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 vermillion. 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 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—

| Lead. | Oxygen. | |-------|--------| | Protoxide of Lead. | 104 parts | 8 parts | | Deutoxide of Lead. | 104 ... | 16 ... |

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:—

| Nitrogen. | Oxygen. | |----------|--------| | Protoxide of Nitrogen. | 14 parts | 8 parts | | Deutoxide of Nitrogen. | 14 ... | 16 ... | | Hypnitors Acid. | 14 ... | 24 ... | | Nitrous Acid. | 14 ... | 32 ... | | Nitric Acid. | 14 ... | 40 ... |

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

| Iron | Oxygen | |------|--------| | Protoside of Iron | 28 parts | | Sesquioxide of Iron | 28 parts | | Ferric Acid | 28 parts |

Here the ratio in the second compound, if we consider the first as in the ratio of 1 : 1, is that of 1 : 15; and in the third, it is 1 : 3. Compounds in which the ratio of 1 : 15 is observed are called sesqui-compounds. In some rare cases we find the ratio of 1 : 25, and of 1 : 35. 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, 15, 2, 25, 3, 35.

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 \(\frac{1}{1000}\)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 \(\frac{1}{1000}\)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 \(\frac{1}{1000}\)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,

| In the first, | 1 | | In the second, | 1 | | In the third, | 1 | | In the fourth, | 1 | | In the fifth, | 1 |

atom of nitrogen unites with

| 1 of oxygen. | | 2 ... | | 3 ... | | 4 ... | | 5 ... |

and the resulting numbers will be as formerly stated.

In the case of such compounds as exhibit the ratio, in their composition, of 1 : 1½, or 1 : 2½, 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

| Iron. | Oxygen. | | --- | --- | | Protioxide of iron | 1 atom | | Sesquioxide or peroxide of iron | 2 atoms |

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 octohedrons; fluor spar appears in cubes, in regular octohedrons, and in regular tetrahedrons; alum forms both cubes and regular octohedrons. This is because the cube, the regular octohedron, 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 octohedron, 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 octohedron, &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 potash, 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 octohedrons. 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 sesquisulphate of a sesquioxide (teresulphate 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 (sesquisulphate 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:

| 1. Silver | Ag | |----------|----| | Gold | Au |

| 2. Arsenious acid (in its unusual form) | AsO₃ | | Teroxide of antimony | SbO₃ |

| 3. Alumina | Al₂O₃ | | Sesquioxide of iron | Fe₂O₃ | | Chromium | Cr₂O₃ | | Manganese | Mn₂O₃ |

| 4. Phosphoric acid | PO₃ | | Arsenic acid | AsO₃ |

| 5. Sulphuric acid | SO₄ | | Selenic acid | SeO₄ | | Chrome acid | CrO₄ | | Manganic acid | MnO₄ |

| 6. Hypermanganic acid | Mn₂O₇ | | Hypochloric acid | ClO₃ |

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 sulphurates, 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 isomorphous. 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, insomuch 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 octohedrons. 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 octohedrons. 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 lampblack 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.

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:

| Gas or Vapour | Specific Gravities | Chemical Equivalents | |---------------|-------------------|---------------------| | Hydrogen | 0·0696 | 1·00 | | Nitrogen | 0·9727 | 14·00 | | Carbon | 0·413 | 6·00 | | Chlorine | 2·4700 | 35·50 | | Iodine | 5·7011 | 127·10 | | Bromine | 5·3330 | 80·00 | | Mercury | 6·9690 | 101·00 | | Oxygen | 1·025 | 16·00 | | Phosphorus | 4·3273 | 64·00 | | Arsenic | 10·9620 | 150·00 | | Sulphur | 6·6480 | 96·00 |

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 $\frac{1}{4}$th of a volume. One volume of hydrogen unites with $\frac{1}{4}$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 $\frac{1}{4}$ 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 $\frac{1}{4}$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 \( \frac{1}{2} \) the size, and those of sulphur \( \frac{1}{6} \) 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 \( \frac{1}{2} \), as the atomic volume of oxygen; and the atomic weight of sulphur, = 32, divided by its specific gravity as gas, = 96 (that of hydrogen = 1), gives the quotient 0.1666 or \( \frac{1}{6} \), 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 \( \frac{1}{2} = 1 \) = the number of atoms in 1 volume of hydrogen; \( \frac{1}{4} = 2 \) = the number of atoms in 1 volume of oxygen; and \( \frac{1}{6} = 6 \) = 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 \( \frac{1}{2} \) and 6, \( \frac{1}{4} \) 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, \( \frac{1}{2} = 0.0694 = 14.409 \) for hydrogen; \( \frac{1}{4} = 1.1026 = 7.2554 \) for oxygen; and \( \frac{1}{6} = 6.9000 = 2.3188 \), for the atomic volumes of hydrogen, oxygen, and sulphur respectively; and these numbers are to each other as 1, \( \frac{1}{2} \), and \( \frac{1}{4} \).

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 \( \div 39.26 = 0.0220 \); in the case of carbon it is 3.5 \( \div 6.04 = 0.5794 \). Finally, in the case of water, the relative number of atoms in a given volume, which may be made the standard, is 1 \( \div 9 = 0.1111 \). If, for convenience, the number for water is made 1000, then that for potassium becomes 9880, 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 191666.

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 acceptation, 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 O₃ 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; PO₄ 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 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,2SO₃, 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 formulæ 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—

\[ \text{KO,SO}_3 + \text{Al}_2\text{O}_3 \cdot 3\text{SO}_3 + 24\text{HO}. \]

This tells us that chemists consider alum as composed of 1 eq. of neutral sulphate of potash, 1 eq. of neutral tere-sulphate of alumina (composed of 1 eq. of alumina, a sesqui-oxide, 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 formulæ 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 Chemistry, as more probable than the other. In like manner, sulphate of potash may be viewed either as KO, SO₃ or K₂SO₄. 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:

\[ \text{K} + \text{HO} = \text{KO} + \text{H} \]

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:

\[ \text{HCl} + \text{AgO} = \text{HO} + \text{AgCl} \]

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:

\[ \text{MnO}_2 + 2\text{HCl} = \text{MnCl}_2 + 2\text{HO} + \text{Cl}_2 \]

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

\[ \text{NaCl} + \text{MnO}_2 + 2\text{SO}_3 = \text{NaO}, \text{SO}_4 + \text{MnO}, \text{SO}_4 + \text{Cl}_2 \]

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:

\[ \text{KO}, \text{NO}_3 + 2(\text{HO}, \text{SO}_3) = (\text{KO}, \text{HO}, 2\text{SO}_3) + \text{HO}, \text{NO}_3 \]

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 | |-------------------|---------------| | K = 49 | S = 15 | | N = 14 | H = 1 | | O₂ = 48 | O₄ = 32 |

\[ \text{KO}, \text{NO}_3 = 102 \] and \( 2(\text{HO}, \text{SO}_3) = 98 \)

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) | |----------------------|------------------------| | K = 49 | N = 14 | | S₂ = 32 | H = 1 | | H = 1 | O₄ = 48 | | O₄ = 64 | |

\[ \text{KO}, \text{HO}, 2\text{SO}_3 = 137 \] \[ \text{HO}, \text{NO}_3 = 63 \]

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—

\[ \text{KO}, \text{SO}_4 + \text{BaO}, \text{NO}_3 = \text{KO}, \text{NO}_3 + \text{BaO}, \text{SO}_4 \]

Here the equivalents are, before the change,

\[ \text{KO}, \text{SO}_4 = \begin{cases} \text{KO} = 48 \\ \text{SO}_4 = 40 \end{cases} \] \[ \text{BaO}, \text{NO}_3 = \begin{cases} \text{BaO} = 76 \\ \text{NO}_3 = 54 \end{cases} \]

After the change, we have

\[ \text{KO}, \text{NO}_3 = \begin{cases} \text{KO} = 48 \\ \text{NO}_3 = 54 \end{cases} \] \[ \text{BaO}, \text{SO}_4 = \begin{cases} \text{BaO} = 76 \\ \text{SO}_4 = 40 \end{cases} \]

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 C₂O₄ HO, or, according to the most recent researches, double this, C₄H₆ 2 HO, which implies, that in the first formula it is a mono-basic acid, containing 1 eq. of basic water, replaceable by bases; and in the second that it is bibasic, that is, has 2 eqs. of replaceable water. Acetic acid is C₂H₄O₂ HO; butyric acid, C₄H₇O₂ HO; benzoic acid, C₆H₅O₂ HO. Oxide of ethyl or ether is C₂H₅O; alcohol, its hydrate, is C₂H₅O₂ 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 C₁₈H₃₅O₂ HO; Meissic acid is C₁₈H₃₅O₂ HO. Then we have hydrocyanic acid, which is C₃NH₂ HO; cyanic acid, C₃NO₂ HO; cyanuric acid, C₆N₃O₃ 3 HO; urea, C₂H₄N₂O₂; gly- Chemistry. Cocaine, \( \text{C}_4\text{H}_8\text{NO}_3 \); quinine, \( \text{C}_2\text{H}_6\text{NO}_3 \); ethylamine, \( \text{C}_2\text{H}_5\text{N} \); phenylamine, \( \text{C}_6\text{H}_5\text{N} \); quinoline, \( \text{C}_8\text{H}_7\text{NO}_2 \); indigo, \( \text{C}_16\text{H}_8\text{NO}_2 \); gelatine, \( \text{C}_9\text{H}_8\text{N}_2\text{O}_3 \); albumen, \( \text{C}_{16}\text{H}_{22}\text{N}_2\text{O}_5 \); etc., 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—

\[ \text{Dry Grape Sugar} + \text{Alcohol} = \text{Carbonic Acid} \]

\[ \text{C}_2\text{H}_2\text{O}_3 = 2(\text{C}_2\text{H}_5\text{O}) + 4\text{CO}_2 \]

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 radicle; for this gives to its compounds the simple formulae of inorganic bodies. The following are some of the admitted compound radicals:

| Symbol | Formula | |--------|---------| | Cyanogen = \( \text{C}_2\text{N} = \text{Cy} \) | | Methylene = \( \text{C}_2\text{H}_2 = \text{Me} \) | | Ethylene = \( \text{C}_2\text{H}_4 = \text{E} \) | | Benzoyl = \( \text{C}_6\text{H}_5\text{O} = \text{Bz} \) | | Acetylene = \( \text{C}_2\text{H}_2 = \text{Ac} \) | | Formyl = \( \text{C}_2\text{H}_2 = \text{F} \) |

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 \( \text{HCy}, \text{CyO}, \text{HO}, \text{Cy}_2\text{O}, \text{3HO} \). Those of ethyle (ether and alcohol) become \( \text{AEo}, \text{HO} \). Acetic acid becomes \( \text{ACo}_2\text{HO} \), and benzoic acid \( \text{BzO}_2\text{HO} \). Ethylamine takes the formula \( \text{NH}_2\text{Ac}, \text{or AdAc} (\text{Ad} = \text{NH}_2\text{ stands for amide}); perchloride of formyle or chloroform, \( \text{C}_2\text{HCl}_3 \), may be also written \( \text{FcCl}_3 \). In this way, even very complex compounds may be expressed as simply as the simplest inorganic bodies. Compare hydrocyanic acid, \( \text{HCy} \) with hydrochloric acid, \( \text{HCl} \); oxide of ethyle, \( \text{AEo} \), with oxide of potassium; acetic acid (hydrated) \( \text{ACo}_2\text{HO} \), with hydrated sulphuric acid, \( \text{SO}_3\text{HO} \); and hyduret of benzoyl, \( \text{C}_6\text{H}_5\text{O}_2\text{H} \) or \( \text{BzH} \), with sulphuretted hydrogen \( \text{SH} \).

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 \( Q \) means quinine, \( M \), morphine. \( T \) stands for tartaric, and \( C \) 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, sulfur, &c.; those of more recent discovery have been named Chemistry, 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 \( \text{um} \), as aurum, argentum, cuprum, plumbum; for gold, silver, copper, and lead. Only one non-metallic body, selenium, ends in \( \text{um} \), 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 \( \text{ic} \) or \( \text{ous} \). Thus we have sulphurous 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 \( \text{ous} \). Thus sulphurous and nitrous acids contain less oxygen than sulphuric and nitric acids.

Should other acids containing the same elements be discovered, the prefix \( \text{hypo} \) is employed in addition. Thus, after sulphurous and sulphuric acid had long been known, other acids were discovered, one of which is called hypo-sulphurous, 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 \( \frac{1}{2} \) eq. 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 deutoxide or binoxide, and in some cases peroxide, meaning the oxide having most oxygen; as deutoxide 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 protoclories, protobromides, deutoclories, &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, deutocloryde or dichloride of mercury, protocloryde of mercury, deutoiodide of mercury, terchloride of antimony, terfluoride of silicon, perchloride of manganese, Chemistry, sulphuret of potassium, phosphuret of lead, carbonet of iron, selenuret of copper, bisulphuret of carbon, bicarbonuret 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 ic, that in ite for an acid in os. 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, glucinium, 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, glycocine, 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, cetyl, acetyl, formyle, benzoyl, and their compounds, are named accordingly; as oxide or chloride of ethyle, terchloride of formyle, hyduret of acetyl, hyduret of benzoyl, 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.

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- 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 silicium.

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, glucinium, 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 4/5th of the atmosphere, 8/9ths of all the water in our globe, and fully 4/5th of alumina and silica, 4/5th of lime, and 4/5th 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 sulphures 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:

\[ \text{Chlorate of Potash. Chloride of Potassium. Oxygen.} \]

\[ \text{KClO}_3 \rightarrow \text{KCl} + \text{O}_2 \]

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

\[ \text{Peroxide of Manganese. Sesquioxide of Manganese. Oxygen.} \]

\[ 2\text{MnO}_2 \rightarrow \text{Mn}_2\text{O}_3 + \text{O}_2 \]

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- 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 watchspring. 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 atmospherical 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, irrespirable.

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 atmospherical 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, however, is this, chemical combination, attended by the evolution of heat and light.

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 aluminium, 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, SO₃; nitrate of soda, NaO, NO₃; these and many others have all the characters of salts. Sulphate of baryta, BaO, SO₃; carbonate of lime, CaO, CO₂; 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 SO₃ form KO, SO₃, 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 K₂O, SO₃, for sulphate of potash. The new one, which is exactly equal to it, is expressed by K₂SO₄, and the group SO₄ 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 $\frac{1}{8}$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 $\frac{1}{4}$ to $\frac{3}{4}$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 wool, 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.

Zinc. Sulphuric Acid. Sulphate of Zinc. Hydrogen. \[ \text{Zn} + \text{H}_2\text{SO}_4 = \text{ZnO} \cdot \text{SO}_4 + \text{H}_2 \]

Zinc. Hydrochloric Acid. Chloride of Zinc. Hydrogen. \[ \text{Zn} + \text{HCl} = \text{ZnCl} + \text{H}_2 \]

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 $\text{SO}_4$, if we represent it by $\text{H}_2\text{SO}_4$ instead of the equivalent formula $\text{H}_2\text{O}$, then the two equations become as like each other as the two operations are. Thus—

Zinc. Sulphuric Acid. Sulphate of Zinc. Hydrogen. \[ \text{Zn} + \text{H}_2\text{SO}_4 = \text{ZnO} \cdot \text{SO}_4 + \text{H}_2 \]

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 $\text{SO}_4$. 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 $\text{SO}_4$. 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, hydroaluminium acids, &c., and the nitrates, phosphates, seleniates, silicates, &c.—and the bromiates, iodiates, fluorates, and sulphurites, &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$_2$SO$_4$; H$_2$NO$_3$; HPO$_4$; KCl; NaBr; CaF; PbS; BaSO$_4$; AgNO$_3$; &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. Hydrogen is highly positive, and has a very strong affinity for oxygen, with which it forms at least two compounds, water, HO, and deutoxide of hydrogen, HO₂; and probably a third, a teroxide or peroxide, HO₃, 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 HO = 9.

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 \( \frac{1}{6} \)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 efficiency, 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, \( \frac{3}{4} \)ds of that loss of volume must be hydrogen and \( \frac{1}{4} \)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 lbs. of oxygen and 1 lb. of hydrogen yield 9 lbs. of water. \( O + H = HO \).

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 $\text{CuO} + \text{H}_2 = \text{H}_2\text{O} + \text{Cu}$.

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 $\text{HO}_2$, at least in this country. On the Continent, where atoms and volumes are believed to agree, the formula of water is $\text{H}_2\text{O}$. 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 $\frac{1}{8}$ 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° Fahr., and boils at 212° Fahr. It is therefore solid at all temperatures below 32°, and gaseous at all above 212°.

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°, when it is about to freeze, but about 39.5°, 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 39.5° 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 39.5°, 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°; 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°, 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°, 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°, and steam at 212°, 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°, the mixture is barely tepid; but if we force 4 oz. of steam at 212° into 16 oz. of water at 60°, 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°, and cannot therefore char or injure them.

Besides boiling at 212°, 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°. For this reason it boils lower on the top of a mountain than at its base. 212° 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° 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 shewn by Dr Marcell'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 HO.SO₄ or H₂SO₄. Water combines also with anhydrous bases, forming hydrates of the bases. Thus anhydrous lime, CaO, combines with water, forming hydrate of lime or slaked lime, CaO.H₂O. Thirdly, water combines with neutral salts, forming hydrates. Thus sulphate of magnesia, MgO.SO₄, combines with 1 eq. of water to form the hydrated salt, MgO.SO₄.H₂O. Lastly, water combines with salts, both anhydrous and hydrated, in another form, which is called water of crystallization. The hydrated sulphate of magnesia, MgO.SO₄.H₂O, takes up 5 eqs. more of water to form the usual crystallized salt, which is represented as follows, MgO.SO₄.H₂O + 5 eq. 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, MgO.SO₄ + KO.SO₄, 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. 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 \( \frac{3}{4} \)ths of the weight of all the soft solids, and about \( \frac{1}{2} \)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, \( \text{HO}_2 = 17 \).

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, \( \text{BaO}_2 + \text{HCl} = \text{BaCl} + \text{HO}_2 \). 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, \( \text{BaCl} + \text{HO}_2 = \text{BaO}_2 + \text{HCl} \). 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 thunder-storms. 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 \( \frac{4}{5} \)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 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 exciters 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, NO = 22.

This compound is obtained by the action of heat on nitrate of ammonia, thus: \(-NH_3HO, NO_2 = 4HO + 2NO\). 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 chemistry, 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 paralysing 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 anaesthesia, 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 \( \frac{1}{3} \) renders the gas a heavy one. For we have—

| 1 vol. oxygen, weighing | = | 1111 | | 1 vol. nitrogen, ... | = | 972 | | 1 vol. nitrogen, ... | = | 972 |

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° 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, NO₂ = 30.

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

\[ \text{Copper, Nitric Acid, Nitrate of Copper, Deutoxide of Nitrogen.} \]

\[ 3 \text{Cu} + 4\text{NO}_3 = 3(\text{CuO}) + \text{NO}_2 \]

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, NO₂. 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.

| 1 vol. oxygen, weighing | = | 1111-1 | | 1 vol. nitrogen, ... | = | 972-2 |

yield 2 vols. deutoxide, ... weighs 1041-6 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 \( \text{NO}_3 + \text{O}_2 = \text{NO}_2 \).

(3.) Hyponitrous Acid, \( \text{NO}_3 = 38 \).

Hardly known in a pure state. It seems to be a liquid, blue at ordinary temperatures, colourless at \( 32^\circ \), 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, \( \text{NO}_4 = 46 \).

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

\[ \begin{align*} \text{Nitrate of Lead} & : \quad \text{Oxide of Lead} : \quad \text{Oxygen} : \quad \text{Nitrous Acid}, \\ \text{PbO}_4 & : \quad \text{PbO}_2 : \quad \text{O} : \quad \text{NO}_2. \end{align*} \]

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.

(5.) Nitric Acid, \( \text{NO}_5 = 54 \).—Hydrated Nitric Acid, \( \text{HO}, \text{NO}_4 \) or \( \text{H}, \text{NO}_5 \).

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

\[ \begin{align*} \text{Nitrate of Potash, Sulphuric Acid} & : \quad \text{Bisulphate of Potash, Nitric Acid}, \\ \text{KO}, \text{NO}_3 & + 2(\text{HO}, \text{SO}_4) = (\text{KO}, \text{HO}, 2\text{SO}_4) + \text{HO}, \text{NO}_5. \end{align*} \]

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 1520, compared to that of water as 1000. 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.

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 medicinæ.

The acid we have described is the hydrated acid, and may be viewed either as composed of water and dry acid, \( \text{HO}, \text{NO}_4 \) or as formed of hydrogen, with the hypothetical radical, \( \text{NO}_4 \); thus, \( \text{H}, \text{NO}_5 \). It was long supposed that the anhydrous acid did not exist in a separate form, and that the 1 eq. of water in \( \text{HO}, \text{NO}_4 \) could not be removed. But it has recently been shown, that the dry acid \( \text{NO}_5 \) may be obtained by the action of chlorine or dry nitrate of silver; thus, \( \text{AgO}, \text{NO}_3 + \text{Cl} = \text{AgCl} + \text{O} + \text{NO}_2 \).

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 \( \text{N}_2\text{O} \)) 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. \( \text{N} \) to 1 eq. \( \text{O} \), or \( \text{N}_2\text{O} \).

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 \( \frac{1}{6} \) 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 \( 60 \div 3 = 20 \) 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 chemistry, 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—

| Gas | Specific Gravity | |---------|------------------| | Air | 1000·0 | | Hydrogen| 69·4 | | Oxygen | 1111·1 | | Nitrogen| 972·2 |

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 \( \frac{1}{28} \) 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. $NH_3 = 17$.

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—

$$NH_4Cl + CaO \cdot H_2O = CaCl + 2H_2O + NH_3$$

or $$NH_4Cl + CaO \cdot H_2O = CaCl + 2H_2O + NH_3$$

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.

(2.) Ammonium. $NH_4 = 18$.

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—

| Salphate of potash | KO, SO$_3$ | | Chloride of potassium | K Cl | | Salphate of ammonia | NH$_4$, HO, SO$_3$ | | Hydrochlorate of ammonium | NH$_4$, H Cl |

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—

| Salphate of oxide of ammonium | NH$_4$ O, SO$_3$, or Am O, SO$_3$ | | Chloride of ammonium | NH$_4$ Cl, or Am Cl | | Salphate of potash | KO, SO$_3$ | | Chloride of potassium | K Cl |

Here we see that ammonium, NH$_4$ or Am, and oxide of ammonium, NH$_4$ O or Am O, can replace potassium and potash (oxide of potassium) without changing the form of the compound. Now oxide of ammonium, NH$_4$ O, is the Chemistry, same thing as ammonia plus water, \( \text{NH}_3 + \text{HO} \), 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, \( \text{NH}_4\text{SO}_4 \), is considered to be sulphate of oxide of ammonium, \( \text{NH}_4\text{O}, \text{SO}_4 \) or \( \text{AmO}, \text{SO}_4 \). When a hydrogen acid such as hydrochloric acid, \( \text{HCl} \), acts on ammonia, it is believed not to combine with it, as expressed in the old formula of sal-ammoniac, \( \text{NH}_4\text{HCl} \), but to react on it, the hydrogen of the acid forming with the ammonia ammonium, with which the chlorine combines, \( \text{NH}_4 + \text{HCl} = \text{NH}_4\text{Cl} = \text{AmCl} \).

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 feebler attraction than the three others, is sufficient to break up the molecule, forming water, and of course, ammonia, \( \text{NH}_4\text{O} = \text{HO} + \text{NH}_3 \). 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, KO | | Oxide of ammonium, \( \text{NH}_4\text{O} \) | | Hydrated oxide of potassium, KO, HO | | Hydrated oxide of ammonium \( \text{NH}_4\text{O}, \text{HO} = \text{NH}_3 + 2\text{HO} \) |

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° Fahr.

(3.) Amide. \( \text{NH}_3 = 16 \).

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, \( \text{NH}_3\text{HO}, \text{C}_6\text{H}_5\text{O}_2 \) or \( \text{NH}_3\text{O}, \text{C}_6\text{H}_5\text{O}_2 \), losing 2 eqs. of water becomes benzamide, \( \text{NH}_3\text{C}_6\text{H}_5\text{O}_2 \). 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 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—

\[ \text{MnO}_2 + 2\text{HCl} = \text{MnCl}_2 + \text{HO} + \text{Cl} \]

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

\[ \text{MnO}_2 + \text{NaCl} + 2\text{SO}_4 = (\text{MnO}_2\text{SO}_4) + (\text{NaO}_2\text{SO}_4) + \text{Cl} \]

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 2500. Under a pressure of about 4 at- 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.** $HCl = 36.5.$

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:

| Sea Salt | Sulphuric Acid | Sulphate of Soda | Hydrochloric Acid | |----------|---------------|------------------|------------------| | NaCl | H2SO4 | Na2SO4 | HCl |

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:

1 vol. chlorine, weighing 2500 1 vol. hydrogen, ... 69.4

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 1210, 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

$$M + HCl = MCl + H,$$ and $$MO + HCl = HIO + MCl.$$

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 formulas, examples of which are placed below each:

| General Formulae. | Acid. | Salt. | |-------------------|-------|------| | Radical chlorine, Cl | HCl | MCl | | Radical of sulphuric acid, SO4 = Su | H2SO4 | M2SO4 | | Radical of nitric acid, NO3 = Nt | HNO3 | MNO3 | | Radical cyanogen, C2N = Cy | HCN | MCy |

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