, but all equally simple when considered as radicals, each having its own symbol. Thus, we have ethyle, $C\_4H\_5\; =\; Ae$; amyle, $C\_5H\_\{11\}\; =\; Ayl$; cetyle, $C\_6H\_\{13\}\; =\; Ct$; phenyle, $C\_6H\_5\; =\; Ph$; and many others.

Of the radicals analogous to sulphur or carbon, benzoyle is a type. Its formula is $C\_7H\_5O\_2$; its symbol Bz; and we have it, as well as sulphur, in combination with—

The analogy is not so close here as in the other cases, but the principle is the same. There are a good many radicals of this type, all forming with hydrogen volatile oils, with oxygen volatile acids. Acetyle, $C\_2H\_3$, or rather formyle, $C\_2H$, is the type of another allied group of radicals, which form with hydrogen volatile oily products, and with oxygen volatile acids of an oily character.

Such are the general characters of some of the best known groups of organic radicals, and it is precisely the compounds connected with these which are best known, and the study of which is most simplified by the admission of these radicals.

ISOMERISM IN ORGANIC COMPOUNDS.

Another remarkable feature in organic compounds is the frequent occurrence of isomerism or polymerism. This is the natural result of the large number of atoms generally present, which of course admits of a great variety in their arrangement, giving rise to isomeric compounds; while the absolute number also varies, being double, or treble, or manifold in one compound of what it is in another, thus producing polymeric compounds. It is obvious that in either case the transformation of one compound into the other must be comparatively easy, since nothing is necessary to be added nor taken away.

As examples of isomerism, that is, of bodies with the same proportion of elements and the same absolute quantity, which makes the atomic weight or equivalent the same, we may mention cyanate of ammonia (of oxide of ammonium), and urea. The former is $AmO,\; CyO\; =\; NH\_4O,\; C\_2NO$. In the latter the rational formula is not known with certainty, but the empirical formula is $C\_2H\_4N\_2O\_2$, which, it will be seen, is the same, absolutely and relatively, as the other. Again, acetate of oxide of ethyle is $AeO,\; AeO\_2\; =\; C\_2H\_3O,\; C\_2H\_3O\_2\; =\; C\_4H\_6O\_2$, while butyric acid is $C\_3H\_7O\_2,\; HO\; =\; C\_4H\_8O\_2$.

These two substances are totally different, and we are able to express the difference in the rational formulae, which shows the first to be a neutral salt of oxide of ethyle, the second a hydrated acid. A still more striking case of isomerism is that of the following substances, in several of which the rational formula is unknown, but all of which have the same atomic weight, and entirely different properties:—

Here we have at all events three totally distinct rational formulae, and in regard to the two first substances it is not that they have the same rational formula, but that we do not yet know the difference, which, as both seem to be hydrated bases but of distinct properties, must of course be in the arrangement of the anhydrous base $C\_4H\_7NO\_2$. There must be a difference, and we might even from analogy conjecture several modes of explaining the difference of properties, but nothing is certainly known of it. It is quite probable that several more substances having the same composition and atomic weight may still be added to this list. These examples of strict isomerism will suffice. As to polymerism, we have an example of it, connected with the four substances just mentioned; for lactamide, a body quite different from all four, has the empirical formula $C\_\{12\}H\_\{17\}N\_2O\_2$, exactly double of theirs. Its rational formula is probably $C\_\{12\}H\_\{17\}O\_2,\; 2NH\_2$, which represents the neutral amide of lactic acid, a dibasic acid. We have also an example of polymerism, connected with acetic ether and butyric acid, the empirical formula of which is $C\_4H\_8O\_2$. That of aldehyde is $C\_4H\_8O\_2$ or exactly one-half, while its rational formula is $C\_4H\_8O,\; HO$, that is, hydrated protoxide of acetyle. Besides this, we have metaldehyde and elaldehyde, polymeric with aldehyde, the precise equivalent of which is doubtful. There are also the three acids, formed of cyanogen and oxygen, namely:—

Many other examples might be given, both of isomerism and of polymerism; but those already given are sufficient to illustrate the principle. There is, indeed, a remarkable class of compounds, namely, the volatile bases, homologous with ammonia, and in which the hydrogen of that substance is partially or entirely replaced by radicals of the ethyle series, which exhibit isomerism carried to an extraordinary point, as will be more particularly explained under the head of these bases. Suffice it here to say, that we may easily form two volatile bases similar in properties, but quite distinct, of the formula $C\_4H\_5N$; three such bases of the formula $C\_6H\_7N$; four of the formula $C\_8H\_9N$; six of $C\_\{10\}H\_\{11\}N$; and so on, the number regularly increasing with the amount of carbon, so that with 20 eqs. of carbon we may have 16 volatile bases, all of the same empirical formula $C\_\{20\}H\_\{21\}N$, and yet all distinct, and each having its own well ascertained rational formula. Nor does this stop here, for with every 2 eqs. of carbon by which the formula is augmented, the number of possible isomeric (not polymeric) volatile bases is increased by two. This will give the reader some notion of the extraordinary fertility of organic chemistry, and of the important part performed by isomerism in the transformation of organic compounds. It is but the other day that these volatile bases were discovered, and already some dozens of them are known.

The examples of isomeric and polymeric transformation are very numerous, and there can be no doubt that this is a principle constantly in action in living organisms, animal or vegetable. The artificial formation of urea is a striking instance. The cyanate of oxide of ammonium, which we

Chemistry. have just seen to be isomeric with urea, spontaneously changes into that substance when its solution is left to itself, and rapidly if warmed. Urea, when heated, is transformed or resolved into ammonia, and hydrated cyanuric acid, polymeric with cyanic acid. That is to say, 3 eqs. of urea yield three of ammonia and 1 of cyanuric acid, $3(C\_2H\_4N\_2O\_2)\; =\; 3NH\_3\; +\; Cy\_3O\_3$, 3 HO. Again, this very cyanuric acid, when heated, is resolved into 3 eqs. of cyanic acid, $Cy\_3O\_3$, 3 HO = 3 (CyO, HO). Cyanic acid, left to itself, is transformed into cyanmelide, a body polymeric with it, possibly having twice its equivalent, $2(CyO,\; HO)\; =\; C\_4H\_2N\_2O\_4$. Cyanmelide, when heated, is resolved, like cyanuric acid, into cyanic acid again, $C\_4H\_2N\_2O\_4\; =\; 2(CyO,\; HO)$. If we now add ammonia, we obtain cyanate of oxide of ammonium, and this can be made again to pass through the whole series of metamorphoses just mentioned. Hydrobenzamide, by boiling with potash, is transformed into the base amarine, isomeric or perhaps polymeric with it. Furfuramide, in the same way, is transformed into the base furfurine, which has exactly double its equivalent. Every case of fermentation, strictly so called, is one of isomeric or polymeric transmutation; not, however, generally into one, but into two or more compounds. Dry grape sugar $C\_\{12\}H\_\{22\}O\_\{11\}$ yields, in the vinous fermentation 2 eqs. of alcohol and 4 of carbonic acid, $C\_\{12\}H\_\{22\}O\_\{11\}\; =\; 2(C\_2H\_5O)\; +\; 4CO\_2$. The same substance, in the lactic fermentation, yields the isomeric lactic acid, $C\_4H\_8O\_3$, 2 HO. Lactic acid, in the butyric fermentation, is resolved into butyric acid, carbonic acid, and hydrogen, $C\_4H\_8O\_3$, 2 HO = $C\_4H\_8O\_3$, HO + 4 CO₂ + H₂. It is easy to see how very important a principle this is, in reference to the vital changes which constitute organic life, animal or vegetable. It is in some such way that the food of plants and animals is assimilated, and becomes converted into those substances of which the animal and vegetable frames are composed. It is more particularly in the animal body, however, that isomeric transformations prevail, because the food of animals is complex, and therefore liable to transformation, and adapted to undergo it. We can even, in some measure, imitate artificially the change by which some parts of the food of animals are dissolved into blood. For we can so treat fibrin as to obtain from it a solution of albumen, much resembling the serum of blood; and albumen is either isomeric with fibrin or very nearly so.

We have seen that great advantage is gained by admitting the existence of organic compound radicals. There is, however, another kind of organic group, which has not the properties of radicals. These are named types, the meaning of which is, that the atoms are grouped together in a certain mode, on which the properties of the compound so entirely depend, that, provided this grouping or arrangement be retained, great changes may be made in regard to the individual elements, without changing the general character of the compound. This leads us to the very remarkable and important law of substitution, which has become so fertile in discoveries of late years. Some writers have imagined that the notion of types was incompatible with that of radicals, but this is not the case. Every radical, nay, every compound of a radical, may also be a type, and we shall find that both ideas are useful in enabling us to classify our knowledge.

The fundamental idea of a type may be found in inorganic chemistry. Hydrochloric acid, HCl, is a type, and so is common salt, NaCl. All compounds, which, like hydrochloric acid, consist of 1 eq. hydrogen and 1 eq. of a powerful negative radical, are acids. All those which consist of 1 eq. of metal and 1 eq. of negative radical, are salts. We can substitute for the chlorine in either, bromine, iodine, cyanogen, or sulphur, and the compound will still have the original character of an acid or a salt. And in the case of salt,

we may substitute for the sodium, potassium, ammonium, barium, iron, lead, silver, &c., and still the result is a salt. In fact, every compound is, or represents, a type, and every chemical change is a substitution. But in inorganic chemistry, the only substitutions which do not destroy the type are those of like for like, of chlorine for iodine, &c., or of one metal for another. It cannot well be otherwise, from the simplicity of inorganic formulae.

But in organic chemistry, where the formulae, and consequently the molecules, are so much more complex, the type acquires a new significance. The elements of all organic compounds being the same, and often in the same amount and proportion, the arrangement or relative position of the atoms comes to be a matter of the utmost importance, as fixing the character of the group, that is, of the type. A certain mode of arrangement gives acid properties; another, the properties of a base; a third, those of an oil; a fourth, those of an ether, and so on. And so powerful is the influence of arrangement in these complex molecules, that substitution is no longer confined to replacing like by like, but extends to the replacement of an element by its equivalent of another, not only not analogous to it, but actually opposite to it.

Naphthaline is $C\_\{10\}H\_8$. The character of the type is, that it is volatile and combustible. Now, we may remove 1 eq. of hydrogen, and replace it, not by anything analogous to hydrogen, but by 1 eq. of chlorine, forming the body $C\_\{10\}H\_7Cl$; chloronaphthase, which has the same general characters. We may go on replacing the hydrogen, atom by atom, by chlorine, and yet the type will remain unaltered. That is, the properties of naphthaline depend chiefly on the way in which the atoms are grouped, and, provided the grouping be unchanged, it does not appear to signify much whether the 8 atoms attached to the 20 of carbon be all hydrogen, or partly hydrogen and partly chlorine. In like manner, bromine may be substituted for the hydrogen, and yet we know that, in general, chlorine and bromine are strongly opposed to hydrogen, being negative, while it is positive. Here, then, we have negative atoms playing the part of positive ones, and this is the peculiar feature of that kind of substitution in organic compounds which we are now considering. And since the negative character of such bodies as chlorine and bromine is thus, as it were, sunk, nay, changed almost to its opposite, it is evident that in naphthaline the nature of the compound molecule must depend far more on the arrangement or grouping, than on the original nature of the atoms it contains. Even the last atom of hydrogen in naphthaline may be thus replaced by chlorine, yielding the body $C\_\{10\}H\_7Cl$, which has still the general characters of the type. The peculiarity of this form of substitution (for we must remember that almost all chemical changes consist in substitution of one element for another, as when the oxide of a metal, MO, is converted into the chloride, MCl, or the sulphuret, MS, or the cyanide, MCy, or the sulphate, $M\_2SO\_4$, &c. &c., where chlorine, sulphur, cyanogen, and $SO\_4$, are successively substituted for oxygen) is still better seen in the action of nitric acid or naphthaline. The first effect is that 1 eq. of hydrogen is oxidized into water; but the oxygen being taken from nitric acid, $NO\_3$, leaves nitrous acid, $NO\_2$, and this complex atom at once takes the place of the hydrogen removed, forming the compound $C\_\{10\}H\_7NO\_2$, which has still the typical characters, although the place of 1 eq. of hydrogen is now occupied by a compound, containing 4 eqs. of oxygen; the body of all others most opposite to hydrogen. And this is so far from being a solitary case, that a large proportion of organic compounds, whether acid, basic, or neutral, form analogous compounds, in which hydrogen is replaced by the same number of atoms of nitrous acid, of chlorine, and of bromine, the general character of the compound remaining the same.

Chemistry. Many examples might be given. Acetic acid, $C\_2H\_4O\_2$, $HO\_2C\_2H\_3$, acted on by chlorine, yields chloracetic acid $C\_2H\_3ClO\_2$, $HO\_2C\_2H\_2Cl$. Aniline, a base, $C\_6H\_5NH\_2$, acted on by bromine, yields the base bromaniline, $C\_6H\_4BrNH\_2$. The neutral oil, benzole, $C\_6H\_6$, yields, with nitric acid, the oil nitrobenzole, $C\_6H\_5NO\_2$.

It is easy to see that in this way a very large number of new compounds may be obtained, and, in fact, such are every day discovered, the experimenter being guided by the laws we have explained. It must be noticed, however, that although the type, generally speaking, is not altered by such substitutions, as we have just seen that the substitution-products from an acid, a base, and a neutral oil, are acid, basic, and oily and neutral, yet the negative energy of the elements substituted for hydrogen is not altogether lost or sunk. For, although we have from aniline, a base, basic substitution products, such as chloraniline, bromaniline, and nitraniline, in all of which 1 eq. of hydrogen is replaced successively by 1 eq. of chlorine, bromine, and nitrous acid, yet when 2 eqs. of hydrogen are thus replaced, the basic properties are much weakened, and when 3 eqs. have been replaced, as in trichloraniline and tribromaniline, these properties are gone, and the compound is neutral. But the very fact that, in this form, 3 eqs. of chlorine, one of which neutralizes the most basic metals, are just sufficient to destroy the basic character of 1 eq. of a feeble base, proves how much the negative character of chlorine, bromine, or nitrous acid is affected by the peculiar position they are made to occupy in the compound molecule, which only allows their negative character to appear to a small extent, when their quantity reaches a certain point.

Such are the general facts concerning types, and the peculiar form of substitution we have endeavoured to explain, by which the type is not materially changed, although hydrogen be replaced by its opposites. These facts are so far from being inconsistent with the existence of compound radicals, that these radicals are themselves, as types, equally capable of this form of substitution with any other compounds. Indeed, in the case of aniline, which is itself a substitution product of another kind, where hydrogen is replaced, not by its opposites, but by its homologues, as we shall presently explain, it is the radical phenyle, $C\_6H\_5$, contained in it, that undergoes the substitution of chlorine, bromine, and nitrous acid for hydrogen.

This leads us to consider that other form of substitution, also peculiar to organic compounds, to which we have just alluded, and to its results, namely, the formation of what are called homologous series, involving principles which, in reference to the classification and understanding of organic compounds, and to their artificial formation, are of far greater practical value than any we have yet expounded. Indeed, we can now see that the progress of science must inevitably reduce the whole of organic chemistry, in which, we must remember, only the same three or four elements are perpetually met with, to a collection of homologous series, in which every compound will have its natural place, indicative at once of its origin, its immediate derivation, and its properties both physical and chemical. This is so much the case, that the student, if he have a clear conception of the nature and relations of the series we call homologous, will have a far better idea of organic chemistry than he could have without this, even if we had space to describe the innumerable organic compounds, which, without this guiding principle, would form, as they have long done, a perfect chaos of isolated facts, which no memory could retain, and to which it would be impossible to give a rational or connected form. For this reason, we shall give a full explanation of this part of the subject, referring the reader to larger works for the voluminous details concerning individual compounds which are altogether inconsistent with our space.

Chemistry. Although we cannot yet include nearly all organic compounds in the series we are about to describe, yet a large number of the more important may be thus included, and thus a catalogue of the known homologous series will form the skeleton, as it were, or groundwork of an arrangement, in which all the parts may be seen to be mutually related. Those substances which do not themselves naturally fall into any of the series will be mentioned as groups, according to their natural affinities, and it will be found that they are in most cases related to the series of which we speak, through some of the products of their decomposition or transformation.

Lastly, after giving a brief account of those substances which cannot yet be included in the homologous series, we shall explain the changes which occur in living organisms, so far as these are known.

1. These series have gradually, and of late rapidly, developed themselves from the researches of modern chemists. They arise from a kind of substitution, the commonest of all in organic chemistry, in which hydrogen is replaced by certain compound radicals, which, being themselves homologous, of course give rise to homologous series, when substituted for hydrogen in various compounds. The origin of these homologous series of radicals themselves is more obscure; but, whatever that origin may be, these radicals are so related to each other as to constitute what may be called the chief or fundamental homologous series. Of these, more than one are known, but we shall select the best known and most important, which is that of the methyl or ethylic radicals, so called because methyl and ethyl are the two first radicals of the series. Let us consider these two. Methyl is $C\_1H\_3$; ethyl is $C\_2H\_5$; and they are, in all respects, closely analogous to one another, and, as has been already stated, to hydrogen and metals. Now what is the difference between them? $C\_1H\_3\; -\; C\_2H\_5\; =\; C\_2H\_2$. Consequently the difference is $C\_2H\_2$ or 2 C H. So that, by adding $C\_2H\_2$ to the formula of methyl, we obtain that of ethyl. To ascertain the true starting point, let us, after subtracting $C\_2H\_2$ from ethyl, which leaves $C\_2H\_3$ or methyl, subtract the same amount, $C\_2H\_2$, from methyl itself; $C\_2H\_3\; -\; C\_2H\_2\; =\; H$. Therefore hydrogen is the origin or point of departure of this series of radicals, and we have already seen that they are analogous to hydrogen. On the other hand, let us add to the formula of ethyl, once more $C\_2H\_2$, and we have $C\_4H\_7\; +\; C\_2H\_2\; =\; C\_6H\_9$, which is the formula of propyl. Another addition of $C\_2H\_2$ to propyl gives $C\_8H\_\{11\}$, which is butyl, and again the addition of $C\_2H\_2$ to butyl gives $C\_\{10\}H\_\{13\}$ = amyl, and so on. Such is the nature of this first or fundamental homologous series, and all other homologous series yet known are formed on the same principle of the addition of $C\_2H\_2$, neither less nor more, at each step, the starting point alone being different. There is every reason to believe that this first series extends from methyl (or from hydrogen), $C\_1H\_3$ to melissyl $C\_\{30\}H\_\{61\}$, although only a few of the radicals, members of this series, are yet known in a separate form.

The first thing to be noticed is, that all the members of the series, being derived from H by the addition of $C\_2H\_2$ in successive steps, contain, and must contain, invariably 1 eq. of hydrogen more than of carbon. Secondly, for the same reason, the number of equivalents of carbon is in all an even number, or divisible by 2, while that of the equivalents of hydrogen is odd. These facts are expressed by giving to the series the general formula, $C\_nH\_\{n+1\}$, in which $n$ signifies 2, or a multiple of 2 by a whole number. The general formula may also be written $C\_\{2n\}H\_\{(2n)+1\}$. But the former is the simpler mode. The formula $C\_nH\_\{n+1\}$ includes all the radicals of this, the methyl or ethylic series. In this, as in all similar homologous series, the compounds

Chemistry. lowest in the scale, that is, with least carbon and hydrogen, are the least dense, the most volatile, and have the strongest affinities; and these properties vary in proportion to the amount of $C\_2H\_2$ added, according to a regular law. The density of the vapour or gas increases, and the boiling point rises with perfect regularity, as we rise on the scale. Methylene and ethylene are gases like hydrogen, at ordinary temperatures; but while methylene requires 20 atmospheres to liquefy it, ethylene is condensed by 2 atmospheres, and the condensed liquid boils under the ordinary pressure at $23^\backslash circ$ or $9^\backslash circ$ below the freezing point. Propylene and butylene are oily liquids, the latter boiling at $226^\backslash circ$, and amylene is an oily liquid boiling at $311^\backslash circ$. Higher in the scale, the radicals are solid, but fusible and volatile; and the higher we go, the higher are the melting and boiling points. This at least is found to be the case in all other homologous series, and is no doubt true in this, although as yet all the radicals high in the scale are not known in the separate state.

2. Such is the first or fundamental homologous series of radicals, homologous with, and analogous to, hydrogen. Now these radicals, in the second and more common form of substitution seen in organic compounds, are substituted for hydrogen, atom for atom, in various compounds, and thus give origin to as many homologous series as there are different compounds in which hydrogen can be replaced by these radicals. Hydrogen, $H$, is the starting point of the radicals; and water, $H\_2O$, is the starting point of their protoxides, so that we have—

and so on, the two series running absolutely parallel. The five protoxides just named are all known, as well as a good many higher in the scale. They are in fact what are called the ethers, oxide of ethylene being common ether. Oxide of methylene or methylic ether is a gas at ordinary temperatures, and the others named are liquids less volatile than ether, while higher in the scale they are solids, as oxide of cetylene, $C\_\{22\}H\_\{33\}O$, which is a crystalline solid, like fat or wax. The general formula of this series is $C\_nH\_\{2n+1\}O$.

3. The third homologous series is that of the hydrated protoxides of the radicals, or the alcohols; of which the starting point may be said to be 2 eqs. of water $H\_2O$, $H\_2O$, which we may suppose to exist, since water can play the part both of acid and base, both negative and positive, and may possibly form double molecules of hydrate of water. At all events we have only to add $C\_2H\_2$ to the first equivalent of water, in successive quantities, to obtain the series of the alcohols, which are—that is, the first 5 of them—

The whole of these alcohols are formed in the peculiar fermentation of sugar, called the vinous or alcoholic fermentation, and all but the first can only be produced in this way, so far as is yet known. When sugar dissolved is placed in contact with yeast, fermentation ensues, and at a certain temperature, if the sugar be pure, only ethylic alcohol and carbonic acid are produced. Dry grape sugar is the substance which ferments. Its formula is $C\_\{12\}H\_\{22\}O\_\{11\}$, and in fermentation, $C\_\{12\}H\_\{22\}O\_\{11\}$ yield 4 $CO\_2$ and 2 $(C\_2H\_2O)$. But when impurities are present, other analogous fermentations, though to a less extent, accompany this one; and in the fermentation of the juice of the grape, but especially of the expressed residue or mark of the grape, and also in that of infusion of grain or of malt, the four other alcohols here named,

and one or two others next above them in the scale, are formed. We do not know exactly what are the products of the fermentations which produce them, but they must differ from those of the vinous fermentation. Thus, 3 eqs. of sugar $C\_\{12\}H\_\{22\}O\_\{11\}$ may yield 4 eqs. of propylic alcohol $4(C\_2H\_2O)$, 4 eqs. of water, 4 $H\_2O$, and 12 of carbonic acid, 12 $CO\_2$. Again, 5 eqs. of sugar may yield 4 of amylic alcohol, 12 of water, and 20 of carbonic acid. But the changes may be more complex. All that we know is, that in some circumstances not yet fully investigated, 5 or 6 different alcohols are formed from sugar. The amylic alcohol was discovered in the alcohol from potatoes and grain a good many years ago, and since then in that from the grape. In the latter, the methylic, propylic, butylic, and caproic alcohols have very recently been detected. Methylic alcohol is obtained, however, chiefly from the distillation of wood, and is often called pyroxic spirit.

The propylic and butylic alcohols are perfectly analogous to the rest, and intermediate in properties between common alcohol and amylic alcohol, or oil of potato spirit, which has long been known. This series exemplifies admirably the nature of a homologous series. The physical properties of the alcohols are graduated in exact proportion to the increased amount of carbon and hydrogen. Thus, the boiling point rises about $34^\backslash circ$ for each step or addition of $C\_2H\_2$; so that knowing the boiling point of one alcohol, we can calculate that of all the rest. This was done with respect to the two newly discovered alcohols before they were obtained, and the result agreed perfectly with the calculation. A good many alcohols are known higher in the scale. Spermaceti yields one of them, cetyl alcohol $C\_\{22\}H\_\{33\}O$, $H\_2O$, and two others have been obtained from wax, namely cerotic or ceric alcohol $C\_\{28\}H\_\{35\}O$, $H\_2O$, and melissic alcohol $C\_\{23\}H\_\{27\}O$, $H\_2O$. These three are crystalline, fusible, volatile solids, but have a high boiling point. General formula $C\_nH\_\{2n+1\}O$, $H\_2O$.

It is unnecessary to repeat in each case the statement, which holds good in all, that the physical properties and chemical affinities, in a homologous series, vary with the amount of carbon and hydrogen; the density and boiling point rising and the affinity diminishing as the carbon and hydrogen increase.

4. The fourth series is that of the hydurets of the radicals of the first series, or positive radicals. The general formula is $C\_nH\_\{2n+1\}H$, $H=C\_nH\_\{2n+2\}$. The first is $C\_2H\_3$, $H$ or $C\_2H\_4$, hyduret of methylene. This is marsh gas, sometimes written $CH\_4$. But $CH\_4$ is polymeric only with the true hyduret. The next is hyduret of ethylene, $C\_4H\_5$, $H=C\_4H\_6$. This series consists of gases, liquids, and solids, and it is believed that some kinds of naphtha, and some kinds of paraffine, belong to it. Naphtha may be $C\_\{10\}H\_\{12\}$, $C\_\{16\}H\_\{14\}$, $C\_\{20\}H\_\{16\}$, and the like, and paraffine may be in some cases, $C\_\{24\}H\_\{48\}$ and $C\_\{30\}H\_\{60\}$. At all events wax yields paraffine, and the substances in wax contain, as we have seen, $C\_\{24\}$ and $C\_\{30\}$. Marsh gas, $C\_2H\_4$, or perhaps $C\_4H\_6$, or a mixture of both, is produced by the decay of dead vegetable matter at the bottom of stagnant water; also in coal mines. It is formed artificially in various processes.

5, 6, 7, 8. It will be seen that these four series are closely related together, the first containing the positive radicals, while the others contain compounds of those radicals. But these radicals form many compounds, and each compound of any one radical, as methylene or ethylene, indicates another homologous series. We have seen the oxide, the hydrated oxide, and the hyduret, each belonging to such a series. Now, the chloride of methylene is one of a fifth series, the general formula of which is $C\_nH\_\{2n+1\}Cl$. The iodide belongs to a sixth, of the general formula $C\_nH\_\{2n+1\}I$; the bromide to a seventh; formula $C\_nH\_\{2n+1\}Br$. All these, at least in the lower part of the scale, are volatile ethereal liquids, and become no doubt solid higher up. These series are exactly parallel to those already mentioned. An eighth series consists of the cyanides, which are remarkable as con-

Chemistry. taining two compound radicals, but otherwise are quite analogous to the rest. These cyanides are interesting from their being either identical or isomeric with the compounds of another series, the nitriles, connected with a second series of radicals, the negative radicals, to be presently mentioned. The cyanides are also interesting, since from them it is possible to reproduce the corresponding alcohols; and in this way we may be able to obtain certain alcohols which could not otherwise be procured.

9, 10. A ninth homologous series is that of the sulphurets of the positive radicals, which, in the lower part of the scale, are ethereal liquids, with an insupportable garlic odour; general formula $C\_n\; H\_\{n+1\}\; S$. A tenth series consists of compounds of the sulphurets with hydrosulphuric acid, of the general formula $C\_n\; H\_\{n+1\}\; S\; +\; HS$. Of these the types are methyl-mercaptan and ethyl-mercaptan, or simply mercaptan, so called from its strong action on oxide of mercury. These two compounds are $C\_1\; H\_3\; S$, $HS$ and $C\_2\; H\_5\; S$, $HS$, while the corresponding sulphurets are $C\_1\; H\_3\; S$ and $C\_2\; H\_5\; S$. The mercaptans have also a strong and offensive garlic odour. It will be seen that the sulphurets are the oxides or ethers in which sulphur has taken the place of oxygen, and the mercaptans are the hydrated oxides or alcohols, in which the same substitution has taken place; for we have—

The two series might be extended as far as any of the others.

11. We now come to another group of homologous series, still containing the same radicals, namely the salts of their oxides, or compounds of the oxides with oxygen acids, or, as they are called, the compound ethers. The ethers are capable of uniting with almost all acids, such as sulphuric, nitric, hyponitrous, carbonic, oxalic, acetic, benzoic, and in fact all organic acids. It is evident that the compounds of each acid with the ethers form a homologous series. We shall merely give as an example of the neutral compound ethers, an eleventh series, the carbonates of the oxides of the second series. Their general formula is $C\_n\; H\_\{n+1\}\; O,\; CO\_2\; =\; C\_\{n+1\}\; H\_\{n+1\}\; O\_3$. Those of methyle and ethyle are volatile ethereal liquids, as are indeed the whole class of compound ethers when low in the scale.

12, 13, 14. A twelfth series is that of the acid sulphates of the oxides of the second series. General formula $C\_n\; H\_\{n+1\}\; O,\; HO,\; 2\; SO\_3$. These are formed when the alcohols are acted on by excess of sulphuric acid. They are strongly acid, and, when neutralized by fixed bases, form what may be considered either as double salts, or salts of acids containing the ethers along with sulphuric acid. The two first of these acid sulphates are $C\_2\; H\_5\; O,\; HO,\; 2\; SO\_3$, called sulphomethyl-ic acid, or double sulphate of water and oxide of methyle; and $C\_4\; H\_9\; O,\; HO,\; 2\; SO\_3$, sulphoethyl-ic or sulphovinic acid, or double sulphate of water and oxide of ethyle. The rest of the series are perfectly analogous to these. When these two, or any of the others are neutralized with potash, which replaces the water, we obtain the sulphomethylate and sulphoethylate of potash, or double sulphates of potash and oxide of methyle or ethyle, which, in fact, belong to a thirteenth homologous series, since every base forms a new series with these acids. The two acids, with their potash and lime salts, may be thus compared:—

The lime salts, of course, form a fourteenth series, and here, as in the case of the neutral compound ethers, there is no limit to the number of series, but that of the acids and bases capable of uniting with the oxides of series 2, and the acids of series 12.

Chemistry. 15. The next, or fifteenth homologous series that we shall specify, is not only in itself a very remarkable one, but at the same time the source of an infinite number of additional series. It still contains the same positive radicals which have been present in all the series hitherto named. In this one they are combined with amide, $NH\_2$, and the result is a series of volatile bases formerly alluded to, homologous with ammonia, which is their starting point, as hydrogen is of the radicals; (for ammonia is amide + hydrogen). They are in the highest degree analogous to ammonia, which is the true type of the volatile organic bases. Their general formula is $C\_n\; H\_\{n+1\}\; NH\_2$, and therefore they contain no oxygen. The formula may also be written $C\_n\; H\_\{n+3\}\; N$, which makes it easy to remember, that the hydrogen in all of them exceeds the carbon by 3 eqs., and that there is 1 eq. of nitrogen, and no oxygen. The following are a few of those in the lower part of the scale. We have added a third column to show the analogy with ammonia more clearly, in which the radicals, as well as the amide, are represented by their abbreviated symbols:—

and so on throughout the whole series of radicals.

No series is more striking than this. Its discovery was predicted by Liebig exactly ten years before it was made, and the properties of the compounds belonging to it plainly indicated. The analogy to ammonia is so perfect, that the base nearest to ammonia, methylamine, can hardly be distinguished from it. It is, like ammonia, a gas, absorbed in large quantity by water, of a pungent smell, almost identical with that of ammonia; it forms white fumes with hydrochloric acid, and its salts exactly resemble those of ammonia. It occurs in various decompositions, and in putrefaction, along with ammonia, and it has been frequently taken for ammonia and described as such before its true nature was known. Ethylamine is only a degree less like ammonia, being at ordinary temperatures a very volatile liquid, the smell of which, while analogous to that of ammonia, yet differs from it. It also has been often overlooked from this resemblance. As in all other series, the compounds higher in the scale are less and less volatile and more oily, and at a certain point become solid and crystalline. Not only do these bases form as many new homologous series as there are acids to combine with them, but with them all the reactions of ammonia may be repeated, giving rise to new compounds. Thus, ammonia forms four or five new bases with platinum; so do these volatile bases, methylamine and ethylamine, so that each of these four or five platinum bases represents a new series, and each of their salts with acids represents another. Thus not only do the sulphates of methylamine, &c., form a series, but the sulphate of each of the four or five platinum bases of ammonia does so likewise. And so of all the other salts. The bases of the series are called amide bases.

16. The sixteenth is also one of volatile bases, but not containing amide. Ammonia is still the type, but in this case it is viewed as composed of $NH,\; H\_2$, or of imide $NH$, with 2 eqs. of hydrogen. The homologous bases of this series consist of imide with 2 eqs. of methyle, or of ethyle, &c.; or of imide with 1 eq. of methyle and 1 of ethyle, or 1 eq. of ethyle, and 1 of propyle, &c. &c. The following list contains a few of them:—

Chemistry. and so on. These bases are very much similar to the others. They are called imide bases, and their general formula is $\backslash text\{NH\}\_2(2(\backslash text\{C\}\_n\backslash text\{H\}\_\{n+1\}))$. It will be seen by the first column, that several of them are isomeric with amide bases, but the second and third columns, which give the rational formulae, show that they are distinct in constitution as they are in properties, although analogous. It must be observed that the general formula of this series $\backslash text\{NH\}\_2(2(\backslash text\{C\}\_n\backslash text\{H\}\_\{n+1\}))$, may also be written $(\backslash text\{C\}\_n\backslash text\{H\}\_\{n+1\})\backslash text\{N\}$, which is the same as that of the preceding series in its second form. The same general form also $(\backslash text\{C\}\_n\backslash text\{H\}\_\{n+1\})\backslash text\{N\}$, includes also the next series, although it also may be expressed so as to show the difference.

Even among the few here given, it will be seen that three are isomeric with bases in the last table, and two of these also with two bases in the first table, series 15. But the rational formulae in the last two columns show that they differ in constitution, as they do also in properties. The bases of this series are called nitrile bases.

The three series just mentioned, of all of which ammonia is the type, include all the known volatile organic bases, natural or artificial, that is, all such as are not decomposed but volatilized, unchanged by a due application of heat. An amide base is easily converted into an imide base by the action of the iodide of one of the radicals of series 1, and an imide base is converted by the same means into a nitrile base. And all of them may be derived from ammonia by the action, in successive steps, of such an iodide. Let us take iodide of ethyle, $\backslash text\{AeI\}$. The first action is $\backslash text\{NH\}\_3\; +\; \backslash text\{AeI\}\; =\; \backslash text\{NH\}\_2\backslash text\{Ae\},\; \backslash text\{HI\}$. That is, the iodine takes 1 eq. of hydrogen, forming hydriodic acid, while the ethyle replaces that eq. of hydrogen, forming the amide base, ethylamine, which combines with the hydriodic acid. The next step is the action of iodide of ethyle on ethylamine, $\backslash text\{NH\}\_2\backslash text\{Ae\}\; +\; \backslash text\{AeI\}\; =\; \backslash text\{NH\}\; +\; \backslash text\{Ae\}\_2,\; \backslash text\{HI\}$; the result is, hydriodate of diethylamine. And in the third stage, when iodide of ethyle acts on diethylamine, we obtain the hydriodate of triethylamine, $\backslash text\{NH\}\; +\; \backslash text\{Ae\}\_2\; +\; \backslash text\{AeI\}\; =\; \backslash text\{N\}\; +\; \backslash text\{Ae\}\_3,\; \backslash text\{HI\}$. Nothing can show more plainly that all these bases are simply ammonia, the hydrogen of which is replaced, in part or in whole, by positive radicals of series 1. It is also evident, that since there are only 3 eqs. of hydrogen in ammonia, we cannot push further the substitution of these radicals for hydrogen, after we have replaced the whole 3 equivalents. If we attempt to do so, we find that we obtain indeed new bases, but of a different type, being no longer volatile, but decomposed by heat, and containing oxygen. The explanation of this is very interesting. We cannot indeed replace more than three atoms of hydrogen in ammonia by our radicals, but we can add to the 3 eqs. of radical a fourth, and no more, or, in other words, we can replace the fourth eq. of hydrogen also in ammonia, which contains 4 eqs. of hydrogen. This leads to the formation of the next series.

18. The eighteenth series is a most remarkable and interesting one, of fixed bases, that is, bases which cannot be volatilized without decomposition, and which no longer are of the type ammonia, but, in composition, of the type of hydrated oxide of ammonium; and as that body is unknown in a separate form, we can compare them to nothing so well as to hydrated oxide of potassium, or caustic potash, to which they are in a most astonishing degree analogous. To illustrate their formation, let us suppose iodide of ethyle to act on triethylamine. Here there is no more hydrogen left to be replaced, for all 3 eqs. are already replaced, and all the hydrogen present is in the form of ethyle. The action is $\backslash text\{N\}\; +\; \backslash text\{Ae\}\_3\; +\; \backslash text\{AeI\}\; =\; \backslash text\{N\}\; +\; \backslash text\{Ae\}\_4,\; \backslash text\{I\}$. The ethyle of the iodide

17. The seventeenth series is again one of volatile bases, Chemistry. of the type of ammonia, and of the general formula $(\backslash text\{C\}\_n\backslash text\{H\}\_\{n+1\})\backslash text\{N\}$, agreeing in this with the two preceding series, as has just been stated. But the more precise formula here is that which is taken from ammonia, viewed, not as amide + 1 eq. hydrogen, nor as imide + 2 eqs. hydrogen, but as nitrogen + 3 eqs. hydrogen, $\backslash text\{N\}\; +\; \backslash text\{H\}\_3$. This being the type, in these bases the whole 3 eqs. of hydrogen are replaced by methyle, or by ethyle, &c., or partly by one and partly by another of the radicals of series 1. The following are some of them. Their general formula, strictly stated, is $\backslash text\{N\}\; +\; 3(\backslash text\{C\}\_n\backslash text\{H\}\_\{n+1\})$.

unites with triethylamine to form a compound metal, tetrathylium, $\backslash text\{N\}\; +\; \backslash text\{Ae\}\_4$, analogous to ammonium, $\backslash text\{NH\}\_4$, and this metal unites with the iodine, forming the iodide of tetrathylium, $\backslash text\{N\}\; +\; \backslash text\{Ae\}\_4,\; \backslash text\{I\}$, exactly similar to iodide of ammonium, $\backslash text\{NH\}\_4,\; \backslash text\{I\}$. To obtain the base, this iodide, which is a crystallizable salt, is acted on by oxide of silver and water. The action is, $\backslash text\{N\}\; +\; \backslash text\{Ae\}\_4,\; \backslash text\{I\}\; +\; \backslash text\{AgO\}\; +\; \backslash text\{HO\}\; =\; \backslash text\{AgI\}\; +\; \backslash text\{N\}\; +\; \backslash text\{Ae\}\_4,\; \backslash text\{O\},\; \backslash text\{HO\}$; and the results are, iodide of silver and hydrated oxide of tetrathylium. The latter is obtained as a crystalline mass by evaporation in vacuo.

In composition, it corresponds to the hydrated oxides of ammonium and potassium. We have—

($\backslash text\{Th\}\; =\; \backslash text\{N\}\; +\; \backslash text\{Ae\}\_4$, tetrathylium). Hydrated oxide of ammonium is not known, as, when we attempt to separate it from the salts of ammonium, it is instantly resolved into ammonia and water. $\backslash text\{NH\}\_4\backslash text\{O\},\; \backslash text\{HO\}\; =\; \backslash text\{NH\}\_3\; +\; 2\backslash text\{HO\}$. The reason of this would seem to be, that the attraction of the oxygen of the oxide for the fourth eq. of hydrogen is so strong as to detach it, that eq. being of course held by a feeble force than the others. If, however, this hydrated oxide could exist in a separate form—and possibly we may some day succeed in obtaining it, perhaps under the influence of intense cold—it would certainly resemble caustic potash, and the hydrated oxide of tetrathylium. The latter can exist in a separate form, just because the attraction of oxygen for the fourth eq. of ethyle is very much less than for hydrogen; that is, at ordinary temperatures; for at higher temperatures hydrated oxide of tetrathylium undergoes precisely the same change as that of ammonium at ordinary temperatures, being converted into ethylamine, corresponding to ammonia, and hydrated oxide of ethyle, corresponding to 2 eqs. of water. $\backslash text\{N\}\; +\; \backslash text\{Ae\}\_4,\; \backslash text\{O\},\; \backslash text\{HO\}\; =\; \backslash text\{N\}\; +\; \backslash text\{Ae\}\_3\; +\; \backslash text\{AeO\},\; \backslash text\{HO\}$. It is on this account that we conjecture that under intense cold the ammonium compound might be permanent. The action of heat on this hydrated oxide of tetrathylium is characteristic of this whole series of bases.

In properties, hydrated oxide of tetrathylium is so very like caustic potash, that were it not for the action of heat, it might, in solution, be confounded with it. It has a caustic alkaline taste, feels soapy to the fingers, as potash does, attracts water and carbonic acid from the atmosphere, precipitates the salts of metals as potash does, converts oils into soaps when boiled with them, and forms insoluble or sparingly soluble salts, with bichloride of platinum and other tests, exactly as potash does. The chief difference is, that besides the caustic alkaline taste, it has a bitter taste; and this is true of the whole class of bases to which it belongs. This and the action of heat distinguish them from potash and soda. But these same characters connect them with the fixed vegetable bases, such as quinine, morphine, strych-

Chemistry, nine, and the like; all of which are bitter, and all of which, when heated, yield a volatile nitrile base, as the base in question yields triethylamine. In short, the bases of this series are of the same type as the fixed vegetable bases, and this being so, it is easy to see that there is no improbability in our being able to form artificially the natural fixed organic bases. Before going farther, we shall here give the formulae of a few of the artificial bases of this series, the general formula of which is $N,\; 4(C\_n\; H\_\{2n+2\})O,\; HO$. They are called ammonium bases.

• Hydrated oxide of ammonium (type) $NH\_4O,\; HO\; =\; AmO,\; HO$
• Hydrated oxide of trimethylium $C\_3\; H\_\{13\}\; NO\_2\; =\; NMe\_4O,\; HO\; =\; TmeO,\; HO$
• ... tetrabutylum $C\_4\; H\_\{29\}\; NO\_2\; =\; NAt\_4O,\; HO\; =\; TthO,\; HO$
• ... tetramethylum $C\_6\; H\_\{41\}\; NO\_2\; =\; NAt\_4O,\; HO\; =\; AymO,\; HO$

To show the analogy in composition with the natural fixed bases, let us take the formula of quinine, which is $C\_\{20\}\; H\_\{12\}\; NO\_2$, which is intermediate between the second and third of those above given. But, in addition to this, there exists a volatile base, found in coal-tar, and formed also by the action of heat on quinine, and bearing to it the same relation that trimethylamine does to hydrated oxide of tetramethylum. It is called quinoline, and is a nitrile base. Now, by the action of iodide of methyl, it is converted into an ammonium base, as follows. Its formula is $C\_\{18\}\; H\_8\; N$. Now, $C\_\{20\}\; H\_\{12\}\; N\; +\; C\_2\; H\_3\; I\; =\; C\_\{22\}\; H\_\{15\}\; N,\; I$. This iodide, acted on by oxide of silver, yields the base, $C\_\{22\}\; H\_\{15\}\; NO,\; HO\; =\; C\_\{20\}\; H\_\{12\}\; NO\_2$, which is either quinine or a base isomeric with quinine. It is such considerations which render the ammonium bases so interesting, and we have dwelt somewhat fully on them, as well as on the amide, imide, and nitrile bases, because of the insight thus obtained into the mode of formation of two very important classes of vegetable products, the volatile bases, and the fixed bases. We have dwelt, however, only on the principles which regulate these very curious and important reactions, and not on the individual compounds, which our space forbids us to do. There is no part of the science which so plainly demonstrates the value and importance of the laws we have endeavoured to explain concerning types, substitution, and homologous series. Yet the whole of the facts in reference to these four series of bases are of quite recent discovery, ethylamine and methylamine dating only from 1840, while the imide, nitrile, and ammonium bases have all been discovered much more recently. There is every reason to expect important practical applications of these discoveries; but even should this not be the case, they have already done more for the science than hundreds of practical applications could ever do.

We shall now turn to a group of homologous series, related to those we have described, but more particularly to a series of negative radicals, derived from the compounds of the positive radicals of series 1.

19. The nineteenth series here specified, is that of the derived or positive radicals themselves. They are little if at all known in the separate state, but may be traced in many compounds, and therefore it is convenient to assume their existence. They are derived, that is, their compounds are derived, from those of the positive radicals, by the removal of 2 eqs. of hydrogen, which is effected by oxidizing agencies, by chlorine, and otherwise. Their general formula is $C\_n\; H\_\{2n-2\}$; and the following are the first in the series—

• Formyle..... $C\_2\; H\; =$ Methyl $C\_2\; H\_3\; -\; H\_2$
• Acetylene..... $C\_4\; H\_3\; =$ Ethyl $C\_4\; H\_5\; -\; H\_2$
• Propionyle..... $C\_6\; H\_5\; =$ Propyl $C\_6\; H\_7\; -\; H\_2$
• Butyryle..... $C\_8\; H\_9\; =$ Butyl $C\_8\; H\_\{11\}\; -\; H\_2$
• Valeryle..... $C\_\{10\}\; H\_\{13\}\; =$ Amyle $C\_\{10\}\; H\_\{15\}\; -\; H\_2$

and so on, regularly.

20. The twentieth series is that of the hydrated protoxides of these negative radicals, the general formula of which is $(C\_n\; H\_\{2n-2\})O,\; HO\; =\; C\_n\; H\_\{2n-1\}O\_2$. The type of this series

is aldehyde, $C\_n\; H\_\{2n-1\}O,\; HO\; =\; C\_n\; H\_\{2n-1\}O\_2$, the hydrated oxide of Chemistry. acetylene. It is called aldehyde, because it is obtained from alcohol by dehydrogenation, for alcohol, $C\_n\; H\_\{2n+2\}O,\; HO\; -\; H\_2\; =\; C\_n\; H\_\{2n-1\}O,\; HO$.

Aldehyde is formed whenever alcohol is acted on by oxygen, the first effect being the removal of 2 eqs. of hydrogen, converted by the oxygen into water. Thus, $C\_n\; H\_\{2n+2\}O,\; HO\; +\; O\_2\; =\; 2HO\; +\; C\_n\; H\_\{2n-1\}O,\; HO$. Aldehyde is a very volatile, pungent, inflammable liquid, having a strong attraction for oxygen, and reducing the salts of silver to the metallic state by this attraction. With 2 eqs. of oxygen it forms pure acetic acid, which is the teroxide of the same radical of which aldehyde is the protoxide. $C\_n\; H\_\{2n-1\}O,\; HO\; +\; O\_2\; =\; C\_n\; H\_\{2n-1\}O\_3,\; HO$.

The whole series of homologous compounds, which, as a class, are called the aldehydes, agree with this one in their derivation, and in their attraction for oxygen, by which they are converted into volatile acids homologous with acetic acid. Several of them are found in nature, and others are formed in various processes. There are two remarkable characters which are found in all aldehydes, even in such as belong to a different series. But here we speak only of that series of aldehydes which is derived from the ethylic series of alcohols, and yields the acetic series of acids. These characters are, that the aldehydes combine with ammonia, forming in several cases permanent, crystallizable compounds; and that they also form crystalline compounds with the sulphites of potash or ammonia. By one or other of these characters aldehydes are detected when mixed with compounds of different properties, and may be purified. But their most important character is that of absorbing oxygen from the air, and producing the volatile and oily acids of the next series.

The following are a few of the aldehydes:—

• Hydrated oxide of formyle... $C\_2\; HO,\; HO\; =\; C\_3\; H\_2\; O\_2$
• Do. acetylene... $C\_4\; H\_3\; O,\; HO\; =\; C\_4\; H\_4\; O\_2$
• Do. propylene... $C\_6\; H\_5\; O,\; HO\; =\; C\_6\; H\_6\; O\_2$
• Do. butyryle... $C\_8\; H\_9\; O,\; HO\; =\; C\_8\; H\_\{10\}\; O\_2$
• Do. valeryle... $C\_\{10\}\; H\_\{13\}\; O,\; HO\; =\; C\_\{10\}\; H\_\{14\}\; O\_2$
• Do. ornanthyle $C\_\{14\}\; H\_\{17\}\; O,\; HO\; =\; C\_\{14\}\; H\_\{18\}\; O\_2$
• Do. capryle... $C\_\{20\}\; H\_\{21\}\; O,\; HO\; =\; C\_\{20\}\; H\_\{22\}\; O\_2$

These and some others are known. The two last-named are found, the former among the products of distillation of castor oil, the latter in the volatile oil of rue.

21, 22. The next, or twenty-first series, is that of the hydrated teroxides of the negative or formylic radicals, which are volatile, and with the exception of two, oily and fatty acids of great interest and importance. Their general formula is $C\_n\; H\_\{2n-1\}O\_3,\; HO\; =\; C\_n\; H\_\{2n-1\}O\_4$. They are found abundantly in nature, generally combined with oxide of lipole or glycerine, forming the fixed oils and fats; but they also occur free, and some of them combined with oxide of ethyle. These acids are formed by the direct oxidation of the aldehydes, which, as we have seen, are themselves formed by the action of oxygen on the alcohols, which removes hydrogen from these compounds. Hence the typical process, which is the conversion of common alcohol into acetic acid, consists of two stages, the production of aldehyde by dehydrogenation, and the oxidation of the aldehyde thus produced. Representing alcohol by $C\_n\; H\_\{2n+2\}O$ for shortness' sake, we have first $C\_n\; H\_\{2n+2\}O\; +\; O\_2\; =\; 2HO\; +\; C\_n\; H\_\{2n-1\}O\_2$; and secondly aldehyde, which is $C\_n\; H\_\{2n-1\}O\_2$, taking up 2 eqs. of oxygen, becomes acetic acid, $C\_n\; H\_\{2n-1\}O\_3\; =\; C\_n\; H\_\{2n-1\}O\_4,\; HO$. It is in this way, and no other, that alcohol, wine, or beer, is converted into vinegar. If the supply of oxygen be deficient, great loss is sustained by the evaporation of the very volatile aldehyde, but with a full supply of oxygen the whole is acidified. As this oxidation, or slow combustion, or decay, of alcohol is set agoing by contact with a ferment, such as yeast, and air at the same time, it has been called the acetic fermentation; but it is merely a case of oxidation or

Chemistry. decay promoted by the presence of a ferment, as will be elsewhere explained.

Of all known homologous series, this one is the best known and the most complete, the series being unbroken from $C\_2$ (in formic acid), to $C\_\{23\}$ (in behenic acid), and at least two acids being known beyond that point. We give here the list of these acids, premising that the two first, which contain, relatively, a very large amount of oxygen, are not oily, but that oily properties begin to appear in the third, and that all the rest are oily and fatty, exhibiting a perfect gradation of physical properties, such as density, fusibility, volatility, &c.; being, in short, a perfect example of a homologous series.

The acids of this truly remarkable series are all volatile, that is, may all be distilled unchanged, but the boiling point rises at each step just as with the alcohols, and to the same extent, namely, about $34^\backslash circ$ for each addition of $C\_2\; H\_4$. The two first are products of oxidation, usually artificial, but formic acid occurs in the ant, and acetic acid in some vegetable juices. The remaining acids are all found in nature, and in general, combined with oxide of lypyle, $C\_3\; H\_2\; O$, or $C\_4\; H\_4\; O\_2$, forming the neutral fixed oils and fats, vegetable and animal. Of the former, olive oil and palm oil are examples; of the latter, tallow and butter. These fats and oils, however, contain also a peculiar oily acid, not of this series, combined also with oxide of lypyle. This is oleic acid, $C\_\{28\}\; H\_\{56\}\; O\_2$, $HO\; =\; C\_\{28\}\; H\_\{56\}\; O\_2$. The most abundant of the oils and fats are those containing oleic acid, stearic acid, palmitic acid, and margaric acid, which seems to be a compound, possibly a mixture of stearic and palmitic acids. The oils containing the acids lower in the scale, from $C\_4$ to $C\_\{20\}$, which are the more volatile of the oily acids, occur only in certain fats, and in small quantities, and give them their peculiar flavour, the fats higher in the scale having neither taste nor smell. The neutral fats are named shortly, after the acids they contain; thus, oleine, stearine, palmitine, margarine, butyrine, valerine, caproine, &c. Solid fats consist chiefly of stearine, palmitine, margarine, &c., with a little oleine; liquid oils of oleine, with some margarine, stearine, &c., dissolved. Butter is a very remarkable fat, containing butine, stearine, palmitine, and myristine, the stearine and palmitine together forming margarine, all of these being solid tasteless fats; and, in smaller quantity, caprine, capryline, caproine, and butyrine, which are oily and sapid, and give to butter its peculiar flavour, especially butyrine, the most abundant of them. It contains also oleine. But it is worthy of notice, that all the acids of the formic series in butter have formulae in which the number of atoms of carbon is divisible by 4, none of the intermediate acids being present. Valerine is the compound which, being present in small quantity in whale oil, as butyrine is

in butter, gives to that oil its peculiar and unpleasant flavour. When the neutral fat oils, such as stearine, palmitine (which constitutes the chief part of palm oil), margarine, and oleine, are boiled with alkalies, they yield soaps, which are compounds of the oily and fatty acids with potash and soda, the former yielding soft, the latter hard soaps. In this process the basic oxide of lypyle is separated, and combining with water, forms a sweetish solution, which by evaporation forms a syrup, called glycerine, or the sweet principle of oils. Oxide of lypyle is $C\_3\; H\_2\; O$, and glycerine is $2(C\_3\; H\_2\; O)\; +\; 3\; HO\; =\; C\_3\; H\_5\; O\_3$. When glycerine is heated in sealed tubes with the oily acids, it again becomes oxide of lypyle and combines with the acids, reproducing neutral oils or fats. When heated by itself glycerine yields an intolerably acrid pungent vapour, which condenses into a liquid called acroleine, or hydrated oxide of acryle, $C\_3\; H\_3\; O$, $HO\; =\; C\_3\; H\_4\; O\_2$. It appears to be either oxide of lypyle, which we have seen to be $C\_3\; H\_2\; O$, or $C\_4\; H\_4\; O\_2$, or polymeric with it, the latter being the more probable view. It attracts oxygen with avidity, and is converted into an acid, acrylic acid, $C\_3\; H\_3\; O\_3$, $HO\; =\; C\_3\; H\_4\; O\_2$, resembling acetic acid. In consequence of the formation of acroleine, the action of heat on oils is an excellent means of detecting the presence in them of oxide of lypyle or glycerine. A drop of any oil containing that compound, heated in the bottom of a test tube, produces the pungent vapour of acroleine which attacks the eyes strongly, and is indeed formed, as all must have perceived, when a candle is blown out so as to leave the wick ignited, that is red-hot, without flame.

The reader will perceive that we have taken this opportunity of giving a brief general sketch of the oils and fats; and have, in fact, appended to the homologous series of the oily acids the description of another series, No. 22, which includes the compounds of those acids with oxide of lypyle or glycerine, that is, the neutral oils and fats. We trust that this brief notice will suffice to give the student an idea of the chemistry of both of these very important series, the oily and fatty acids, and the neutral oils and fats. It will also serve to illustrate the advantage of studying homologous series, since otherwise this numerous class of bodies would present only a mass of confusion.

23. The next, or twenty-third series we shall name is that of the salts formed by the acids of series 21 with oxide of ammonia. We shall not, however, enter into details, but only state, that from this series two others arise by the separation of water. As an example of the series itself, we take acetate of ammonia, $NH\_4\; O$, $C\_4\; H\_7\; O\_2$. General formula $NH\_4\; O\; +\; (C\_4\; H\_7\; O\_2)\_2$.

24. The twenty-fourth series is derived from the preceding by the separation of 2 eqs. of water, and includes the amides of the acids of series 21. Example acetamide, which is acetate of ammonia, minus 2 eqs. of water, $NH\_4\; O$, $C\_4\; H\_7\; O\_2$ - $2\; HO\; =\; NH\_2$, $C\_4\; H\_7\; O\_2\; =\; C\_4\; H\_5\; NO\_2$. General formula $NH\_2\; +\; C\_4\; H\_5\; NO\_2$.

25. The twenty-fifth series is derived either from the twenty-third, by the separation of 4 eqs., or from the twenty-fourth by that of 2 eqs. of water, by which all oxygen is removed from both, and the result is a series of very remarkable volatile liquids, called nitriles, and containing the elements of one of the negative radicals, with 1 eq. of nitrogen. Take for example acetonitrile, which is thus derived from acetate of ammonia, $NH\_4\; O$, $C\_4\; H\_7\; O\_2$ - $4\; HO\; =\; C\_4\; H\_3\; N$, or from acetamide; $NH\_2$, $C\_4\; H\_5\; NO\_2$ - $2\; HO\; =\; C\_4\; H\_3\; N$.

The most important point in regard to this series is, that in composition they are either nitriles of the formyl radicals, or cyanides of the methyl radicals one step lower in the scale. Thus acetonitrile is either $C\_4\; H\_3\; N$, or $C\_2\; H\_3$, $C\_2\; N$, which latter formula is cyanide of methyl.

Now it is not yet certain whether in all cases the nitriles, as obtained from the ammonia salts of the acids of the formic series, are really the cyanides of ethylic radicals, or only

Chemistry. isomeric with them. In some cases they certainly appear to be identical with these cyanides as otherwise obtained, but in others they seem to be different. Supposing them to be the cyanides, then it will be possible from any acid of the series No. 21 to obtain the cyanide of the ethylic radical, having 2 eqs. of carbon less than the acid, as from acetic acid cyanide of methyl. Now from the cyanides we can reproduce the alcohol from which they are derivable. And thus we may probably be able to form artificially alcohols which otherwise might never be known. For example, the alcohol with 30 eqs. of carbon is unknown; but from the ammonia salt of the acid with 32 eqs. of carbon, palmitic acid, we can obtain the body palmitonitrile $C\_\{32\}H\_\{31\}N$, which is probably the cyanide of the radical $C\_\{30\}H\_\{29\}$. And if so, from this cyanide it might be possible to form the alcohol in question, which contains the radical $C\_\{30\}H\_\{29\}$. The general formula of the nitriles is $C\_nH\_\{2n-1\}N$ or $C\_nH\_\{2n+1\}\; +\; C\_2N$.

26. The next, or twenty-sixth series, will be that of the terchlorides of the formylic radicals, of which chloroform, the terchloride of formyle is the type. The properties of chloroform are now well known. At present no other member of this series has been studied, but judging from that one they merit investigation. General formula $C\_nH\_\{2n-1\}Cl\_3$.

27. The oil of olefiant gas, or of the Dutch chemists, represents a twenty-seventh series. It is the hydrochlorate of a protochloride of acetylene, $C\_4H\_3Cl,\; HCl\; =\; C\_4H\_2Cl\_2$. Of this series also hardly any others are known. The oil in question has a singular resemblance to chloroform in its taste and smell, as well as in its action on the system when inhaled. General formula, $C\_nH\_\{2n-1\}Cl,\; HCl$.

28. The twenty-eighth series is that of the hydurets of the formylic radicals, the type of which is olefiant gas, or hyduret of acetylene, $C\_4H\_3,\; H\; =\; C\_4H\_2$. This is a series of combustible gases, liquids, and solids, formed abundantly in the distillation of organic substances, such as wood, coal, or animal matter, along with the hydurets of the methylic radicals, typified by marsh gas $C\_4H\_4$. It is probable that several species of naphtha and of paraffine belong to this series, as well as to series 4. The compounds of this series, No. 27, are absorbed by chlorine even in the dark, which is not the case with those of series 4.

29. The next or twenty-ninth series is very remarkable; it is that of which oxalic acid is the type, and consists of dibasic acids, having a singular relation to the acids of series 21. Oxalic acid is $C\_2O\_4,\; 2HO\; =\; C\_2H\_2O\_4$. Now, if we compare this with the formula of formic acid, $C\_2H\_2O\_4$, we perceive that they differ exactly by $C\_2O\_4$, that is, by 2 eqs. of carbonic acid; so that, to obtain the formula of the acids of this series, we have only to add $C\_2O\_4$ to those of the acids of series 20, thus—

It will be seen that up to $C\_\{20\}$, there are only two blanks in this series; and it is a proof of their close relation to the oily volatile acids, that every one of these, except perhaps the last, is formed, along with the parallel oily acids, in the oxidation of fats and fatty acids by means of nitric acid. It is only very recently that these relations have been recognised and the series established; and as it includes oxalic acid, one of the commonest acids in plants, we are entitled to expect that the progress of science will develop series in which the other acids of acid fruits and leaves will find their place. We have another proof of the close relation between

these two series of acids, the formic series and the oxalic series, in the fact that oxalic acid, when heated, yields formic and carbonic acids, which, in the above table, are shown to exist in it by their elements at least. This is, indeed, one of the best methods of obtaining formic acid. It seems probable that we shall succeed in applying the same method to obtain from the other acids of the oxalic series the oily acids parallel to them, as, for example, cenanthylic acid, from suberic acid, and the rare pelargonic acid from sebacic acid. This, however, has not yet been done. Sebacic acid is formed when oleic acid is distilled, and the production of it by heat is an excellent test of the presence of oleine in any oil or fat. The oxalic series of acids are all dibasic, and their general formula is $C\_nH\_\{2n-2\}O\_4\; =\; C\_nH\_\{2n-4\}O\_2,\; 2HO$.

30. The next, or thirtieth series, is that of the cyanates of the oxides in series 2. The type is cyanic acid, which is remarkable for its transformation into urea, when combined with ammonia. Besides this, when heated with potash, it yields carbonic acid and ammonia, $C\_2NO,\; HO\; +\; 2KO,\; HO\; =\; 2(KO,\; CO\_2)\; +\; NH\_3$. Now, the homologous cyanates of oxides of methyle, ethyle, &c., will undergo the same transformations. By the latter, heating with potash, they yield, instead of ammonia, the homologous bases, methylamine, ethylamine, &c., which were discovered by Wurtz in this way. The former metamorphosis, that is, with ammonia, gives rise to the next series, which is a very remarkable one. It is unnecessary to dwell longer on these cyanates, the general formula of which is $(C\_nH\_\{2n+1\})O,\; C\_2NO$.

31. The thirty-first series is that of which urea is the type. And here we have another example of the comprehensiveness of the principle of homologous series. Urea has long been considered as an isolated body, without any visible relation to others, and now we find that, viewed in the light of the principle alluded to, it is a type of a numerous class of compounds, the discovery of which is due to that principle. The following are the first members of this series:—

The perfect analogy which here prevails is fully seen, if we consider hydrated cyanic acid as cyanate of oxide of hydrogen, and the cyanates of oxide of methyle, &c., as compounds in which methyle, &c., replace that hydrogen. The rational formula of urea is not certainly known, but as it is basic, we may suppose it to represent 2 equivalents of ammonia, in which 2 equivalents of hydrogen are replaced by CO, or carbonic oxide, thus: $N\_2H\_4\; =\; 2$ eqs. ammonia, and urea = $N\_2C\_2O\_2$ } or $N\_2(CO)\_2$ } we shall thus have—

The compounds of this series, homologous with urea, are perfectly analogous to it in properties. Urea, the type, is found ready formed in urine, and is produced artificially by the above-mentioned transformation of cyanate of ammonia. It crystallizes in prisms, which in form as well as in taste resemble nitre. It is soluble in alcohol, and forms with acids salts, being a weak base. The nitrate and oxalate

Chemistry. of urea are sparingly soluble, and crystallize readily. The action of heat on urea has been already explained, as well as its production, under the head of isomeric and polymeric transformations.

32. The next series we shall mention is that which consists of compounds of the aldehydes with ammonia. Only one of them, that composed of acetic aldehyde and ammonia, is well known; but there is no doubt of the existence of several others, the tendency to form such compounds being a character of the aldehydes. The best known of these compounds, aldehydammonia as it is called, is $\backslash text\{NH\}\_3\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{C\}\_4\backslash text\{H\}\_7\backslash text\{O\}\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_7\backslash text\{NO\}\_2$. It crystallizes readily, and undergoes some remarkable transformations. The general formula of this series is $\backslash text\{C\}\_n\backslash text\{H\}\_\{2n-1\}\backslash text\{O\}\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{NH\}\_3$. One of its transformations is that caused by sulphurous acid, of which 2 eqs. unite with 1 of aldehydammonia, or 1 eq. of aldehyde unites with 1 eq. of bisulphite of ammonia, to form the compound $\backslash text\{NH\}\_3\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{C\}\_4\backslash text\{H\}\_7\backslash text\{O\}\; \backslash cdot\; 2\backslash text\{SO\}\_3\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_7\backslash text\{NS\}\_2\backslash text\{O\}\_4$. This compound is isomeric with taurine, a very remarkable substance obtained from bile. It is probable that this is one of a new homologous series.

33. The type of the next series is derived from formic aldehyde, by the action of hydrocyanic acid and water, $\backslash text\{C\}\_2\backslash text\{H\}\_2\backslash text\{O\}\_2\; +\; \backslash text\{C\}\_2\backslash text\{NH\}\; +\; 2\backslash text\{HO\}\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_3\backslash text\{NO\}\_4$. This last formula is that of a remarkable base, found in bile and elsewhere in the animal body, and from its sweet taste called glycine, glycoll, or sugar of gelatine. We prefer the first name. It is best obtained by boiling hippuric acid, an acid found in urine, with strong hydrochloric acid, when two eqs. of water are taken up, and it is resolved into benzoic acid and glycine. Hippuric acid is $\backslash text\{C\}\_8\backslash text\{H\}\_8\backslash text\{NO\}\_4$, and with 2 HO, it yields $\backslash text\{C\}\_\{11\}\backslash text\{H\}\_8\backslash text\{O\}\_4$, benzoic acid, and $\backslash text\{C\}\_4\backslash text\{H\}\_7\backslash text\{NO\}\_2$, glycine. Glycine is a base, and forms crystallizable salts with acids, but it also forms crystalline compounds with bases and with neutral salts. Of the series of glycine three only are as yet known, which are—

The last column represents these compounds as amides of the acids which constitute the next series. Alanine is obtained by the action of hydrocyanic acid on aldehyde. Leucine is found in the animal body, as is also glycine. It cannot be doubted that other compounds of this series will be discovered, and they are likely to be compounds of much interest.

34, 35. The next series consists of acids, and the type is lactic acid. They are in so far connected with the aldehydes, that they consist of formic acid, coupled with the aldehydes. Thus we have—

The two first acids, if regarded as monobasic, will be written thus—

And their ammonia salts, minus 2 eqs. of water, according to the usual law, will give the amides, glycolamide and lactamide, which have the composition of glycine and alanine, but seem to be only isomeric with them.

Glycolate of ammonia, $\backslash text\{NH\}\_3\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{C\}\_4\backslash text\{H\}\_5\backslash text\{O\}\_2\; -\; 2\backslash text\{HO\}\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_3\backslash text\{O\}\_4\; \backslash cdot\; \backslash text\{NH\}\_3$
Lactate of " " $\backslash text\{NH\}\_3\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{C\}\_6\backslash text\{H\}\_7\backslash text\{O\}\_3\; -\; 2\backslash text\{HO\}\; =\; \backslash text\{C\}\_6\backslash text\{H\}\_5\backslash text\{O\}\_4\; \backslash cdot\; \backslash text\{NH\}\_3$

The reason why these two amides are not identical with glycine and alanine seems to be that the acids are really dibasic, and that therefore the true formulae of the acids, as well as the amides, are double those just given.

Glycolic acid..... $\backslash text\{C\}\_4\backslash text\{H\}\_5\backslash text\{O\}\_2\; \backslash cdot\; 2\backslash text\{HO\}$ Glycolamide $\backslash text\{C\}\_4\backslash text\{H\}\_3\backslash text\{O\}\_4\; \backslash cdot\; 2\backslash text\{NH\}\_3$ Chemistry.
Lactic acid..... $\backslash text\{C\}\_6\backslash text\{H\}\_7\backslash text\{O\}\_3\; \backslash cdot\; 2\backslash text\{HO\}$ Lactamide $\backslash text\{C\}\_6\backslash text\{H\}\_5\backslash text\{O\}\_4\; \backslash cdot\; 2\backslash text\{NH\}\_3$

It is probable that the amides form a distinct series by themselves. There is, besides, another series, strictly isomeric with the glycine series, namely, the hyponitrites of the oxides of series 2.

The analogue of the first of these in the glycine series is unknown. The others are analogous to and isomeric with glycine, alanine, and leucine. This may be called series 35.

36. Another series, isomeric with 33 and 35, is that of which the type is urethan or carbamate of oxide of ethyle. Carbamic acid is an acid amide, or bicarbonate of ammonia minus 2 eqs. of water. $(\backslash text\{NH\}\_3\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; 2\backslash text\{CO\}\_2)\; -\; 2\backslash text\{HO\}\; =\; \backslash text\{C\}\_2\backslash text\{H\}\_2\backslash text\{NO\}\_3$. This last is carbamic acid, and with oxide of methyle it forms a compound isomeric with glycine. $\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{O\}\; +\; \backslash text\{C\}\_2\backslash text\{H\}\_2\backslash text\{NO\}\_3\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_7\backslash text\{NO\}\_4$. The compound with oxide of ethyle is isomeric with alanine; that with oxide of amyle is isomeric with leucine.

There is still another compound, sarcosine, a base, which is isomeric with alanine, but whether it be one of a series we do not yet know.

37. Analogous to the last series is that of the compounds of oxamic acid with the oxides of series 2. They are beautifully crystallized bodies, as are also those of series 36, and are formed by the action of ammonia on the oxalates of the oxides of methyle, ethyle, &c. Oxamic acid is acid oxalate of ammonia, minus 2 eqs. of water. $(\backslash text\{C\}\_2\backslash text\{O\}\_4\; \backslash cdot\; \backslash text\{HO\}\; \backslash cdot\; \backslash text\{NH\}\_3\backslash text\{O\})\; -\; 2\backslash text\{HO\}\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{NO\}\_4\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{NO\}\_4\; \backslash cdot\; \backslash text\{HO\}$, and with oxide of methyle, this acid, oxamic acid, forms the first of the new series. $\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{O\}\; +\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{NO\}\_4\; =\; \backslash text\{C\}\_6\backslash text\{H\}\_7\backslash text\{NO\}\_4$. This is sometimes called oxamethylan. The next one is oxamethan or oxamate of oxide of ethyle, $\backslash text\{C\}\_4\backslash text\{H\}\_5\backslash text\{O\}\; \backslash cdot\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{NO\}\_4\; =\; \backslash text\{C\}\_8\backslash text\{H\}\_7\backslash text\{NO\}\_4$.

38. The next series is one consisting of certain newly discovered acids, the two first of which are known. These contain glycolic and lactic acids, coupled with benzoic acid, and thus form a connecting link between the extensive group of allied homologous series now under consideration, and that group to which benzoic acid, its congeners, and their derivatives belong. These acids are—

Benzoglycolic acid, $\backslash text\{C\}\_\{15\}\backslash text\{H\}\_8\backslash text\{O\}\_6\; =\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{O\}\_2\; +\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{O\}\_2$
Benzolactic acid, $\backslash text\{C\}\_\{17\}\backslash text\{H\}\_\{10\}\backslash text\{O\}\_6\; =\; \backslash text\{C\}\_6\backslash text\{H\}\_4\backslash text\{O\}\_2\; +\; \backslash text\{C\}\_4\backslash text\{H\}\_2\backslash text\{O\}\_2$
Benzoleucic acid, $\backslash text\{C\}\_\{23\}\backslash text\{H\}\_\{14\}\backslash text\{O\}\_6$, is said to exist, but has not yet been studied. The amides of these acids probably form a new series, only one of which is yet known.

39. This is hippuric acid, $\backslash text\{C\}\_\{15\}\backslash text\{H\}\_8\backslash text\{NO\}\_4$, which is the amide of benzoglycolic acid; that is, benzoglycolate of ammonia, $(\backslash text\{NH\}\_3\; \backslash cdot\; \backslash text\{C\}\_\{15\}\backslash text\{H\}\_8\backslash text\{O\}\_4)$, minus 2 eqs. HO, which gives $\backslash text\{C\}\_\{15\}\backslash text\{H\}\_6\backslash text\{NO\}\_2$. This is a very remarkable acid, found in large quantity in the urine of herbivorous animals. When boiled with strong hydrochloric acid, it is first resolved, like other amides, into ammonia and the acid, which is here benzoglycolic acid, 2 eqs. of water being taken up. But the benzoglycolic acid is itself resolved into benzoic acid and glycolic acid, 2 eqs. of water being taken up, and the glycolic acid, acting on the ammonia, produces, with the separation of 2 eqs. of water, glycolamide or glycine. So that the ultimate results of the operation are benzoic acid and glycine. Hippuric acid as yet stands alone, but as we already know homologues both of benzoglycolic acid and of glycine, it cannot be doubted that we shall in time discover homologues also of hippuric acid.

40. The next series we shall mention is that of which acetone is the type. When the salts of acetic acid are decomposed by heat, they yield carbonic acid, which remains combined with the base, and acetone which distils over. In

Chemistry. the case of acetate of lime we have $\backslash text\{Ca\; O,\; C\}\_2\backslash text\{H\}\_3\backslash text\{O\}\_2\; =\; \backslash text\{Ca\; O,\; CO\}\_2\; +\; \backslash text\{C\}\_2\backslash text\{H\}\_3\backslash text\{O\}$. The true formula of acetone appears to be double of this, or $\backslash text\{C\}\_3\backslash text\{H\}\_6\backslash text\{O\}\_2$, and it is regarded by some as a hydrated oxide analogous to alcohol; that is, as $\backslash text\{C\}\_3\backslash text\{H\}\_6\backslash text\{O,\; HO\}$. This analogy is far from close, but all the acids of the acetic series, at least above acetic acid, seem to form singular compounds, which are isomeric with the aldehydes of the acids next above each. Thus acetone is isomeric with propylic aldehyde. Various opinions are entertained as to the real nature, and even as to the formulae of these compounds, but for the present we adopt the formulae which give to all of them 2 eqs. of oxygen.

We now come to a group of homologous series, of very recent discovery, and as yet but little known, save in the case of one or two, but which indicate the existence of a very large number of compounds of the most remarkable composition and properties. These are the compounds in which the elements of the radicals of series 1 are combined with a certain class of metals, giving rise to new and most singular radicals or compound metals having most energetic affinities, and in their compounds very analogous to metals. We can only briefly mention the characters of these compounds.

41. The first series of this group is that which contains zinc. When iodide of methyl or ethyl is heated with zinc in a closed tube, there is formed a crystalline mass, consisting of iodide of zinc, along with the new radical, zinco-methyl in the former, zinco-ethyl in the latter case. On distilling the mass in hydrogen gas, iodide of zinc is left, and the new radicals distil over. They are volatile, fetid, spontaneously inflammable liquids, and appear to act as energetic positive radicals, analogous to metals, combining with oxygen, chlorine, sulphur, &c. They decompose water, producing oxide of zinc and the carbohydrogen of series 4. Thus with zinco-methyl, which is $\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{Zn\}$, and water $\backslash text\{HO\}$, we have $\backslash text\{ZnO\}$ and $\backslash text\{C\}\_2\backslash text\{H\}\_4$; zinco-ethyl is $\backslash text\{C\}\_3\backslash text\{H\}\_7\backslash text\{Zn\}$, and zinco-amyl which has been formed, is $\backslash text\{C\}\_\{10\}\backslash text\{H\}\_\{11\}\backslash text\{Zn\}$. No other radicals of this series are yet known.

42. The next series contains antimony. It is formed by the action of an alloy of potassium and antimony on iodide of methyl or ethyl. The formula of stibiomethyl is $\backslash text\{Sb\; Me\}\_2\; =\; \backslash text\{Sb\; C\}\_2\backslash text\{H\}\_5$. That of stibethyl is $\backslash text\{Sb\; Ac\}\_2\; =\; \backslash text\{Sb\; C\}\_3\backslash text\{H\}\_7$. These properties resemble those of zinco-ethyl, and they are powerful radicals, of the class of metals.

43. This series also contains antimony. The formula of the first compound is $\backslash text\{Sb\; Me\}\_2\; =\; \backslash text\{Sb\; C\}\_2\backslash text\{H\}\_5$, and it is called stibiomethyl. It is analogous to ammonium and potassium, and forms a hydrated oxide, resembling caustic potash.

44. The next series contains tin, and the compounds are called stannomethyl and stannoethyl. Their formulae are $\backslash text\{Sn\; C\}\_2\backslash text\{H\}\_5\; =\; \backslash text\{Sn\; Me\}$ and $\backslash text\{Sn\; C\}\_3\backslash text\{H\}\_7\; =\; \backslash text\{Sn\; Ac\}$. They are analogous to the preceding.

45. In this series, 2 eqs. of the preceding one seem to be condensed into one, so that the formulae are, $\backslash text\{Sn\}\_2\backslash text\{C\}\_4\backslash text\{H\}\_6\; =\; \backslash text\{Sn\}\_2\backslash text\{Me\}\_2$, and $\backslash text\{Sn\}\_2\backslash text\{C\}\_6\backslash text\{H\}\_\{10\}\; =\; \backslash text\{Sn\}\_2\backslash text\{Ac\}\_2$. These also are powerful quasi-metallic radicals.

46. In the next series lead is the metal combined with the organic radicals. A compound has been described, consisting of $\backslash text\{Pb\}\_2\backslash text\{Ac\}\_2$, which, like the others is a positive radical, forming a base with oxygen, and salts with chlorine, iodine, &c. There seem to be several other radicals, formed of lead and ethyl, in different proportions. They are all liquid and fuming, and burn when kindled, producing thick fumes of oxide of lead. Each of the lead radicals will of course belong to a distinct series.

47. Tellurium also combines with ethyl to form one or more radicals. The best known is tellurethyl, $\backslash text\{Te\; Ac\}\; =\; \backslash text\{Te\; C\}\_3\backslash text\{H\}\_7$. Another, which belongs to a different series, is $\backslash text\{Te\; Ac\}\_2$. They are both very fetid and poisonous liquids, and powerful radicals.

48. Arsenic forms similar compounds, both with ethyl

and methyl. The best known of them is kakodyle, $\backslash text\{As\; Me\}\_2\; =\; \backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{As\}$. This is a crystalline volatile substance, having all the chemical relations of a metal. Its protoxide is a volatile, fetid, spontaneously inflammable, and frightfully poisonous liquid, which is strongly basic. Its teroxide is an acid, and is devoid of smell and of poisonous action. Its compounds with chlorine, iodine, sulphur, cyanogen, &c., are all volatile, fetid, and poisonous. There are several other compounds of arsenic with methyl and ethyl, forming so many separate series, but as yet they are little known.

49. The next series we shall mention is typified by kakoplatyle, which consists of kakodyle, platinum, and water, or perhaps oxygen and water. It is also a positive radical, of the formula $\backslash text\{C\}\_2\backslash text\{H\}\_5\backslash text\{As\; Pt\; O,\; HO\}$, and forms a hydrated oxide, a chloride, iodide, sulphuret, &c. &c.

50. In the next series, methyl and ethyl are combined with phosphorus. There are several compounds with each, and they appear to correspond to the compounds of phosphorus with hydrogen. The compound $\backslash text\{P\; Me\}\_2$, which corresponds to kakodyle, is like it a very fetid and poisonous liquid. Besides this, there are the radicals $\backslash text\{P\}\_2\backslash text\{Me\}$, $\backslash text\{PH\}\_2\backslash text\{Me\}$ and $\backslash text\{P\; Me\}\_3$, all of which are analogous to the first. Each of course indicates a separate series.

We have only indicated the existence of these very remarkable compounds of metals and phosphorus with radicals of the methyl series. Already they are very numerous, and they have all striking properties and strong affinities. New compounds of this class are daily discovered, and almost every new one is the first of a new series. Besides the metals we have named, bismuth and potassium are said to form similar compounds, not yet described. It will be seen that all the metals which have yet been found to unite with the methyl radicals are more or less volatile, and all the compounds are so, whereas the compounds of most of these metals with oxygen, chlorine, sulphur, &c., are fixed in the fire. Most of these compound radicals or compound metals are spontaneously inflammable in the air, from their strong attraction for oxygen, and most of them are poisonous in the highest degree, apparently because they bring the metals they contain in contact with the system in a very fine state of division.

We have now indicated most of the established homologous series, connected with the methyl radicals. It will be seen that these include a large number of important organic compounds, such as alcohol, ether, compound ethers, volatile and oily acids and their salts, volatile and fixed bases and their salts, carbohydrogens, amides, &c. There are, however, many compounds which cannot as yet be classified in this way. But before turning to these, we must mention another group of homologous series, not directly connected with the methyl radicals, but rather in some points running parallel to the series already described. This is the benzoic group, so called from benzoic acid, which is a characteristic feature of the group.

1. The first series of this group is that of which benzoic acid is the type. This acid is found ready formed in gum benzoin, and is also produced by the oxidation of hyduret of benzoyl, by the decomposition of hippuric acid, so that it often occurs in the urine of herbivora, and among the products of the oxidation of such bodies as fibrin, albumen, &c. Its formula is as follows, with those of its homologues:—

The third acid is hardly known, but some of its derivatives are known, and are perfectly homologous with those of benzoic acid. Toluic acid is connected with substances found in balsam of Tolu, and cumic acid with those found in oil of cumine. These acids are all crystallized,

Chemistry. volatile, soluble in water and in alcohol, and approach in characters to resinous bodies.

2. In the next series are those compounds of which the type is hyduret of benzoyle. This is an oily liquid, found in the oil of bitter almonds with hydrocyanic acid. These substances, with others, are formed in the fermentation of amygdaline, a substance peculiar to bitter almonds. The hyduret when pure is $C\_\{14\}H\_5O\_2\; =\; C\_\{14\}H\_5O\_2\; \backslash cdot\; H$. It absorbs 2 eqs. of oxygen from the air, and is soon converted into benzoic acid: $C\_\{14\}H\_5O\_2\; +\; O\; =\; C\_\{14\}H\_5O\_3$. It may be viewed as the hyduret of the radical benzoyle, $C\_\{14\}H\_5O\_2$; and this radical, with 1 eq. of oxygen, forms dry benzoic acid, $C\_\{14\}H\_5O\_2\; +\; O\; =\; C\_\{14\}H\_5O\_3$. At the same time the hydrogen is converted into water, which combines with the dry acid to form the crystals $C\_\{14\}H\_5O\_2\; \backslash cdot\; H\; +\; O\_2\; =\; C\_\{14\}H\_5O\_3\; \backslash cdot\; HO$. The oil of cumine, $C\_\{20\}H\_\{11\}O\_2$, may be viewed as the hyduret of cumyle, $C\_\{20\}H\_\{11\}O\_2\; \backslash cdot\; H$, and is homologous with it. These hydurets may be regarded as the aldehydes of the acids they yield.

3. The next series may be that of the radicals, benzoyle, cumyle, &c. They are hardly at all known.

4. The next is that of the amides of the acids of series 1. Benzamide, toluylamide, cuminamide, &c. Benzamide is benzoyle + amide, or $C\_\{14\}H\_5O\_2\; \backslash cdot\; NH\_2$, and the others are analogous.

5. The next may be that of chloride of benzoyle, $C\_\{14\}H\_5O\_2\; \backslash cdot\; Cl$.

6. The next, that of cyanide of benzoyle, $C\_\{14\}H\_5O\_2\; \backslash cdot\; Cy$.

7. The next may be that of sulphuret of benzoyle, $C\_\{14\}H\_5O\_2\; \backslash cdot\; S$. Many more might be added, but these will suffice to illustrate the compounds in which benzoyle, $C\_\{14\}H\_5O\_2$, may be traced.

8. The next series contains products of decomposition of the acids of this series. When benzoic acid is heated with excess of lime, it is resolved into 2 eqs. of carbonic acid, containing all its oxygen, and a remarkable carbohydrogen, containing all the hydrogen, $C\_\{14\}H\_5O\_2\; +\; 2\; CaO\; =\; 2\; (CaO,\; CO\_2)\; +\; C\_\{14\}H\_5$. This last is benzoyle, or hyduret of phenyle, $C\_\{14\}H\_5\; \backslash cdot\; H$. It is a somewhat fragrant liquid, which is found also in coal tar. Four homologous liquids are known, which are toluole, $C\_\{14\}H\_5$; xylene, $C\_\{16\}H\_\{10\}$; cumole, $C\_\{18\}H\_\{14\}$; and cymole, $C\_\{20\}H\_\{18\}$.

9. The next series is that of which the radical phenyle, $C\_\{14\}H\_5$, is the type. They are not known in the separate state, but enter into many compounds, the bodies of the last series being their hydurets. They have some analogy to the methylic radicals.

10. The next series is that of the hydrated oxides of the phenylic radicals. Hydrated oxide of phenyle, the type, $C\_\{14\}H\_5O\; \backslash cdot\; HO\; =\; C\_\{14\}H\_5O\_2$, is found in coal tar, and often called carbolic acid. It much resembles creosote, and it is most probable that the compound next to it, $C\_\{14\}H\_5O\_2\; \backslash cdot\; HO\; =\; C\_\{14\}H\_5O\_3$, is creosote itself.

11. We next come to the compounds of the phenylic radicals with amide, which, as in the case of the ethylic radicals, are volatile bases. The first of them is phenylamine or aniline, $C\_\{14\}H\_5N\; =\; C\_\{14\}H\_5\; \backslash cdot\; NH\_2\; =\; NH\_2\; \backslash cdot\; Ph$ ($Ph\; =\; C\_\{14\}H\_5$). It is an oily base, distinguished by the great tendency of its salts to crystallize. Its homologues are solid but fusible, and quite analogous to it. We have—

Aniline is obtained by heating indigo with potash, and is also found, with some of the others, in coal tar and animal oil, that is animal tar oil. In this oil, obtained by distilling bones, there is another homologous series of volatile bases, discovered by Dr Anderson, isomeric with these, but

far more volatile. The first of them, picoline, is isomeric Chemistry. with aniline, lutidine with toluidine, &c.

12. There are no compounds which yield a greater number of substitution products than those we are here reviewing. In the first place, besides the amide bases above given, there are two other series, imide and nitrile bases, of which the types are diphenylamine, $NH\; \backslash cdot\; Ph\_2$, and triphenylamine, $N\; \backslash cdot\; Ph\_3$. These also have their homologues, and form two new series. Then aniline, by replacement of its own hydrogen by chlorine, bromine, &c., and nitrous acid, yields such compounds as chloraniline, dichloraniline, trichloraniline, bromaniline, dibromaniline, tribromaniline, nitrilaniline, dinitraniline, and others, in which 1, 2, or 3 eqs. of the hydrogen of the radical phenyle in aniline are replaced by the elements just mentioned. To take an example or two, we have—

Most of these are basic and crystallize well. The homologues of aniline yield similar products, so that each is the commencement of a series.

13. Besides this, the hyduret of phenyle, or benzole and its homologues, also yield similar substitution products with chlorine, bromine, and nitrous acid. Thus we have, with others,

These compounds, each of which represents or typifies a new homologous series, give rise also to various reactions, producing other compounds. But even without this, we have here another large group of homologous series.

14. Another considerable group consists of the various substitution products derived from benzoic acid itself. For example, there is nitrobenzoic acid. Benzoic acid, $C\_\{14\}H\_5O\_2$, or rather $C\_\{14\}H\_5O\_3\; \backslash cdot\; HO$, has one of the 5 eqs. of hydrogen in the anhydrous acid replaced by nitrous acid, and gives the new acid, $C\_\{14\}H\_5O\_3\; \backslash cdot\; HO$. Then there is bromobenzoic acid, as well as others, each of which typifies a series.

Here also might be added two other series, namely, that of benzoglycolic acid and that of its amide, which is hippuric acid; but these have been already noticed among the series connected with the ethylic radicals. These two series, in fact, serve to connect the benzoic with the ethylic group of homologous series.

Hyduret of benzoyle gives rise to a very large number of new compounds, by the various reactions it undergoes with different substances, such as bases, ammonia, sulphuret of ammonium, &c. &c. Among these new products are several remarkable bases, such as amarine, sophine, and picrine. It is not known that these belong to homologous series, but it is probable.

Allied to the benzoic group are some others, of which but few members are yet known.

Thus oil of anise and oil of estragon yield when oxidized by nitric acid an acid, anisic acid, $C\_\{14\}H\_5O\_2$. Oil of Gaultheria procumbens contains, in union with oxide of methyle, an acid called salicylic acid, because it can be formed artificially from salicine, the bitter principle of willow-bark. Sali-

Chemistry. clyc acid is $C\_\{14\}H\_6O\_4$, and is therefore homologous with anisic acid.

There are also the hydurets of salicylic and of anisic, which bear the same relation to these two acids as hyduret of benzoyl does to benzoic acid. Hyduret of salicylic is the oil of spiraea, and is isomeric with benzoic acid, $C\_\{14\}H\_8O\_3$, $HO\; =\; C\_\{14\}H\_7O\_2$, H.

There are many interesting compounds connected with salicylic acid and hyduret of salicylic, and these probably belong to homologous series, as the two bodies themselves do.

Again there is oil of cinnamon, which yields hyduret of cinnamyl, $C\_\{18\}H\_\{10\}O\_2$, H, and this by oxidation becomes cinnamic acid, $C\_\{18\}H\_\{10\}O\_2$, $HO\; =\; C\_\{18\}H\_9O\_2$. This acid is very analogous to benzoic and salicylic acids. Heated with lime, it yields the carbolydogen $C\_\{18\}H\_8$, analogous to benzene or hyduret of phenyl, along with 2 eqs. of carbonic acid. It is probable that these compounds are each members of new homologous series, and that these series are nearly allied to those of the benzoic group.

Having thus very briefly indicated the existence and nature of a large number of homologous series, we must shortly notice those compounds which cannot as yet be reduced to such series, although we have every reason to expect that in time they may be so classified.

These we must notice very briefly, but as they form certain well-marked natural groups, it will be possible, in small space, to convey a general notion of their nature and properties.

1. Organic Acids.

These are numerous, and it is but in very few cases that we know, as yet, anything about their constitution, that is, about the radicals they contain. We have seen that something is known, in that respect, of formic and acetic acids, of the oily acids homologous with these, of oxalic acid, and of benzoic acid and its congeners.

The commonest of the vegetable acids not yet noticed are the following, oxalic acid being prefixed, as having a great analogy in properties with most of them, and as being of all the simplest in composition. These acids are all crystallizable, soluble in water, and, with one or two exceptions, sour to the taste. As a general rule the number of eqs. of oxygen exceeds that of hydrogen; and lastly, they are either monobasic or polybasic, as indicated by the number of eqs. of basic water in each:—

Citric acid is found in the juice of the fruits of the orange or citron tribe, and in other acid fruits, as in the currant. Malic acid occurs in unripe fruits of the apple and pear tribe, especially in the fruit of the mountain ash or service tree. Tartaric acid is found in the grape as acid tartrate of potash, and racemic acid, which is isomeric with it, occurs only in the grapes of certain districts. Oxalic acid occurs in wood sorrel, Oxalis acetosella, and in the leaf stalks of rhubarb; also in other plants, as acid oxalate of potash; in lichens, Sedum sempervivum, &c., as oxalate of lime. Fumaric acid occurs in fumaria, aconitic acid in aconite and equisetum, meconic acid only in opium, the juice of the poppy, and kinic acid in cinchona bark. Tartaric acid and gallic acid, which are not sour, but astringent, are found in nutgalls, oak bark, and other astringent vegetables.

The frequency of polybasic, especially dibasic and tribasic acids, among vegetable acids, gives rise to many acid salts,

such as the acid tartrate and acid oxalate of potash, and to Chemistry. many double salts, as the tartrate of potash and soda, and the tartrate of potash and antimony, Rochelle salt, and tartar emetic.

The neutral salts of vegetable acids with lime, oxide of lead, and oxide of silver, are usually insoluble, or if not, sparingly soluble. As there is a general analogy in composition, especially in regard to the excess of oxygen over hydrogen, so there is a general analogy in properties. But the astringent acids above named, and others like them, strike a deep blue or green colour, approaching to black, with the salts of iron; and kinic acid, in which the oxygen only equals the hydrogen, is a very weak acid.

Like all vegetable products, these acids, and all similar to them, are formed by the plant from carbonic acid and water, which contain the elements of all of them. But if we take as many eqs. of carbonic acid as the acid contains of carbon, and as many eqs. of water as it contains of hydrogen, which of course is the smallest quantity that can yield a given acid, we shall find that these materials always contain more oxygen than the acid formed from them. In other words, the plant in forming them deoxidizes carbonic acid and water, more or less, and thus oxygen, so removed, is given out as gas. Thus, oxalic acid, $C\_4H\_2O\_4$, cannot be formed from less than 4 eqs. of carbonic acid and 2 of water, which are $C\_4H\_2O\_6$, so that in this case 2 eqs. of oxygen are given out. All the other acids contain a smaller proportion of oxygen than oxalic acid, and therefore, in their formation, more oxygen is given out. Moreover, they may be formed either from carbonic acid and water, or from oxalic acid and water, or from any acid containing more oxygen than themselves; and it is probable that it is in this way by a succession of steps, that the plant produces all the vegetable acids, of which only the chief have been given above.

2. Neutral and Mild Organic Compounds.

Kinic acid, in which the hydrogen and oxygen are equal, leads to the next group, in all of which that is the case. They are few in number, but very abundant, widely distributed, and of great importance, as they constitute the chief mass of the vegetable kingdom. They are mild in character, and generally tasteless or sweet. They are as follows:—

In consequence of the equality between the oxygen and hydrogen in these compounds, they may be viewed as formed of carbon plus water; or, in other words, in forming them from carbonic acid and water, either directly or indirectly, that is, by successive steps, the plant gives out exactly as much oxygen as was contained in the carbonic acid from which they were derived. Hence their formation follows that of the acids, and they are probably formed in general from these acids, which would account for the uniform presence of the latter in vegetable juices.

Another fact, depending on the circumstance that they all contain 12 eqs. of carbon, and differ only in the amount of water, or its elements, is this: that all before grape sugar in the table may be converted, even artificially, into that kind of sugar. The conversion of starch into sugar constantly takes place in germination, as in malting, and that of both starch and woody fibre into sugar occurs in the ripening of fruits. In these processes, it is effected by the contact of a ferment. It consists in the addition of water or of its elements. The same result is obtained artificially by boiling them with diluted sulphuric acid, and also, in the case of starch, by adding an infusion of malt.

Chemistry. Woody fibre is insoluble in all solvents, and perfectly different. Its most remarkable character is that of forming gun-cotton, or nitro-cellulose, when acted on by nitric acid. In that case, 2, 3, or more eqs. of nitric acid are taken up, while water is in some cases expelled, in others not. Gun-cotton is, in some instances at least, $C\_\{12\}H\_2O\_4\; \backslash cdot\; 4NO\_3$, so that if formed from cellulose $a$, 4 eqs. of nitric acid are taken up; if from cellulose $b$, 2 eqs. of water are given out at the same time. But there are several varieties of gun-cotton.

Woody fibre, by long digestion with alkalies or acids, is partially converted into a substance which, like starch, strikes a blue with iodine.

Starch consists of small grains, which are said to be hollow sacs, formed of an insoluble membrane, containing a soluble jelly. Others say that the grains are formed of successive layers of insoluble matter. Hot water causes them to swell and burst, allowing the contents to escape. The leading characters of starch are the becoming deep blue with iodine, and the being converted into sugar and rendered soluble by acids and by ferments. Before becoming sugar, it passes through an intermediate stage, in which it is dissolved as dextrine or gum.

Cane sugar is known by its crystals, and by being more soluble and sweeter than grape sugar. In contact with acids or a ferment, it becomes grape sugar, and then, but not before, it undergoes fermentation.

The action of the ferment appears to depend on its being in a state of decomposition, consequently of molecular motion; and this motion, communicated to the molecules or rather to the atoms of the sugar, suffices to destroy the existing equilibrium and produce a new one. Sugar undergoes several different fermentations, which we shall mention under grape sugar.

Gum is tasteless, viscid, soluble in water, insoluble in alcohol. By the action of nitric acid it is converted into mucic acid, $C\_\{12\}H\_2O\_\{14\}\; \backslash cdot\; 2HO$, and not, like sugar, into oxalic acid.

Grape sugar is distinguished from cane sugar by being less soluble and less sweet, and also by being easily decomposed into brown products by heating with diluted alkalies. It is the only kind of sugar which undergoes fermentation; all the others are first converted into grape sugar and then ferment.

When the ferment is yeast, or vegetable fibrine in a state of decomposition, the vinous fermentation takes place in solution of sugar. The juice of the grape and infusion of malt both contain sugar and fibrine, and only require to be exposed to the air. But if the air be carefully filtered, by being made to pass through tubes filled with cotton, no fermentation takes place. Hence, and because certain low organisms appear in fermenting liquids, it is probable that the change is induced in the ferment, not as was supposed by the oxygen of the air, but by the introduction of the minute spores or germs of these organisms, which find an appropriate soil in the fibrine. The vinous fermentation is expressed by the equation—

We have already stated that other allied fermentations take place, by which propylic, butylic, amylic, and other alcohols are formed along with common alcohol, but in small quantity. In these, water is probably also one of the products.

When caseine is the ferment, sugar, at the temperature of from $70^\backslash circ$ to $90^\backslash circ$, is converted into lactic acid, or undergoes the lactic fermentation, which is this:—

With the same ferment at a rather higher temperature, lactic acid undergoes the butyric fermentation, especially if first neutralized. The change is—

There is another fermentation not yet fully understood, but which occasionally accompanies the lactic fermentation. In this, sugar disappears, and we find only gum and mannite. Mannite is $C\_6H\_8O\_6$, or $C\_\{12\}H\_\{16\}O\_\{12\}$; and as gum is $C\_\{12\}H\_\{14\}O\_\{11\}$, it is not easy to see where the mannite gets the excess of hydrogen. Probably some other substance is formed which has escaped notice as yet. It must contain an excess of oxygen, and is therefore probably an acid.

Sugar of milk has the same formula as dry grape sugar, and passes into that form by contact with acids and ferments.

In consequence of its being the only source of the compounds of ethyle, such as alcohol and ether, as well as of other alcohols and ethers, sugar is closely connected with the ethylic group of homologous compounds. We know nothing of its intimate constitution, except that, although resolved into alcohol and carbonic acid by fermentation, it does not seem to contain carbonic acid, nor consequently alcohol, ready formed. For by the action of permanganate of potash it is entirely converted into oxalic acid and water, which implies the addition to its elements of 18 eqs. of oxygen. Now, since carbonic acid is the ultimate product of the full oxidation of carbon in all organic compounds, it is hardly conceivable that oxalic acid, which is an inferior stage of oxidation, should be the sole product of the oxidation of carbon in a substance where any part of that carbon was already in the form of carbonic acid. We cannot, therefore, regard sugar as a compound of ethyle, and for the present must consider it as an appendix to the ethylic series, without knowing the precise relation between them.

Besides the kinds of sugar we have named, all of which pass into grape sugar, and in that form undergo fermentation, there appears to be a kind of sugar which is uncrystallizable, but which is probably rendered so by the presence of impurities. Molasses, or the syrupy uncrystallizable mother liquor from the cane juice is rendered uncrystallizable by the action of bases on the sugar during the evaporation, which produces two brown acids, glucic and melassic acids, the presence of which deprives a great part of the sugar of its power of crystallizing. This source of loss is now greatly diminished by carefully neutralizing with acid the lime used in clarifying the cane juice, and also by evaporating in vacuo, at a low temperature. The uncrystallizable sugar in molasses, and that in the residues of the cane, may be fermented, and yields the spirit called rum, which when new contains a trace of butyric acid, derived, no doubt, from a small portion of sugar having undergone the lactic and butyric fermentations. This butyric acid, on keeping, gradually combines with oxide of ethyle from a portion of alcohol, producing butyric ether, and the spirit thus acquires its very peculiar and fragrant odour, as well as the pleasant flavour of pine-apple. In Germany, rectified potato spirit, being first purified by rectification from all the oils which make the crude spirit so coarse in flavour, is converted into highly-flavoured rum by the addition of a very minute quantity of butyric ether artificially prepared. The presence of this ether in the pine-apple, to which it gives the flavour, is one of the few instances of the occurrence of a compound of ethyle as a natural product, and even here it derives its origin from sugar.

It is entirely in the form of sugar that all the starch in seeds, which usually contain it, becomes available in germination for the growth of the first stem and leaves; and it is in the same form that the starch in the food of man and animals enters the circulation, and is enabled to perform its proper function of supporting, by its slow oxidation, the animal heat. Here we see the extreme importance of the tendency in an organic substance like starch to undergo

Chemistry. isomeric transformation, for starch and grape sugar, differing only by 2 HO, may be held as isomeric. In the insoluble form of starch, this important element of food is stored up till required; and whether in germination or in digestion, for these processes are almost exactly the same, is rendered soluble and conveyed into the circulation or the juices of the animal or the plant. The reader will also observe that cellulose or woody fibre is most probably formed from sugar by another transformation of the same kind, in which water is separated.

3. Neutral Compounds; bitter, acrid, coloured, or yielding colours with ammonia.

In this group, which is a very numerous one, but formed of compounds generally limited to families, genera, or even species of plants, we include such bodies as the following. In their formation, the deoxidizing process has been carried a step farther than in the last group, for not only all the oxygen of the carbonic acid (or a quantity equal to it), but also a part of that of the water, has been given out. In the vegetative process, therefore, they are naturally produced after, and in all probability from, the substances of group 2.

We must observe that all of these formulae are not absolutely ascertained, and that few of these substances have been as yet fully studied. But the general character of their formulae is no doubt correct, and some, such as mannite, orcin, salicine, pectine, and phloridzine, are well known. As to their properties, mannite, as it is close to sugar in composition, so is it also in taste and mildness. Antiarine and elaterine are acrid and virulent poisons; quassine, salicine and phloridzine are bitter and febrifuge; pectine and pectic acid, a body closely allied to it, form the vegetable jelly of the juices of plants, such as the apple, currant, or carrot juices, and in fact are almost universally present. Hæmatoxyline is the red colouring principle of logwood, and is a type of a class hardly studied as yet. Orcine, which crystallizes beautifully, is a colourless principle found in some lichens, which, with ammonia, gives rise to the fine and permanent blue dye called archil, and is also a type of an interesting class of compounds recently investigated with great success by Schunck and others, particularly by Stenhouse. The rest are devoid of marked characters. This list is only a selection from a very large number of similar bodies, which, for the most part, have yet to be investigated, and on which, therefore, we need not dwell.

4. Oxygenated Volatile Oils, and Volatile Acids derived from them.

These we have already noticed under the head of the benzoic group. They are strongly marked in character, and in forming them the deoxidizing process has been carried still farther, as may be seen in the following list, which includes only the most important of them.

It will be seen that in every case the oil, or in that of cumarine the resinoid crystalline body, differs from the related acid only by 2 eqs. of oxygen. These oils and resinoid bodies are moreover fragrant (cumarine is the odorous principle of the tonka bean), and both they and the acids, especially the latter, approach to resins in properties, as well as in composition.

5. Volatile Oily and Fatty Acids, with the oils and fats containing them.

As the growing plant pushes its deoxidizing agency still further, fixed oils and fats are produced, a class of compounds both important and abundant. We already know their proximate constituents, namely, the acids of the formic or acetic series (excepting only the two acids lowest in the scale, which are not oily) on the one hand, and on the other the basic oxide of lipyle, or of glyceryle, or glycerine. It is unnecessary to repeat the list of acids, but we shall quote a few for illustration, with the oxide of lipyle.

We have already explained that fixed oils and fats consist of such acids as these, combined with oxide of lipyle, which, when the oil is saponified by boiling with potash or soda, separates in the shape of glycerine. The only exceptions yet known are those of spermaceti, which is composed of cetyl acid, isomeric with palmitic acid, combined with oxide of cetyl, $C\_\{16\}\; H\_\{32\}\; O$, and wax, which consists partly of free cerotic acid, partly of palmitic acid combined with oxide of cetyl, $C\_\{16\}\; H\_\{32\}\; O$; and in some kinds of wax of melissic acid and oxide of melissyle, $C\_\{18\}\; H\_\{36\}\; O$. Drying oils, such as oil of linseed, of walnut, &c., consist of oxide of lipyle, united to a peculiar oleic acid, which rapidly absorbs oxygen and dries up into a resinous mass or varnish. The neutral fixed oils and fats, composed as we have stated, are produced by a very high degree of deoxidation, when compared with the preceding groups, and are therefore formed in an advanced stage of the vegetative process. Hence they abound in seeds. In some of them, as in wax, the oxygen is reduced to a small fraction. But, taken as a whole, the group may be represented as sugar or starch minus oxygen, for sugar is $C\_\{12\}\; H\_\{22\}\; O\_\{11\}$, and the average composition of fixed oils and fats is represented by the proportion $C\_\{12\}\; H\_\{15\}\; O$, or $C\_\{12\}\; H\_\{15\}\; O\_2$. Hence, if fats are formed in the animal body from starch or sugar, as it seems certain they are, it must be by a process of deoxidation, although the general nature of the animal vital process is that of oxidation.

6. Resins and Camphors.

This group is small in point of numbers, but very abundant and widely diffused. In it the oxygen is still farther diminished, as may be seen in the following list.

Among these bodies isomerism and polymerism prevail

Chemistry. to a great extent. The general character of the group is that they are very inflammable, fusible, insoluble in water, soluble in alcohol, frequently acid, but weak acids. Camphor is very volatile.

7. Non-oxygenated Volatile Oils.

In this group, which is both numerous and widely diffused, no oxygen is left. Isomerism and polymerism predominate among them, so that the following short list includes the most frequent proportions.

Most of the volatile or essential oils of this class have a strong aromatic smell, and are indeed the source of the smell of the plants in which they occur. They are usually obtained by distilling the plants, flowers, seeds, or exudations from trees, with water, with the vapour of which the oils readily pass over. Many of them occur in the exudation from trees, especially from many of the coniferous, mixed with resins, constituting what is called turpentine or balsam. When this is distilled with water the resin is left behind, and in this way common resin is obtained along with oil of turpentine, from the turpentine of commerce. Balsam of copaiva and Canada balsam are turpentine oils which, containing more oil, are more fluid. These oils, exposed to air, absorb oxygen, and dry up into a resinous mass. Indeed most resins in composition are the oxides of the volatile oils which occur with them. Hence the use of oil of turpentine as a varnish and in oil painting. True varnishes, however, are solutions of resins in oil of turpentine, alcohol, naphtha, pyroxilic spirit, and other solvents.

The groups we have thus briefly sketched include all those vegetable compounds which consist of carbon, oxygen, and hydrogen only. But there are many and very important compounds which contain also nitrogen, and these we shall now endeavour briefly to indicate according to their natural affinities.

In order that these substances should be formed, the plant must be supplied with nitrogen, and this is done chiefly in the form of ammonia, or of nitric acid, both of which are conveyed to plants in the water which reaches their roots. In germination, however, there is another source of ammonia, namely, the decomposition of the albuminous or sanguineous matter in the seed. Now it is remarkable that in germinating plants, before true leaves have been formed, and, what is the same thing, in etiolated plants, or those grown in the dark, the juice is found to contain a large proportion of a crystallizable nitrogenized compound, namely, malamide or asparagine. It would seem, therefore, as if these were the first nitrogenized body produced by the plant. Its origin is obvious. The plant first produces some acids, and among these malic acid. This meets with ammonia, and, water being separated, malamide is formed, or we may simply suppose it to be produced from carbonic acid, water, and ammonia. In the first case, malic acid, $C\_4\; H\_4\; O\_4$, $2\; NH\_3$, losing 2 eqs. of water, yields malamide or asparagine, $C\_4\; H\_8\; N\_2\; O\_4$. In the second, 8 eqs. of carbonic acid, 4 of water, and 2 of ammonia, yield 1 eq. of malamide and 12 of oxygen; so that, whether malic acid be first formed, or malamide be directly produced, if it be formed from carbonic acid, ammonia, and water, it must be by deoxidation, as in all the preceding groups. This is no doubt the case in growing plants where it occurs, but in germination it may be formed, at least in part, by the decomposition of fibrin, albumen, or casein. All other compounds of amide, or those of imide, or, in short, all de-

rived from ammonia, are formed in growing plants in the Chemistry. same way, that is by deoxidation. There are thus formed several classes of compounds, neutral amides, such as asparagine or malamide, amygdaline, and similar bodies, of which white or colourless indigo, the source of the coloured indigo, is one; the volatile bases, which may be either amide, imide, or nitrile bases; and the fixed bases, which are either ammonium bases, or coupled compounds containing such bases. The formation of the volatile and fixed bases in the various forms of amide, imide, nitrile, and ammonium bases, we have already explained in their proper place. We shall here unite in one short list a few nitrogenized vegetable compounds of the kinds we have mentioned, all of which are produced in plants, either directly from carbonic acid, ammonia, and water, by deoxidation, or indirectly from ammonia acting on substances already formed by deoxidation from carbonic acid and water, as we have explained above with reference to malamide.

This list will give an idea of the composition of the greater number of crystallizable or volatile nitrogenized products found in plants. The number of fixed bases is very large, but they all resemble those quoted, which are the active principles of the plants in which they occur; morphine of opium, where it is associated with several other bases, cocaine, papaverine, narcotine, thebaine, and narcaine; quinine of cinchona bark, which also yields cinchonine and quinine, and strychnine of nux vomica, which also contains brucine. Besides these, there are veratrine in veratrum, atropine in belladonna, aconitine in aconite, digitaline in digitalis, daturine in datura, colchicine in colchicum, and others of less interest. Nicotine and cocaine, the active and very poisonous principles of tobacco and hemlock, are in all respects analogous to the volatile bases of the ethylic and benzoic series already noticed. Amygdaline is a bitter substance, peculiar to the bitter almond, which, in contact with water and the albuminous compound of the almond, undergoes a peculiar fermentation, and is resolved into several bodies not entirely known, but among them are hyduret of benzoyle and hydrocyanic acid, which together form the oil of bitter almonds of commerce.

White indigo is the substance present in the juice of indigofera, or at all events is produced by the action of ammonia on a substance there present, and as soon as it is formed, attracts oxygen, and is converted into indigo, $C\_\{16\}\; H\_\{10\}\; NO\_2$, the white indigo losing 1 eq. of hydrogen. Indigo, when acted on by different reagents, yields a large number of very interesting products of decomposition. With 2 eqs. of oxygen it forms isatine, $C\_\{16\}\; H\_8\; NO\_2$, and this yields a whole series of substitution products, in which chlorine and bromine are substituted for its hydrogen, besides many derivatives from these. When heated with potash, isatine yields carbonate of potash, hydrogen, and aniline, or phenylamine, $C\_6\; H\_5\; N$; so that the products of decomposition of indigo and its derivatives fall into the phenylic series. By the action of nitric acid, indigo is converted into two acids, according to the strength of the nitric acid. One of these belongs to the salicylic series, and is nitrosalicylic or indigotic acid, $C\_\{10\}\; H\_8\; NO\_3$; the other is a yellow, intensely bitter, crystallizable acid, which forms with potash a salt nearly insoluble. It has been called carbazotic, picric, nitropicric, and nitrophenic acid; but it is simply hy-

Chemistry. drated oxide of phenyle, or carbolic acid, $C\_\{12\}H\_8O,\; HO$, in which 3 eqs. of hydrogen are replaced by 3 of nitrous acid, $C\_\{12\}H\_3NO\_3$, } $O,\; HO$. This again connects indigo with the phenylic series. The same acid, which might perhaps be best named trinitrocarbolic acid, is formed in many other cases of the oxidation of organic compounds by nitric acid, as, for example, from aloes, and from several resins. But we can only indicate the existence of the numerous derivatives of indigo in this place.

Caffeine is found, not only in coffee, but also in tea; and in Paraguay tea or guarana, used for the same purpose as tea or coffee. It has considerable analogy in composition, and also in the products of its decomposition, with certain animal products connected with uric acid, and it is probably in virtue of this that it acts as a kind of stimulant.

There are one or two volatile oils, such as those of garlic and assa-fetida, which contain sulphur. One, if not both of these, is sulphuret of allyle, $C\_4H\_7S$; allyle being a radical either isomeric or identical with propionyle, the radical of propylic acid. Oil of mustard and oil of cochlearia are the sulphocyanide of allyle, $C\_4H\_7S,\; C\_2N\_2S\_2$, and contain, therefore, both sulphur and nitrogen. In the formation of these oils in plants, carbonic acid, water, ammonia, and sulphuric acid are employed, and, as in all other cases, oxygen is given out.

There are various substances of no great importance which we have not specifically mentioned, but which all find their place in one or other of the groups of vegetable products we have named. Such are various colouring matters, and various volatile and crystallizable bodies, allied to the volatile oils.

But there remains one most important group of compounds formed by plants alone, and essential to the food of animals. This is the group of the albuminous or sanguigenous bodies, the only substances from which the animal system can form blood. These are three in number—albumen, fibrine, and caseine; and they are of all vegetable products the most complex, containing not only the five elements already mentioned (carbon, hydrogen, nitrogen, oxygen, and sulphur), but also—as essential constituents, without which, though in small proportion, they cannot even exist—mineral salts, especially phosphates. Moreover, the number of atoms in these compounds is far larger than in any we have mentioned, amounting, indeed, to hundreds. It is evident that one chief object of vegetation is the formation of these substances, and also that from their complexity they must be the last formed; and in fact they abound in the seeds, tubers, and other parts capable of reproducing the plant, and after the formation of which the plant dies down, either for a season or permanently. They are always accompanied by the two other classes of compounds, which with these constitute not only the first food of the young plant, but the food of animals, namely, starch, sugar, gum, or cellulose, on the one hand, or oils and fats on the other; these two constituting the respiratory part of animal food, as the three more complex products constitute the sanguigenous part of that food.

We may either conceive these compounds to be formed directly from carbonic acid, water, ammonia, and sulphuric acid, with the separation of oxygen; or, what is much more probable, we may suppose them formed from compounds already of a certain degree of complexity, such as sugar, with the aid of ammonia and sulphuric acid, or finally from sugar or some such substance, acted on by some nitrogenized compound, such as malamide. The precise steps of the process are unknown, but it is obvious that, whether formed directly or indirectly, they must be ultimately derived from the original food of plants—carbonic acid, water, ammonia, sulphuric acid, and the mineral salts derived from the soil—and that this cannot take place without the separation of a

large amount of oxygen. The nearest approach we can make to the formulae of these compounds is the following:—

Albumen and fibrine..... $C\_\{124\}H\_\{120\}N\_\{22\}S\_2O\_\{49\}$
Caseine..... $C\_\{285\}H\_\{225\}N\_\{26\}S\_2O\_\{49\}$

besides the phosphates. It is very difficult to ascertain with certainty such formulae; but these, with respect to the proportions, represent the results of analysis; and as to the absolute number of atoms, we assume 2 eqs. of sulphur, because we find that these bodies contain sulphur in two different states, which implies at least 2 atoms.

These substances are essential to the growth of plants, as the vegetable cell, even if formed of cellulose alone, cannot be formed without the presence of one of these bodies. But in fact they also take part frequently in the formation of the cell wall, and the primordial utricle, or lining membrane of the cell, seems to consist chiefly of them.

Vegetable fibrine and albumen are isomeric; or, at least, the best analyses have not been able to show more difference between them than between different analyses of one of them. When dissolved in the vegetable juices, the fibrine coagulates spontaneously, the albumen when the liquid is boiled, and the caseine on the addition of an acid after the coagulation of the albumen, if all three or two of them be present together. In the solid form they are found, fibrine in the seeds of grasses and cerealia, albumen in those of nuts, almonds, and the kernels of stone fruit; and caseine in the seeds of the leguminosæ. The fibrine in grain is accompanied by starch and a little sugar, and in some cases also a little oil; the albumen is almost always associated with oil, as in almonds, walnuts, hazelnuts, and many other seeds; and in the seeds of leguminosæ the caseine is mixed chiefly with starch. In edible roots and tubers there is the same mixture. In the potato, fibrine and starch; in carrots, turnips, &c., fibrine, sugar and pectine; in pulpy fruits, fibrine, starch, sugar, and pectine, are the chief ingredients. In short, the vegetable cannot live and grow, and bear fertile seeds, without at the same time providing food for animals, not only in seeds, roots, and tubers, but also in leaves, leaf-stalks, leaf-buds, flower-buds; and, indeed, every part which is full of juice. And this leads us to consider the composition of the animal body.

But first we must remind the reader, that we have seen that plants live on carbonic acid, water, ammonia, sulphuric acid (in the form of sulphates), and phosphates, chlorides, alkalies, and other mineral matters—the three first compounds derived from the air, the rest from the soil. Secondly, that in germination, which takes place in the dark, oxygen is absorbed along with water, the albuminous matter in the seeds enters into decomposition, and becomes a ferment, by which means the starch is converted into sugar and dissolved, along with the greater part of the albumen, which is also rendered soluble by the fermentative action of the decomposed portion, while malamide appears, probably derived from the same source, and carbonic acid is given out. And thirdly, that as soon as the first leaves are formed, and light obtains access, the true, proper, vegetative process begins, in which, from the articles of their food above enumerated, plants produce acids, neutral substances, mild or acrid, or bitter or coloured, bases, oils and fats, resins, volatile oils and volatile acids; and finally, sanguigenous compounds, the formation of all of these being dependent on light, and being accompanied by the continual evolution of oxygen. Such is the nature of the vegetative process, carried on in virtue of the energy derived from the solar rays, and it is marked by three essential characters; the first is, that light is absolutely essential to it; the second is, that it is essentially a constructive process, in which the least complex compounds forming the food of plants are built up into more and more complex forms, till at last the sanguigenous bodies, the most complex of all, are produced; and the third, that it is a process of deoxidation of the food

Chemistry. of plants to a greater or less extent, and occasionally complete; but a deoxidation of a kind unknown in the laboratory, where we can only deprive bodies of oxygen by using others which seize and retain the oxygen, whereas the plant retains the carbon, hydrogen, and nitrogen, &c., and liberates the oxygen. The power required for this must be prodigiously great, and must far surpass that of our most energetic agencies; yet the vegetable cell effects it, without the help either of heat or of any other appliance. There can be no doubt that light is the source of this astonishing power; but, be this as it may, it is characteristic of vegetation.

Let us now turn to the animal system. We have stated that the food of animals is composed of two kinds of substance, the sanguineous, and the respiratory. Both must be rendered soluble in order that they may enter the circulation, and this is effected in the process of digestion. It is remarkable, that this first animal process agrees with the first vegetable one, namely, with germination. In mastication, the food is divided and mixed with saliva, oxygen being added from the air, and being inclosed by the viscosity of the saliva. In the stomach, the gastric juice is added, which contains a little free acid, some albuminous matter dissolved, and the usual animal salts. The dissolved albuminous matter, acting as a ferment, at the temperature of the body, renders the sanguineous portion of the food soluble, while either the same ferment, or, as is now believed, one peculiar to the saliva, converts the starch, if any be present, into sugar, so that it also is dissolved. The dissolved sanguineous matter is gradually converted into blood, and from that fluid the various tissues are formed. The fibrine of flesh and the albumen of the blood are identical with vegetable fibrine and albumen; but the fibrine of the blood and the albumen of eggs are different. The two former, as we have already stated, are $C\_\{216\}H\_\{120\}N\_\{27\}S\_2O\_\{16\}$. Now, when one of them is employed to produce blood fibrine, or hematofibrine, which is $C\_\{228\}H\_\{120\}N\_\{27\}S\_2O\_\{22\}$, it may be in this manner that 2 eqs. albumen (or fibrine) with 2 eqs. of water are converted into 1 of hematofibrine, 1 of gelatine, $C\_\{22\}H\_\{22\}N\_\{12\}O\_\{12\}$, and 1 of cholic acid, one of the acids of bile, $C\_\{32\}H\_\{45\}NS\_2O\_4$. Or it may be that 2 eqs. albumen and 4 of water yield 1 eq. hematofibrine, 1 gelatine, 1 cholic acid, the other acid of bile, $C\_\{32\}H\_\{45\}NO\_\{12\}$, and 2 of sulphuric acid. Lastly, these two processes may be combined, so that—

By this it will be seen that the formation of blood fibrine is accompanied most probably by that of gelatine, necessary for membranes and bones, and that of the chief constituents of bile. Indeed, blood fibrine may be regarded as ordinary fibrine and albumen, half converted into gelatine and bile, for 1 eq. of it with 18 of water, may yield 3 of gelatine and 1 of cholic acid.

This example will illustrate the fact, that while the animal body cannot produce any sanguineous body from food in which none of them is present, it can transform one into the other, as albumen into fibrine, fibrine into albumen, or both into casein, or, as we have just seen, albumen or fibrine into hematofibrine; but that, where such changes are not isomeric transformations, they are a commencement of destruction, and their production, even if more complex, as is the case with hematofibrine, is not a pure case of construction as in the plant, but one in which with one more complex body others much less complex are produced. In the same way, when the food contains casein, as milk does, and when hematofibrine is formed from casein, it is accompanied by the production of chondrine, the substance of which cartilage, so necessary to the young animal, is formed.

1 eq. casein with 10 of water may yield 1 of hematofibrine, Chemistry. and 1 of chondrine, $C\_\{72\}H\_\{38\}N\_8O\_\{32\}$.

In this way, then, the animal body, if supplied with albumen, fibrine, or casein, can produce from them, or any one of them, the materials necessary, in addition to themselves, for the formation of blood, muscular fibre, membranes, cartilage, bones, &c., even although these should be in part more complex than the food. But it is beyond the power of the animal body to produce any one of these substances from food which does not contain at least one of them, and for that food it is absolutely dependent on vegetables. Although we do not know the details, there can be no doubt that the formation of nervous matter or cerebral substance, is effected on the same principles. In this stage, water, as we have seen, is taken up, but oxygen is not required, nor is it given out. It is after the formation of blood and of the tissues, by some such changes as those we have specified, that the true vital process peculiar to animals begins. This process is exactly the reverse of the vegetative one; for while the latter, acting by deoxidation of the least complex among compounds which are its food, constructs or builds up those that are more and more complex, till it produces the most complex of all, that is, the food of animals, giving out during the whole succession of changes a large amount of oxygen; the former takes up, in respiration, a large amount of oxygen, acts by oxidation, and beginning with the most complex compounds, destroys them, and breaks them up into such as are less and less complex, ending at last with the very same substances as form the food of plants, and giving out all the time, in respiration, carbonic acid, in quantity equal, or very nearly so, to the oxygen absorbed, the difference of oxygen being given out as water.

These two great processes are exactly opposed, and, in point of fact, exactly balance each other, so that the air remains always of the same composition, although animals are constantly removing oxygen and replacing it by carbonic acid, because plants, on the other hand, are constantly removing carbonic acid, and replacing it with oxygen.

As to the products of the oxidation and destruction of the tissues in the animal body, they are very numerous, and only partially known. Such bodies as gelatine, chondrine, and the like, are among them, and it is to be observed that these products of destruction of the tissues, even when they approach in complexity the sanguineous bodies, are not capable of forming blood. Thus gelatine alone can never support animal life, not being convertible into blood, as fibrine, albumen, and casein are.

After such complex products of oxidation as gelatine and chondrine, there come probably a number of intermediate compounds, which, however, are as yet unknown. But at a certain stage in the oxidizing process bile is formed, which is composed of the two acids, cholic acid $C\_\{32\}H\_\{45\}NS\_2O\_4$, and cholic acid $C\_\{32\}H\_\{45\}NO\_\{12\}$, both combined with soda. We have already noticed these acids under the series of glycine, which may be formed from cholic acid, while taurine, a compound containing all the sulphur of the bile, is obtained from cholic acid. In both acids, these substances are coupled with cholic acid, $C\_\{45\}H\_\{39\}O\_\{39\}\; =\; C\_\{45\}H\_\{39\}O\_3\; \backslash cdot\; HO$.

Cholic acid, by boiling with water, is converted into a resinous acid, isomeric with the anhydrous cholic acid, which is called choloidic acid; and when this is further boiled with acids, it yields a very insoluble resin, called dyslysine, which is choloidic acid minus 3 eqs. of water, or $C\_\{45\}H\_\{32\}O\_4$.

Chemistry. Such are the chief products of bile. It is probable that all the sulphur of the tissues takes the form of choleic acid, that is, passes into the bile, before being expelled from the body as sulphuric acid by farther oxidation. The bile, which is mixed with the digested mass of food as it leaves the stomach, is apparently reabsorbed after producing some important effect in digestion, or in the conversion of the food into perfect chyle or blood, and being thus thrown into the circulation, it is destroyed by oxidation. We have already seen how the acids of bile may be formed from albumen or fibrin, along with hematofibrin, and also from the latter body by the addition of water; but they are also formed by oxidation. For example:—

Or cholic acid may be formed, beside various other processes of oxidation, from chondrine and gelatine, as follows:

In all these three examples, it will be seen that the biliary products are accompanied by such as are urinary, urea, uric acid, and water, and also by carbonic acid. There are, no doubt, many other modes of decomposition and intermediate products, and some of these products we know, as for example, kreatine $C\_8H\_\{21\}N\_3O\_4$, and hippuric acid $C\_\{18\}H\_\{21\}NO\_4$, the former being found in the juice of muscle and in urine, the latter in urine. Now we can easily see how they may be formed, from gelatine for example, for—

Any of these substances, fully oxidized, will yield carbonic acid, ammonia, water, and, if sulphur be present, sulphuric acid. Thus—

When uric acid is formed, it must, in warm blooded animals, be farther oxidized, so as to yield soluble compounds, or else by its insolubility it is deposited, producing calculous disease. In general, all the uric acid but a very small portion is thus oxidized, and the results are (uric acid = $C\_\{16\}H\_4N\_4O\_2$)—

or

It will be seen that the less complete oxidation of uric acid produces, besides urea, oxalic acid and allantoin. Oxalic acid does occur, and when it meets with lime is apt to form a calculus of the insoluble oxalate of lime. Allantoin is found in the allantoinic fluid or foetal urine, and in the urine of very young animals, in whom respiration, and consequently oxidation, is somewhat imperfect. But when the oxidation is complete, urea or ammonia, carbonic acid, and water, are the products.

Many more examples might be given of the mode in which the changes in the animal body may be supposed to be effected, and in some such modes they must be effected. But

Chemistry. as we do not know all the different stages of the oxidation, nor all the products, we only give these representations as possible or probable, in regard to the details, although in principle certain; for all the facts we know prove that in the animal body every change is the result either of oxidation, more or less complete, or of a transformation with or without the addition of water, but never, as in plants, of deoxidation with liberation of oxygen. On the contrary, oxygen is constantly absorbed, carbonic acid constantly given out, and the complex substance of the tissues is constantly broken up into less and less complex compounds, till the final result is the food of plants.

It is in the very performance of their proper functions that the tissues are oxidized, and as fast as a portion of any tissue, being oxidized, is, while doing its work, broken up, the blood replaces it by a new portion derived from the food. At the same time the blood carries off the broken up or effete tissues, and out of them produces, according as the oxidation is more or less advanced, gelatine and the like, bile, juice of muscle, or urine. It is the blood which conveys to every part the oxygen necessary for the destruction of the tissues, that is, for the performance of their functions, which is their destruction. This is the reason why great efforts of any kind, muscular or otherwise, are attended with great waste of substance, and require an increased supply of food. It is the blood also, which, after it has carried the oxygen to act on the tissues, so as to enable them to perform their part, and in so doing to be destroyed, pro tanto; and after it has replaced the effete tissues which have, in becoming effete, fulfilled their office, by fresh material, also carries away not only the solid and liquid products of oxidation, but also the carbonic acid so largely formed, which it conveys to the lungs, and there exchanges for oxygen. But there is another source of carbonic acid besides the oxidation of the tissues. This is the oxidation or slow combustion of the respiratory food, of the sugar, &c., and the oils or fats, some of which are always present in food, as provided by Nature. And it is by this oxidation of the carbon and hydrogen of these matters as well as those of the tissues that the animal heat is kept up, without which none of the vital processes could be carried on. Now both the oxygen required for this purpose and the carbonic acid produced are carried to and fro by the blood, which at the same time has to perform three most essential functions, or rather four; first to enable the tissues, by oxidizing them, to perform their functions; secondly, to repair the waste of tissue; thirdly, to concoct or assist in concocting, out of the first products of oxidation, the various secretions and excretions; and lastly, to effect by the oxygen it carries the oxidation also of the respiratory food, or the production of animal heat, and to convey to the lungs the carbonic acid which is formed, and exchange it for oxygen.

The blood is admirably fitted for these purposes. It consists of water, holding in solution, besides salts, albumen and fibrin, and in this solution are suspended the red and white blood corpuscles. These seem to have much to do in the conveyance of the gases, which probably adhere to them. At all events, their colour changes from dark to bright red, according as the blood is loaded with carbonic acid (venous), or with oxygen (arterial). But that the blood may absorb so much carbonic acid, and yet give it out easily, it must be alkaline, and yet not too alkaline, which would retain the carbonic acid. This is accomplished by the presence of the peculiar phosphate of soda, tribasic with 2 eqs. of soda, which, acid in composition, is alkaline in character; namely $PO\_3$, $2\; NaO,\; HO$. It absorbs carbonic acid as well as carbonate of soda, and gives it off much more readily and completely. For another reason, the blood must be alkaline; for if it were acid, or even neutral, neither fibrin nor albumen could remain dissolved, nor could the necessary changes take place. Moreover, it must be phosphate of soda in the blood, and not that

Chemistry. of potash, for potash tends to form the salt $\backslash text\{PO\}\_4\backslash text\{K\}$, $\backslash text\{K\}\_2\backslash text\{O\}$, $2\backslash text\{H\}\_2\backslash text\{O\}$, which is an acid salt. This very salt exists in the juice of muscle, which is almost in absolute contact with the blood, and in the gastric juice, but never in the blood, though the fluids which contain it are only separated from it by the most delicate possible membrane. The presence of these two fluids, blood and juice of muscle, one alkaline, the other neutral or slightly acid, in such close proximity, is probably connected with the existence of currents of electricity in the body, which have been demonstrated especially in muscular action. Two such fluids, with a porous solid between them, will always create an electric current.

It is obvious that for two chief reasons the supply of oxygen to the animal body must be abundant. It has to oxidize and destroy the tissues, that is, to enable them to perform their functions; and it also has to consume the respiratory food, and ensure the regular supply of animal heat. The effects of a deficient supply of oxygen are easily seen. The tissues being less thoroughly oxidized, the blood must become loaded with intermediate products. Among others, uric acid must occur, and give rise to calculous and gouty diseases. And this is the case. But deficiency of oxygen may arise from two causes; either from an absolute defect of oxygen, from diminished respiration, sedentary habits, which diminish respiration, and other analogous causes, or, what is more common, excess in food, whereby a full ordinary supply of oxygen becomes insufficient. This is the principal reason why excess in eating causes disease. Another cause may contribute to this, namely the deficiency of alkaline salts in the food, which is apt to occur if too much animal food and too little vegetable food be taken. The alkaline salts of vegetable acids are converted into carbonates in the system, and the presence of these greatly promotes oxidation; hence their absence or deficiency is injurious. For the same reasons, the strong wines of the south, which contain hardly any tartar, that is, acid tartrate of potash, are apt to cause calculus and gout, if habitually used; while on the Rhine, where wine is the only beverage, but the wines are weaker, and strongly charged with tartar, these diseases are unknown, save as imported.

When from any cause the supply of oxygen in proportion to the sanguineous food is too small, the animal body possesses the singular power of obtaining a supply of oxygen from sugar or starch, which, by losing oxygen, are converted into fat. In this way the animal body can produce fat from sugar, like the plant, but the oxygen, instead of being given out, is employed to make up for the deficiency of that element. At the same time, as fat is formed along with it, the animal becomes fat. This explains why stall-fed animals fatten sooner than such as are outside; the former having a deficient supply of oxygen, that is, deficient respiration, compared with the latter; it also explains how the most fattening food is such as contains, with a certain proportion of sanguineous matter, a large amount of starch or sugar. In this point, the practice of experienced feeders is entirely according to theory.

The food of animals and of man must contain a due proportion of sanguineous and respiratory matter. For man, the best proportion is that found in grain and in milk, which are almost the only articles of food on which, alone, that is, on either of them without addition, life and health can be sustained. In them, the proportion is 1 part of sanguineous matter to $4\backslash frac\{1\}\{2\}$ or 5 of respiratory matter. Lean meat contains too little respiratory matter, although it contains a good deal of fat or oil diffused through it. Pease and beans and cheese are also too rich in sanguineous matter, while potatoes and rice contain a great excess of respiratory food, which is the case also with very fat meat, such as bacon, &c. Hence, we usually add potatoes or rice to lean meat, and bacon or fat pork to pease and beans. Working men fed on potatoes require a very large supply of that food, so poor

in sanguineous matter, for it contains but 1 part of fibrine Chemistry. to 8, 10, or 11 of starch, to supply the daily waste of tissue, which starch cannot supply. With a large amount of potatoes they may thrive, but in that case much starch passes off unchanged in the excreta. On the other hand, when men are fed exclusively on flesh, they require a very large supply of it, as do carnivorous animals, because, besides supplying the waste of tissue, a large part has to be oxidized to yield the animal heat, for which it is far less fitted than starch. To burn off, or oxidize, so much sanguineous matter requires a very large supply of oxygen, and this is the reason why carnivorous savages must lead a life of continual exertion, as they generally do in hunting for their food. For the same reason, carnivorous animals in confinement are always in motion except when asleep. The exercise increases the frequency and depth of respiration, and thus promotes oxidation. Lastly, the respiratory food is required, not merely to yield animal heat by its slow combustion, but also to yield fat by its deoxidation, which fat or oil assists in the formation of tissues from sanguineous matter.

Such, in a very few words, are the general laws which regulate the food of animals. As to the excreta, they consist, for the most part, of the gases and vapours formed by oxidation, or carbonic acid and water, excreted by the lungs and skin, and ammonia, chiefly excreted in the form of salts in the urine; of the soluble salts which have served their purpose in the economy, or are the result of oxidation, as phosphates of potash, soda, and ammonia, sulphates, chlorides of potassium and sodium, &c.; of soluble organic excreta, as urea and kreatine, with traces, in man, of uric and hippuric acids, which, with the soluble salts and other less known substances, are excreted in the urine; and, finally, of the insoluble salts, as phosphates of lime, magnesia, and iron, along with any undissolved or insoluble matter in the food, as woody fibre, excess of starch, resinous matters, and certain fetid products of the oxidation of sanguineous bodies which are also insoluble, but have not been fully studied, all of which insoluble matters form, with a certain amount of water, the solid or semisolid excreta. In cold-blooded animals, such as reptiles, where oxidation is very imperfect, there is no liquid urine; the whole excreta form one mass, which, in serpents for example, consists partly of undigested matters, bones, hair, feathers, and the like, but chiefly of a white salt, urate of ammonia. The same is true, to a great extent, of carnivorous birds, such as sea fowl, whose excreta consist of the bones of fishes and urate of ammonia. Exposed to the air, they became partly altered, the uric acid being in part converted into oxalic acid and other products, and the result is guano, which owes its value as a manure to the phosphates it contains, to the salts of ammonia, and to the uric acid, which is a source of ammonia when further decomposed.

We have seen that the animal process is the reverse of the vegetable one; that it begins where the other ends, with the sanguineous and respiratory substances, and ends where the other begins, in the excreta, gaseous, liquid, and solid, which amount to carbonic acid, ammonia, water, sulphuric acid, or sulphates, phosphates, chlorides, &c., in short the very substances which are the food of plants. Urea, on being expelled in the urine, is soon transformed into carbonic acid and ammonia; for $\backslash text\{C\}\_2\backslash text\{H\}\_4\backslash text\{N\}\_2\backslash text\{O\}\_2\; +\; 2\backslash text\{H\}\_2\backslash text\{O\}\; =\; 2(\backslash text\{NH\}\_4\backslash text\{CO\}\_2)$. This change takes place under the influence of a ferment, and the ferment in the urine is the mucus suspended in it; for if this be at once separated by filtration, the urea does not ferment.

We have noticed the composition of the best known among the constituents of the animal body, and the products of the various changes which occur during life. The various secretions are formed chiefly of water, with certain animal substances dissolved, generally in small quantity, and

Chemistry. certain salts. Milk, which is important as a naturally prepared food, adapted more especially to the growth of the young animal, contains a considerable amount of casein dissolved, which is identical with the casein of peas, beans, and other leguminous plants. The respiratory food in milk consists of fat or oil, called butter, which we have already described under the head of the volatile oily acids of the acetic series, and of a peculiar form of sugar, called lactine or sugar of milk, the composition of which is $C\_\{12\}H\_\{22\}O\_\{11\}$, the same as that of dry grape sugar, with which, however, it is only isomeric, not identical.

When milk is left to itself, the oil rises to the surface as cream. But soon after, the casein enters into decomposition, becomes a ferment, and induces in the sugar the lactic fermentation, by which lactic acid is formed. This goes on till so much free acid is formed as checks the fermentation; which is, however, immediately renewed if the acid be neutralized by soda or chalk. The coagulation of the casein instantly follows the formation of lactic acid, that is, as soon as the alkaline reaction proper to milk is neutralized, and it becomes distinctly acid. This is why milk when sour is found coagulated. The other method of coagulating milk, by means of rennet, an infusion of the lining membrane of the calf's stomach, depends on the dissolved sanguineous matter in the rennet acting as a ferment, and coagulating the casein rather by an action of contact, as it converts starch into sugar, than by the formation of lactic acid, for the coagulation by rennet is complete before the milk becomes perceptibly acid. The whey or serum, separated from the curd, contains all the sugar, various salts, especially phosphate of potash, and a small quantity of an albuminous matter not fully studied.

Cheese is the coagulated casein, more or less perfectly separated from the whey, and containing more or less of the oil or butter, according as made from cream, from milk as drawn, or from skimmed milk, when only traces of oil are left. The different flavours and qualities of cheese also depend in part on the pastures, in part on the mode of manufacture followed in different places.

The gastric juice has been mentioned as resembling the juice of muscle, in being acid from the acid phosphate of potash, $PO\_3$, $KO$, $2HO$. Its power of dissolving fibrin, albumen, &c., depends on its being acid, and on the presence of a ferment, or dissolved sanguineous matter in a state of decomposition. We can imitate this solvent action out of the body by placing flesh in a solution of 1 part of hydrochloric acid in 1000 of water at the temperature of the body. A small part of the flesh, or of some of the membranes about it, is first dissolved, and by degrees the whole is softened and converted into chyme, in which the greater part of the fibrin is dissolved, and the oil suspended, giving it a milky aspect.

The pancreatic juice seems to have a special action in rendering fats and oils soluble, or at least capable of being absorbed. It is said to set free the acids which are more soluble than the fats themselves. Its composition is not perfectly known, but it resembles that of the saliva, which, as we have seen, is supposed to be the special agent in the conversion of starch into sugar. The bile and the urine we have already described. The latter is a solution of urea with salts, and certain little known products of the changes in the body, which are commonly grouped together as the extractive matter, besides traces of kretaline, kretinine, a base differing from kretine in containing 4 eqs. of water less, hippuric acid, and uric acid. In the extractive matter of cow's urine there have been found a substance, taurylic acid, very analogous to, and probably homologous with, carbolic acid, also carbolic acid itself, and two other acids, both volatile and resembling carbolic acid. Carbolic acid is $C\_6H\_5O$, taurylic acid $C\_\{14\}H\_\{18\}O\_2$, damaluric acid is $C\_\{14\}H\_\{18\}O\_2$, and damalic acid $C\_\{22\}H\_\{24\}O\_4$. All these are, like carbolic

acid, which occurs in tar, products of incomplete oxidation Chemistry. or combustion, as tar itself is; and their occurrence is a strong illustration in favour of the view which regards the body as a furnace, in which the food is the fuel, the carbonic acid, water, and ammonia are the products of complete combustion, and the soot and ashes combined are represented by the solid and liquid excreta.

The juice of muscle or of flesh is remarkable for being either neutral or acid, although in close proximity to the alkaline blood with only a thin and permeable membrane between them; also for containing potash where blood contains soda; and for the occurrence of kretine in much greater quantity than in the urine. It also contains albumen, coagulable by heat.

Uric acid has been much studied, in reference chiefly to its products when oxidized. It yields a very large number of remarkable derivatives, but for the present these, although interesting to the chemist, do not admit of any application so important as to render it necessary to detail them here. We have stated what the most practically important results of its more or less complete oxidation are, and in the human body it is only a trace of it which, from its sparing solubility, can enter the urine, the rest being converted into urea chiefly with allantoine and oxalic acid in one case, and in the more frequent one, carbonic acid and water, or carbonic acid and ammonia.

Now that we have traced the two great processes of organized life, the vegetable and animal, and have seen how they are mutually related, and how they balance, and are dependent on each other, we can easily understand the principles which ought to regulate agriculture. Vegetation is entirely a chemical process, and subject therefore to chemical laws, although the vital force which modifies their action be beyond our reach. The plants, as we have shown, nay, the single cell, can effect changes which are beyond the power of the chemist, with all his appliances of heat, electricity, and chemical agents. But still these are chemical changes. If, then, vegetation be a chemical process, the foundation of a rational agriculture must be laid in chemistry.

Now the food of plants consists of two parts, the atmospheric and the terrestrial. The former is but little in our power, and therefore it is to the latter, that is, to the soil, that we must direct our attention.

We have already specified those elements which the plant derives from the soil, and which are fortunately present in almost every soil. They are sulphur, in the form of gypsum, phosphates of lime and magnesia, oxide of iron, carbonate of lime, potash, soda, silicic acid, and in some cases iodides and fluorides. But some of the most essential, especially the phosphates, are often present in so small a proportion as not to suffice for the purposes of agriculture.

The first principle that may be laid down is this, that to render a soil generally fertile, all the mineral substances which are really essential to the crops must be present, and in sufficient quantity. The absence of one essential ingredient renders the others, however abundant, totally useless. No plant can grow in a soil destitute of phosphates, or of sulphates, or of alkalies. Some plants may grow, indeed, in the absence of one important mineral element, as, for example, of potash, but only on condition that some other analogous substance, such as soda in some cases, and lime in others, replaces it. There is no means of remedying the absence of phosphates or of sulphates, because nothing else can replace them.

The next point to be noticed is, that the crops raised for food in agriculture exhaust the soil of certain constituents to a far greater extent than any natural vegetation can do. In all cultivated plants, the edible part, whether seeds, roots, or leaves, has been brought to a most unnatural development, for the sake of the food they yield. Now, a great

Chemistry. part, and the most valuable part of that food consists of sanguineous matter; and this, as we have already stated, cannot exist without a certain amount of phosphates of lime and magnesia. There is no such thing known as albumen, fibrin, or casein, without phosphates. Now of all the useful constituents of the soil, the phosphates are the least abundant, so that in many cases a cultivated crop or two exhausts the soil of all its available phosphates. The same thing is true of any other constituent required for the cultivated crop and not very abundant in the soil.

The next principle is this, that if the soil by nature can produce an average cultivated crop, this degree of fertility can only be kept up in one way, that is, by restoring to the soil exactly what we have removed of mineral matter in the crop. For this is another difference between natural vegetation and cultivated crops, that in the latter case a great part of the crop, and that by far the richest in mineral matter, especially in phosphates, is exported or abstracted from the land. This is the principle on which the use of manures depends, for manures are nothing but different means of restoring to the soil what we have extracted from it.

The next remark we would make is, that common or farm-yard manure is evidently, from its origin, a most excellent form of manure; for, consisting of straw which is part of the crop, and of the solid and liquid excreta of animals fed on another part of it, the latter, as we have explained, represent the ashes of that part of the crop on which the animals have fed. But the ashes of any crop are exactly the mineral substances which it required and extracted from the soil, and in the very proportions required; so that the ashes of any crop must be the best manure for another crop of the same, provided the first crop was a good one.

Unfortunately only a part of the crop is consumed on the spot and converted into manure, and therefore the farmer must have recourse to other modes of restoring what has been removed, or of supplying what may be deficient.

The most natural quarter to which he can apply is the excreta of large town populations, both of men and of animals, which have consumed that part of his crops which was exported. But hitherto this invaluable manure has been, for the most part, carefully washed down into the sea, and thus lost for ever to the land, except in so far as a part of it is recovered, at a most disproportionate cost, in the shape of sea-ware, and still more of guano. For the town manure, conveyed into the sea, there promotes the growth of sea plants of all kinds; on these small animals feed; on these again fish; on these larger fish, and on these perhaps sea-fowl, whose excreta, after ages of exposure, are imported as the most precious manure, while at the same moment we are recklessly casting away the very best manure for our crops, being the ashes or mineral part of these crops, contained in the excreta of towns.

Another manure, imported also at great cost, and like guano very likely to fail us ere long, is bones or bone dust. As we have already said of guano, in speaking of uric acid, that its value chiefly depends on the phosphates it contains, so is the value of bones measured by the phosphates. But the animals whose bones we purchase feed on our crops, or on others similar to ours, and every particle of phosphate in them has been extracted from the soil in these crops. In bones, therefore, we are only restoring, as in guano, at great cost, a part of what we have removed from the soil. And what we gain in bones is lost to the countries exporting them.

In short, every particle of manure ought to be carefully preserved, whether on the farm, or in large cities where it is now thrown away to the value of millions annually. Experience and observation have taught this truth long ago to the Chinese, who do not allow the smallest portion of that to be wasted which has been extracted from the soil; and yet probably they have no theory, no science to guide them. When shall we, who can see why this ought to be done,

act with the same common sense as the Chinese have long exhibited? Chemistry.

Besides the phosphates, for the sake of which farm-yard manure, guano, and bone-dust are chiefly valued, there are other substances which must be restored to the soil. Sulphate of lime or gypsum is fortunately present in considerable proportion in most soils, but where it is deficient one crop may exhaust the soil of it. It is easily obtained, being a common rock, or mineral.

The alkalies, although almost always present in considerable proportion, are yet apt to fail, because it is only that portion which is soluble that can avail the plant. Now the potash is all derived from felspar, but felspar is quite insoluble, and it is only in so far as it is decomposed that it yields its potash. This decomposition is effected by exposure to air, and one of the chief advantages of ploughing and fallow is to facilitate the decomposition of the disintegrated felspar, which is the chief ingredient of clay soils, and forms part of all soils. Some kinds of felspar are much more slowly decomposed than others, and but a small quantity of potash is available at any one time when such kinds prevail. One crop may entirely exhaust the available potash, and the most liberal supply of phosphates and sulphates will be of little use unless the supply of potash is at the same time attended to. Hence the use of nitre as a manure, or at least one use of it, and of carbonate or sulphate of potash.

But there is another expedient, which acts by increasing the supply of potash. This is the use of lime. The true action of lime is not, as has been supposed, to destroy the organic matter remaining in the soil, but to assist in the decomposition of felspar, which is rapidly decomposed when in contact with quicklime and water. This is the reason why lime is more advantageous in stiff clay soils, which are soils full of felspar.

The burning of very stiff clay also promotes the subsequent action of the weather on the felspar.

In grain crops and in grass much silica is required for the straw. Now silica abounds in all soils, except perhaps some chalky soils. But silica can only enter the plant in the form of alkaline silicates, and therefore the decomposition of felspar which directly supplies silicate of potash is doubly important.

A question naturally arises here. What is the value of organic matter in the soil or in manures. Now it is at once evident that it cannot be essential to the growth of plants as some have stated, that the soil, or the manure, or both, should contain organic matters, or mould, which is organic matter half decayed. For the first plants must have grown in a soil without a trace of mould. Moreover, direct experiment has often proved that plants will thrive in a purely mineral soil, provided it contain all the mineral elements essential to the growth of the plant. In that case the plant obtains from the air exclusively, through water, the carbonic acid and ammonia from which it obtains the whole of its carbon and nitrogen.

But fertile soils do contain mould and other organic matters, as do also most manures. Are these of no use?—far from it. The advantages are these. The slow and constant decay, or oxidation of the mould, &c., gives rise to a constant supply of carbonic acid in the soil itself. A part of this is conveyed to the plant dissolved in water along with the carbonic acid of the air. Another part of it being also dissolved by the water of the soil, dissolves out of the soil carbonates and phosphates of lime, magnesia, and iron, and is the chief means of introducing these into the juice of the plant. And in both ways, in supplying carbon and in supplying mineral matter, this carbonic acid derived from the carbonaceous matter in the soil greatly accelerates the growth of the plants, and very much shortens the period required for them to reach maturity. This, in our northern latitudes, is a matter of the last importance. But even in-

Chemnitz dependent of this advantage in point of time, it is probable
Chemosh. that a very large part of the phosphates, &c. enters the plant
by virtue of the carbonic acid formed in the soil.

The advantages derived from the presence of ammonia in the manure, or of substances yielding it either in the manure or in the soil, is analogous. The plant, if supplied with all the necessary mineral matter, will, indeed, obtain the ammonia, without which it cannot avail itself of these from the atmosphere. But the proportion of ammonia in the air is so small, that a long time in our climate is required for this, and any addition to the supply of ammonia has the effect of enabling the plant to attain maturity sooner.

While, therefore, the plant is not absolutely dependent on the presence of organic matter in the soil or in the manure, there is a decided advantage in its presence there.

Those who have employed a special manure, such as gypsum, or bone-dust, or guano, with good effect, are apt to make sure of the same result on repeating the application. But they are often completely disappointed; for the effect of the first application, say that of gypsum, to a soil deficient in it, has been to produce a heavy crop for which gypsum alone was wanting. But it must be remembered that the heavier the crop the more probable is it that the raising of it has exhausted the soil for the time of some one of the other essential elements, perhaps of the phosphates, in which case a second application of gypsum is dead loss.

Again, it often happens that when a special manure has been found very successful, the same farmer or his neighbour immediately applies it to the first field he has in hand, and perhaps derives no benefit whatever from it. The reason is, that a special manure can only do good where the soil is deficient in the substance supplied by that manure. Gypsum will produce astonishing effects on soils destitute of that substance, but containing all the other elements of fertility; but if added to a soil which, like very many, contains already far more gypsum than is required for many crops, it is merely thrown away.

The advantage of rotation of crops is very obvious. Both the phosphates, which are usually minutely diffused in rocks,

and silicate of potash, so essential to grain crops, are only rendered gradually available by the gradual disintegration and decomposition of the felspar and other rocks. If, then, by one crop, one mineral element has been more exhausted than another, let us suppose this to be the case with the available or soluble silicate of potash, a grain crop having been raised, a repetition of the same crop would fail for want of a sufficient supply of this material. But a green crop, requiring hardly any, if any, silicate of potash, would find sufficient phosphates, gypsum, lime, and alkalies, and would allow the action of the weather to go on gradually rendering available a new supply of the silicate. This example will illustrate the principle. In the application of it, as of the other principles we have laid down, experience is the best guide, because there are still very many points, the theory or principle of which is quite obscure, but in regard to which the experienced and intelligent farmer acts with confidence. We cannot attempt to do more here than briefly to indicate the general principles of the application of chemistry to agriculture. The agricultural reader must be referred to works expressly written on this most important subject, such as those of Davy, Liebig, Johnston, and others.

We have now brought to a conclusion the brief and imperfect sketch which we proposed to give of organic chemistry. Our limited space made it absolutely necessary to condense and select the matter presented to the reader. Moreover, the science is in a state of transition, and of very rapid progress, which renders the task doubly difficult. We have endeavoured to lay down, as much as possible, leading principles, and to avoid mere details. We have not even dedicated a section to the destructive distillation of organic substances; but most of the products will be found alluded to under various heads, as those of the hydrets of the ethylic and acetic radicals, the volatile bases of the ethylic and benzoic series, acetic acid, &c. &c.

We can only trust that the difficulty of condensing so large and varied a multitude of facts and their relations into so small a space will excuse the many imperfections of the execution. (w. g.)

CHEMNITZ, or CHEMNITIUS, MARTIN (1522-1589), a famous Lutheran divine, the disciple of Melancthon, was born at Britzen in Brandenburg. He was employed in several important negotiations by the Lutheran princes, and was professor at Brunswick for 30 years. He wrote Examen Concilii Tridentini, Frankf. 1585, 4 vols. fol. and 4to; A Treatise on Indulgences, 8vo, Geneva, 1599; Harmonia Evangelica, 1600, and Theologiae Jesuitarum principia capita, Rochelle, 1589.

CHEMNITZ, a town of Saxony, in the circle of Zwickau, stands in a beautiful and well-watered valley on the river Chemnitz, an affluent of the Mulde, 35 miles W.S.W. of Dresden. It is the first manufacturing town in Saxony. In point of population it ranks third, having (1849) 30,753 inhabitants, of whom 30,036 are Lutherans. The cotton goods, especially stockings, for which it is chiefly celebrated, and to which it owes its present prosperity, rival even those of England in quality and cheapness; one factory, the largest in Saxony, having 18,600 spindles. It is also celebrated for the making of spinning machinery. Chemnitz is a place of considerable trade, exporting a great part of its industrial products to the United States; and has manufactures of linens, bleaching and dye works, and tanneries. A railway connects it with Riesa, and thence with Leipzig, Dresden, Berlin, &c. The town is neat, clean, and well built, containing many fine edifices, among which may be mentioned the great church, town-house, and cloth-hall. Chemnitz was for four centuries a free imperial city.

CHEMOSH, or CHAMOSH, the name of a national god

of the Moabites and of the Ammonites, whose worship was introduced among the Israelites by Solomon (1 Kings xi. 7). No etymology of the name which has been proposed, and no attempt which has been made to identify this god with others whose attributes are better known, are sufficiently plausible to deserve particular notice. Jerome's notion that Chemosh is the same as Baal Peor has no historical foundation; and the only theory which rests on any probability is that which assumes a resemblance between Chemosh and Arabian idolatry (cf. Beyer, Addit. ad Selden, p. 322; Pocock, Specimen, p. 307). Jewish tradition affirms that he was worshipped under the symbol of a black star; and Maimonides states that his worshippers went bareheaded, and abstained from the use of garments sewn with the needle. The black star, the connection with Arabian idolatry, and the fact that Chemosh is coupled with Moloch, favour the theory that he had some analogy with the planet Saturn.

CHEMOSIS, an inflammation of the eyes, causing the cornea to redden and swell, so as to impede vision.

CHENAUB, the ancient Acesines, a river of the Punjab, which has its source in Lat. 32. 48., Long. 77. 27., in the British district of Lahoul, S. of Ladakh, or Middle Thibet. Its course is first north-westerly to Kishtawar, in the dominions of Gholab Singh, the present ruler of Cashmere; thence it proceeds S.W. to Riasi, where it leaves the mountains and enters the plain of the Punjab. At Akmur, 50 miles below this point, it becomes navigable—at least for timber rafts. From Akmur it continues a south-west-