May be defined, The study of such phenomena or properties of bodies as are discovered by variously mixing them together, and by exposing them to different degrees of heat, alone, or in mixture, with a view to the enlargement of our knowledge in nature, and to the improvement of the useful arts: or, It is the study of the effects of heat and mixture upon all bodies, whether natural or artificial, with a view to the improvement of arts and natural knowledge.

The science of chemistry is undoubtedly of very high antiquity; and, like most other sciences, its origin cannot be traced. In scripture, Tubal Cain, the 8th from Adam, is mentioned as the father or instructor of every artificer in brass or iron. This, however, does not constitute him a chemist, any more than a founder or blacksmith among us has a right to that title. The name of chemist could only belong to him, whoever he was, who first discovered the method of extracting metals from their ores; and this person must necessarily have lived before Tubal Cain, as every blacksmith or founder must have metals ready prepared to their hand. Nevertheless, as Tubal Cain lived before the flood, and the science of chemistry must have existed before his time, some have conjectured, that the metallurgic part of chemistry, on account of its extreme usefulness to mankind, was revealed to Adam by God himself.

Be this as it will, Siphoas, an Egyptian, is considered by the chemists as the founder of their science. He was known by the Greeks under the name of Hermes, or Mercurius Trismegistas; and is supposed to have lived more than 1900 years before the Christian era. A numerous list of this philosopher's works is given by Clemens Alexandrinus; but none of them are now to be found, nor do any of them appear to have been written professedly on chemistry.

Two illustrious Egyptians, of the name of Hermes, are recorded by ancient authors. The elder supposed to be the same with Mizraim, the grandson of Noah, the Hermes of the Greeks, and Mercury of the Romans. The younger Hermes lived a thousand years afterwards, and is supposed to have restored the sciences after they had fallen into oblivion, in consequence As the science of chemistry is supposed to have been well known to the Egyptians, Moses, who was skilled in their wisdom, is hence ranked among the number of chemists; a proof of whose skill in this science is thought to be, his dissolving the golden calf made by the Israelites, so as to render it potable.

Of all the Greeks who travelled into Egypt, in order to acquire knowledge, Democritus alone was admitted into their mysteries. The Egyptian priests are said to have taught him many chemical operations; among which were the art of softening ivory, of vitrifying flints, and of imitating precious stones.

Very little, however, can be gathered from any of the more ancient authors concerning the progress of this science; but as there is no instance of any nation in the world which was totally destitute of metals, we are assured that the metallurgic part of chemistry must have been known to some persons in every age, and every nation; nor have we the least reason to imagine that the knowledge of metallurgy, in these early ages, was at all inferior to what it is at present. In the sacred writings, allusions are very frequently made by the prophets to the methods of refining silver, and particularly to the blowing of bellows upon the surface of it, in order to drive off the baser metals scorified, and reduced to a cax by the violent action of the heat. By the manner in which these things are spoken, it would seem that the knowledge of them was pretty general, as much indeed as it is just now.

In the fourth and fifth centuries, some of the Greek writers speak of an art, as then generally known, of transmuting the baser metals into gold; and, in the end of the 13th century, when the learning of the eastern countries was brought into Europe by the Arabians, the pretensions to a knowledge of transmutation of metals began likewise to spread into this quarter of the world. The art itself, called Alchemy, is supposed to have been of Egyptian original; and it is probable, that when the ancient Greek philosophers travelled into Egypt, they brought back some of the allegorical language of the Egyptian art ill understood, which afterwards passed into their mythology.

The science of alchemy seems to have been the earliest branch of philosophic chemistry, as being originally speculative; whereas, in all other parts of chemical knowledge, facts seem to have preceded reasoning. Success in alchemy was thought impossible, without a previous knowledge of the nature, essence, and principles of metals; whence they are produced in the mines, whence they received their increase, &c.

The general principles of metals were supposed to have been two, viz. mercury and sulphur; of both which, particularly of the latter, there were different kinds. In gold it was pure, red, fixed, and incombustible; but of different qualities in the other metals. The principles of gold they imagined to be scattered in certain other bodies; and these they laboured to collect and unite, in order to the composition of the precious metal. The alchemists did not stop here; they pretended to a product of a higher order, called the elixir, the medicament for metals, the tincture, the philosopher's stone; which by being projected on a large quantity of any of the inferior metals in fusion, should change them into pure gold; being laid upon a plate of silver, copper, or iron, and moderately heated, should change into gold all the places to which it was applied; and on being properly treated with pure gold, should change it into a powder of the same virtue and efficacy with itself; and which, by continued coction, should have its virtue more and more exalted, so as at last to be able to change into pure gold 272,330 parts of base metal.

Pretensions of this kind carry along with them such an air of absurdity, that it is not at present worth while to say anything further concerning them, than that they have at last fallen into universal discredit, though not sooner than the last century.

In the sixteenth century, chemistry was first introduced into physic by Paracelsus, who added a new species of folly to that of his predecessors. This was an imagination, that by alchemy an universal medicine might be discovered, by which the human life could be prolonged to any length of time whatever. But, though Paracelsus, and his disciple Van Helmont, both pretended to have been in possession of this remedy, the one died at the age of 48, and the other at that of 63 years.

This notion of an universal remedy, absurd as it was, found many partisans; and the books of the chemists were now filled with receipts for making portable gold, elixirs of life, panaceas, &c., but written in unintelligible language.

Happily the taste for knowledge, which at last began to succeed the jargon and ignorance of preceding ages, awakened men to inquiries concerning the most important and essential operations of chemistry.

Agricola is one of the first and best authors on the subject of metallurgy. Being born in a village in Mithia, a country abounding in mines and metallurgic works, he described them exactly and copiously. He was a physician, and contemporary with Paracelsus, but of a character very different. His writings are clear and instructive, as those of Paracelsus are obscure and useless. Lazard, Erker, Schindler, Schubert, Henkel, &c., have also written on metallurgy, and described the art of effaying metals. Anthony Nevil, Dr Merret, and the famous Kunkel, (who discovered the phosphorus of urine) have described very fully the arts of making glas, enamels, imitations of precious stones, &c.; but their writings, as well as those of succeeding chemists, are not free from the illusions of alchemy; for true it is, that an obstinate and inveterate malady never disappears at once, without leaving traces behind. Soon after, however, the alchemical phrenzy was attacked by many powerful antagonists, who contributed to rescue the science of chemistry from an evil which at once disgraced it, and retarded its progress. Among these, the most distinguished are Kircher a Jesuit, and Connington a physician, who wrote with much success and reputation. About the middle of the last age, several of the parts of chemistry began to be collected, examined, and compared, with a view to discover their principles, and observe their relations, so as to unite them into one body of rational doctrine. This, which we may consider as the foundation of chemistry, considered as a science, was first begun by James Berner, physician to the king of Poland, who arranged into order the principal chemical experiments, and added rational explanations. His work is intitled Philosophical Chemistry. The phenomena of this science are there referred to the system of alkalies and acids, established by Tachenius, but abused by being too far extended. A valuable treatise on chemistry was also published by Bohmius, professor at Leipzig; and Becher, principal physician to the electors of Mentz and Bavaria, has laid down the most satisfactory theory ever published; and which has been adopted by almost all succeeding philosophical chemists, particularly Stahl, who has shewn it to be the most enlightening, and the most conformable to the phenomena of chemistry of any thing of the kind ever published.

From this time the science of chemistry has advanced prodigiously. A true theory, arising from experiments already made, has led to numberless new ones; and the labours of the learned Boerhaave, Boyle, Newton, &c. have contributed to lay a foundation for the improvements made by the celebrated chemists of the present time, from whose new discoveries it is to be hoped, that the science will still continue to advance, many useful arts to be improved, and the operations of nature to be more and more understood.

PART I. THEORY OF CHEMISTRY.

According to the definition we have given of this science, the theory of it ought to consist in a thorough knowledge of all the phenomena which result from every possible combination of its objects with one another, or from exposing them in all possible ways to those substances which chemists have found to be the most active in producing a change. So various, however, and so widely extended are the objects of chemistry, (comprehending all terrestrial bodies whatever), that a knowledge of this kind is utterly unattainable by man. The utmost that can be done in this case is, to give some account of the phenomena which accompany the mixtures of particular substances, or the appearances they put on when exposed to heat, which have been already so well ascertained that they may now be laid down as rules, whereby we may, with a good deal of certainty, judge of the event of our experiments, even before they are made.

Here we must observe, that though the objects of chemistry are as various as there are different substances in the whole system of nature, yet they cannot all be examined with equal ease. Some of these substances act upon others with great violence; and the greater their activity, the more difficultly are they themselves subjected to a chemical examination. Thus, fire, which is the most active body in nature, is to little the subject of examination, that it hath hitherto baffled the ingenuity of the greatest philosophers to understand its composition; (see Fire.) This substance therefore, though it be the principal, if not the only agent in chemistry, is not properly an object of it, because it cannot be made a subject of any chemical operation.

It hath been customary to consider all bodies as composed of certain permanent and unchangeable parts, called elements; and that the end of chemistry was to resolve bodies into these elements, and to re-compose them again by a proper mixture of the elements when so separated. Upon this supposition the alchemists went; who, supposing that all bodies were composed of salt, sulphur, and mercury, endeavoured to find out the proportions in which these existed in gold, and then to form that metal by combining them in a similar manner. Had they taken care to ascertain the real existence of their elements, and, by mixing them together, composed any one body whatever, though but a grain of lead, the least valuable of all metals, their pretensions would have been very rational and well founded; but as they never ascertained the existence of such elementary bodies, it is no wonder that their labours were never attended with success.

Another set of elements which were as generally Mr Boyle's received, and indeed continue to be so in some measure to this day, are fire, air, earth, and water.—This doctrine of elements was strenuously opposed by Mr Boyle; who endeavoured to prove, that fire was not an element per se, but generated merely from the motion of the particles of terrestrial bodies among one another, (see Fire); that air was produced from the substance of most solid bodies; and that water, by a great number of distillations, was converted into earth. His arguments, however, concerning fire were not at all conclusive; nor does the expulsion of air from fixed bodies prove that any of their solid parts were employed in the composition of that air, as later discoveries have shewn that air may be absorbed from the external atmosphere, and fixed in a great number of solid substances. His attention concerning water deserves much consideration, and the experiment is well worth repeating; but it does not appear that he, or any other person, ought to have relied upon the experiment which was intended to prove this transmutation. The fact was this. Having designed to try the possibility of reducing water to earth by repeated distillations, he distilled an ounce of water three times over himself, and found a small quantity of earth always remaining. He then gave it to another, who distilled it 197 times. The amount of earth from the whole distillations was six drachms, or \( \frac{1}{2} \) of the quantity of water employed; and this earth was fixed, white, and insoluble in water.—Here it is evident, that great suspicions must lie against the fidelity of the unknown operator, who no doubt would be wearied out with such a number of distillations. The affair might appear trivial to him; and as he would perhaps know to which side Mr Boyle's opinion inclined, he might favour it, by mixing some white earth with the water. Had the experiment been tried by Mr Boyle's own hand, his known character would have put the matter beyond a doubt.

Even the existence of earth as an element, appears as disputed. as dubious as that of the others; for it is certain that there is no species of earth whatever, from which we can produce two dissimilar bodies, by adding their other component parts.—Thus, the earth of alum has all the characters of simplicity which we can define in any terrestrial substance. It is white, insipid, odorless, and perfectly fixed in the fire; nevertheless, it seems to be only an element of that particular body called alum: for though alum is composed of a pure earth and vitriolic acid joined together, and Epsom salt and selenite are both composed of a pure earth combined with the same acid; yet by adding oil of vitriol to the earth of alum, in any possible way, we shall never be able to form either Epsom salt or selenite. In like manner, though all the imperfect metals are composed of inflammable matter joined with an earthy basis; yet by adding to earth of alum any proportion we please of inflammable matter, we shall never produce a metal; and what is still more mortifying, we can never make the earthy basis of one metallic substance produce any other metal than that which it originally composed. See Earth.

A little consideration upon the subject of elements will convince us, not only that no such bodies have ever yet been discovered, but that they never will; and for this plain reason, that they must be in their own nature invisible.—The component parts of any substance, may with propriety enough be called the elements of that substance, as long as we propose carrying the decomposition no farther; but these elements have not the least property resembling any substance which they compose. Thus, it is found that the compound salt called sal ammoniac is formed by the union of an acid and an alkali: we may therefore properly enough call these two the elements of sal ammoniac; but, taken separately, they have not the least resemblance to the compound, which is formed out of them. Both the acid and alkali are by themselves so volatile as to be capable of distillation into an invisible vapour by the heat of one's hand; whereas, when joined together, they are so fixed as almost to endure a red heat, without going off. If, again, we were to seek for the elements of the acid and alkali, we must not expect to find them have any properties resembling either an acid or an alkali, but others quite different; and if we could discover anything which was the common element of all bodies, we believed to find a substance which had no property in common with any other in the whole system of nature, and consequently behoved to be imperceptible.

To the above mentioned four elements, viz. fire, air, earth, and water, a kind of fifth element has generally been added, but not usually distinguished by that name, though it has apparently an equal, if not a greater, right to the title of an element than any of the others. This substance is called the phlogiston, or inflammable principle; on which the ignition of all bodies depends. Some have imagined this substance to be the same with fire, or the matter of heat and light; but very absurdly: for the phlogiston is always displaced, and to appearance destroyed by fire; which it could not be if itself were either heat or light. See Phlogiston.

Before we proceed to give a general theory of the changes which happen upon the mixtures of different bodies together, or exposing them singly to heat, we must observe, that all of them depend on certain qualities in bodies, by which some of them are apt to join together and to remain united while they have an opportunity. The cause of these qualities is totally unknown; and therefore, philosophers, after the example of Sir Isaac Newton, have expressed the apparent effect of this unknown cause by the word attraction. From them the word has been adopted by the chemists; and is now generally used in speaking of the phenomena which are observed in the mixture of different substances.

This attraction is not equally strong between all substances; in consequence of which, if any body is compounded of two others, and another is presented to it which has a greater attraction for one of the component parts than they have for one another, the substance will be decomposed. A new compound is then formed by the union of that third substance with one of the component parts or elements (if we please to call them so) of the first. If the attraction between the body superadded, and either of the component parts of the other, is not so strong as that between themselves, no decomposition will ensue; or if the third substance is attracted by both the others, a new composition will take place by the union of all the three.

The objects of chemistry, as we have already observed, are so various, that an enumeration of them all is impossible. To clear the mind therefore, when speaking of them, and render more useful any thing that is said or wrote on chemistry, it is necessary to divide them into different classes, comprehending in each class those bodies which have the greatest resemblance to one another, and to which one common rule applies pretty generally.—The division formerly used, was that of vegetables, animals, and minerals; but this has been thought improper, as there are many substances in each of those kingdoms which differ very widely from one another, and which are by no means subject to the same laws. The most approved method, at present, of arranging the objects of chemistry is into Salts; earths; metals; inflammable substances; waters; animal, and vegetable, substances.

Sect. I. Salts.

1. Salts are either fusible, that is capable of abiding the fire, and melting in a strong heat, without being dissipated; or volatile, that is, being dispersed in vapour with a small heat. Their other properties are, that they are soluble in water; not inflammable, unless by certain additions; and give a sensation of taste, when applied to the tongue.

The most general characteristic of salts is, that they are all soluble in water, though some of them much more difficultly than others. Most of them have likewise the property of forming themselves, in certain circumstances, into solid transparent masses of regular figures, different according to the different salt made use of, and which are termed crystals of that salt. In this state they always contain a quantity of water; and therefore the utmost degree of purity in which a salt can be procured, is when it has been well crystallized, and the crystals are freed of their superfluous moisture. moisture by a gentle heat. They generally appear then in the form of a white powder.

In the solution of salts in water, the first thing observable is, that the water parts with the air contained in it; which immediately rises to the top, in the form of bubbles. This, however, is most remarkable when the salt is in the dry form we have just now mentioned, because there is always a quantity of air entangled among the interstices of the powder, which rises along with the rest; and this discharge of air is sometimes so great, as to be mistaken for an effervescence. From this however, it is essentially different. See Effervescence.

Another thing observable in the solution of salts is, that a considerable change happens in the temperature of the water in which they are dissolved; the mixture becoming either a good deal warmer or colder than either the salt or the water were before. In general, however, there is an increase of cold, and scarce any salt produces heat, except when it has been made very dry, and deprived of that moisture which it naturally requires; and thus the heating of salts by being mixed with water may be explained on the same principle with the heat produced by quicklime. See Quicklime.

After salt has been dissolved in a certain quantity by water, no more of that salt will be taken up, unless the water is heated; and as long as the heat continues to increase, the salt will be dissolved. When the water boils, at which time it has attained its greatest heat, and will take up no more salt, it is then said to be saturated with that salt. This, however, does not prevent it from taking up a certain quantity of another salt, and after that perhaps of a third, or fourth, without letting go any of the first which it had dissolved. How far this property of water extends, has not yet been ascertained by experiments.

To the above rule there is only one exception known as yet; namely, common sea-salt: for water dissolves it in the very same quantity when cold, as when boiling hot. It has been said by some, that all deliquescent salts, or those which grow moist on being exposed to the air, had the same property: but this is found to be a mistake.

This property of solubility, which all the salts possess in common, renders them easily miscible together; and the property by which most of them shoot into crystals renders those easily separable again which have no particular attraction for one another. This is likewise rendered still more easy by their requiring different proportions of water, and different degrees of heat, to suspend them; for by this they crystallize at different times, and we have not the trouble of picking the crystals of one out among those of the other.

The manner in which the solution of salts in water is effected, is equally unaccountable with most of the other operations of nature. Sir Isaac Newton supposed that the particles of water got between those of the salt, and arranged them all at an equal distance from one another: and from this he also accounts for the regular figures they assume on passing into a crystalline form; because, having been once arranged in an orderly manner, they could not come together in disorder, unless something was to disturb the water in which they were suspended; and if any such disturbance is given, we find the crystals are by no means so regular as otherwise they would have proved. Others have thought that these figures depend on a certain polarity in the very small particles into which the salt is resolved when in a state of solution. These things, however, are merely conjectural; neither is it a matter of any consequence to a chemist whether they are right or wrong.

Though solution is that operation which salts undergo the most easily, and which should seem to affect them the least of any, a repetition of it proves nevertheless very injurious to them, especially if it is followed by quick evaporation; and the salt, instead of being crystallized, is dried with a pretty strong heat. Newman relates, that a pound of sea-salt was reduced by 13 solutions, and evictions, to half an ounce; and even that was mostly earth. Where solution is required, therefore, it ought always to be done in close vessels, in which also the subsequent evaporation should be performed, (see Evaporation); and in all cases where crystallization is practicable, it ought to be preferred to violent evaporation.

The two great divisions of salts are into acids and alkalies. The first of these are known by their peculiar taste, which is called acid, or sour. They are not found in a solid form; neither are any of them, except the acids of vitriol, of tartar, of phosphorus, and of borax, capable of being reduced to solidity. The others, when highly concentrated, that is, brought to the utmost degree of strength of which they are capable, always become an invisible vapour, permanently elastic, until it comes in contact with water, or some other substance with which they are capable of uniting. For such acids the name of salts seems less proper, as we can scarcely say that a vapour, which is already much more fluid than water, can be dissolved in that element.

The acids are divided into the mineral, the vegetable, and the animal; expressing their different origin, or where they are most commonly to be found. The mineral acids are commonly reckoned three; the vitriolic, the nitrous, and the marine. To this the acid of borax ought to be added; but its weakness makes it much less taken notice of as an acid than the others. A Swedish chemist, however, Mr Scheele, hath lately added a new acid to the number of the mineral ones, under the name of the fluor acid.

The vegetable kingdom affords only two distinct species of acids. The one appears fluid, and when concentrated to the utmost degree becomes an invisible vapour. This is produced from fermented liquors, under the name of vinegar. An acid similar to this, and which is thought not to be essentially different from it, is found in the juices of certain fruits, as lemons, &c., and is extracted from most vegetables by distillation with a strong fire. The other is likewise a consequence of fermentation; and crusts on the bottom and sides of casks in which wine is put to depurate itself. In its crude state it is called tartar; and when afterwards purified, is called the cream, or crystals, of tartar.

The animal acids, which have hitherto been discovered, vered, are only two: the acid of ants, and that of urine, which is also the acid of phosphorus. The first of these is volatile; and consequently must be supposed a vapour; when in its strongest state: the other is exceedingly fixed; and will rather melt into glass, than rise in vapours. Besides these, it is said an acid is contained in blood; in wasps, bees, &c.; but no experiments have as yet been made on these to determine this matter with any degree of precision.

The alkalies are of two kinds; fixed and volatile. The fixed kind are subdivided into two; the vegetable, and mineral or fossil alkali. The vegetable is so called, because it is procured from the ashes of burnt vegetables; the fossil, because it is found native in some places of the earth, and is the basis of sea-salt, which in some places is dug out of mines in vast quantity. They are called fixed, because they endure a very intense degree of heat without being dissipated in vapour, so as even to form a part of the composition of glass. The volatile alkali is generally obtained by distillation from animal substances. In its pure state this alkali is perfectly invisible; but affects the sense of smelling to such a degree, as not to be approached with safety.

The acids and alkalies are generally thought to be entirely opposite in their natures to one another. Some, however, imagine them to be extremely similar, and to be as it were parts of one substance violently taken from each other. Certain it is, that when separated, they appear as opposite to one another, as heat and cold. Their opposite action indeed very much resembles that of heat and cold, even when applied to the tongue; for the alkali has a hot, bitter, burning taste, while the acid, if not considerably concentrated, always gives a sensation of coldness. In their action too upon animal substances, the alkali dissolves, and reduces the part to a mucilage; while the acid, if not very much concentrated, tends to preserve it uncorrupted.

If an alkaline salt, and moderately strong acid in a liquid state, be mixed together, they will immediately unite; and, provided the alkali has not been deprived of its fixed air, their union will be attended with a very considerable effervescence (see Art. No. 13). If the alkali has been deprived of air, no effervescence will ensue, but they will quietly mix together; but if a due proportion of each has been added, the liquor will neither have the properties of an acid nor an alkali, but will be what is called neutral. The bringing the liquor into this state, is called saturating the acid, or alkali, or combining them to the point of saturation.

If the liquor after such a saturation be gently evaporated, a saline mass will be left, which is neither an acid nor an alkali, but a new compound formed by the union of the two, and which is called a perfect neutral salt. The epithet perfect is given it, to make a distinction between the salts formed by the union of an acid and an alkali, and those formed by the union of acids with earthy or metallic substances; for these will likewise unite with acids, and some of the compounds will likewise crystallise into regular figures; but, because of their weaker union with these substances, the salts resulting from combinations of this kind are called imperfect.

All acids, the volatile sulphurous one excepted, change the blue infusions of vegetables, such as violets, to a red; and alkalies, as well as some of the imperfect neutrals, change them green. This is the nicest test of an acid or alkali abounding in any substance, and seems the most proper method of determining whether a solution intended to be neutral really is so or not.

Though between every acid and alkali there is a very strong attraction, yet this is far from being the same in all; neither is it the same between the same acid and alkali in different circumstances of the acid. When the acids are in a liquid state, and as free as possible of inflammable matter, between which and the nitrous and vitriolic acids there is a very strong attraction, the vitriolic acid will expel any of the rest from an alkaline basis, and take its place. Thus, if you combine the acid of sea-salt, or marine acid, to the point of saturation, with the fossil alkali, a neutral salt will be formed, which has every property of common salt; but, if you pour on a certain proportion of the vitriolic acid, the acid of sea-salt will immediately be expelled; and the liquor, upon being evaporated, will contain not the neutral salt formed by an union of the marine acid with the alkali, but another confining of the vitriolic acid joined with that alkali, and which has quite different properties from the former.

When the acids and alkalies are applied to one another in a liquid state, the vitriolic acid always shows itself to be the most powerful; but when applied in a solid form, and urged with a violent heat, the case is very much altered. Thus, the acid of borax, commonly called sal soda, is so weak as to be disengaged from its basis by every acid applied in a liquid form, that of tartar alone excepted; but if even the vitriolic acid combined with an alkali be mixed with this weak acid, then effervescing, and at last urged with a vehement fire, the vitriolic acid will be disengaged from its basis, and rise in vapours, leaving the weaker acid in possession of the alkali. The same thing happens on adding the phosphoric or urinous acid, to combinations of the vitriolic or other acids with alkaline salts.—When the acids are in a liquid state, then, the most powerful is the vitriolic; next, the nitrous; then the marine; then vinegar; acid of ants; and lastly the sal soda and tartar, which seem to be nearly equal in this respect. As for the fluor acid, no great number of experiments have as yet been made upon it, and Dr Priestley hath rendered it very probable that this new acid is no other than the vitriolic.—If they are applied in a solid form, the most powerful are the sal soda, and phosphoric acid; then the vitriolic, nitrous, marine, and vegetable acids.

When they are reduced to vapour, the case is exceedingly different; for then the marine acid appears to be the most powerful, and the vitriolic the least of any. It is impossible, however, to preserve the vitriolic acid in the form of vapour, without combining it with a certain quantity of inflammable matter, which must necessarily destroy its strength. Dr Priestley, however, found, that the marine acid, when reduced to vapour, was capable of diffusing the nitrous acid from a fixed alkali. The acids have the property of uniting themselves to many other substances besides fixed alkalies, and forming neutral compounds with them. Of these the chief is the principle of inflammability, or phlogiston. In the vitriolic, nitrous, and phosphoric acids, the attraction for this principle is very strong; so great, that the two former will even leave a fixed alkali to unite with it. In the marine acid it is less perceptible; in the liquid vegetable or animal acid still less; and in the acid of tartar, and fat fedatius, not at all.

Besides this, all acids will dissolve metallic and earthy substances; with these, however, they do not in general unite so firmly as with alkaline salts; nor do they unite so strongly with metals as with earths.

In general, therefore, we may expect, that after having dissolved a metal in any acid whatever, if we add an earthy substance to that solution, the acid will quit the metal which it had before dissolved, to unite with the earth. In this case the solution will not be clear, as before; but will remain muddy, and a quantity of powder will fall to the bottom. This powder is the metallic substance itself, but deprived of one of its component parts; and in this case it is said to precipitate in the form of a calx.

If to this new solution of the earthy substance in an acid liquor, a volatile alkaline salt, not deprived of its fixed air, is added, the acid will quit the earth, and unite with the alkaline salt. The earth thus disengaged will again precipitate, and lie at the bottom in fine powder, while the volatile alkali and acid remain combined together, and the liquor again becomes clear.

The attraction between volatile alkalies and acids is considerably less than between fixed alkalies and the same acids. If, therefore, a fixed alkali be now added to the liquor, the volatile alkali will be separated, and the acid will unite with the fixed alkali. The volatile alkali indeed, being perfectly soluble in water, cannot precipitate, but will dissolve its separation by the pungent smell of the mixture; and upon evaporating the liquor, the volatile alkali will be diffused, and a saline mass consisting of the acid and fixed alkali will remain.

Lastly, if the acid employed was the nitrous, which has a strong attraction for the principle of inflammability, if the saline mass be mixed with a proper quantity of inflammable matter, and exposed to a strong heat, the acid will leave the alkali with vast rapidity, combine with the inflammable matter, and be destroyed in flame in a moment, leaving the alkali quite pure.

Though the above-mentioned effects generally happen, yet we are not to expect that they will invariably prove the same, whatever acid is made use of; or even that they will be the same in all possible variety of circumstances in which the same acid can be used.—The acid of tartar is one exception, where the general rule is in a manner reversed; for this acid will quit a fixed alkali for an earth, especially if calcined, and even for iron. If lead, mercury, or silver, are dissolved in the nitrous acid, and a small quantity of the marine acid is added, it will separate the stronger nitrous acid, and fall to the bottom with the metals, in form of a white powder.—The vitriolic acid, by itself, has a greater attraction for earthy substances than for metals; and greater still for fixed alkaline salts than for either of these: but if quicksilver is dissolved in the nitrous acid, and this solution is poured into a combination of vitriolic acid with fixed alkali, the vitriolic acid will quit the alkali to unite with the quicksilver. Yet quicksilver by itself cannot easily be united with this acid.

**Sect. II. Earths.**

These are divided into five classes: 1. Absorbent, General alkaline, or calcareous earths. 2. Argillaceous earths, vison, or clays. 3. The flinty. 4. The fusible earths; and, 5. The talcs.

1. The first class comprehends all those that are capable of being converted into lime. They are found of various degrees of hardness; but none of them are capable of totally resisting the edge of a knife, or striking fire with steel. They are found to consist of a very friable earth, joined with a large quantity of air, and some water. They effervesc with an acid when poured on them; by which they are distinguished from all other kinds of earth, except the argillaceous. When calcined by a strong fire, they part with the water and air which they contained, and then acquire a great degree of causticity, lose their power of effervescing with acids, and become what is called quicklime. They are soluble in acids, but not equally so in all. The vitriolic and tartaric acids form compounds with them very difficultly soluble; the felsites, formed by the vitriolic acid and calcareous earth, requiring, according to Mr Beaumé, an ounce of water to dissolve a single grain of it. The solubility of the tartaric felsite, hath not yet been determined.—With the other mineral acids, the calcareous earths become easily soluble; and by proper management form concretes which appear luminous in the dark, and are called phosphores.

2. The argillaceous earths differ from the calcareous, in not being convertible into quicklime. When mixed into a paste with water, and exposed to the fire, they shrink remarkably, crack in many places, and become excessively hard. By being gently dried in the open air before they are turned, they do not crack, and thus may be formed into vessels of any shape. Of this kind of earth are formed all the brown sort of earthen ware. The purest kind of argillaceous earth naturally found, is that whereof tobacco-pipes are made.

All the argillaceous earths are soluble in acids. With the vitriolic they dissolve into a gelatinous tough liquor very difficultly crystallizable; but which, on the addition of some fixed or volatile alkali, may be fitted into crystals of the salt called alum. With the other acids they form affrighting salts of a similar nature.

The attraction between the argillaceous earths and acids is very weak, yielding not only to alkaline salts both fixed and volatile, but even to some metals, particularly iron; but these earths have as yet been but little the subject of chemical examination in this way. They have a remarkable property of absorbing the colouring matter of cochineal, Brazil-wood, &c., as have also the calces of some metals. See Lake.

Both the calcareous and argillaceous, and indeed all earths earth when pure, resist the utmost violence of fire; but when mixed together will readily melt, especially if in contact with the burning jewel. Dr Lewis having made covers to some crucibles of clay and chalk mixed together, found that they melted into a yellow glass, before the mixtures in the crucibles were fused in the lead. But though they melted thus readily when in contact with the jewel, it was with great difficulty he could bring them to a transparent glass when put into a crucible. See Glass.

The other species of earths, viz., the flinty, fusible, and talcy, being no other way the subjects of chemistry than as they are subservient to the making of glass, all that can be said of them will most properly come under that article. For their different species, see Mineralogy.

Besides the above mentioned species of earths, there are others which may be called anomalous, as having some resemblance of the calcareous and argillaceous, and yet being essentially different from them. These are the white earth called magnesia alba, the earth of burnt vegetables, and that produced from burning animal substances.

Magnesia alba was at first prepared from the thick liquor remaining after the crystallization of nitre; and is now found to be contained in the liquor called bittern, which is left after the separation of common salt from sea-water. In the former case it was united with the nitre, in the latter with the vitriolic acid. It is also found naturally in the soft kind of stone called fleatices or "soap-stone;" and in the concrete used for taking spots out of cloths, called French chalk. It differs from the calcareous earths, in not acquiring any causticity when deprived of its air, of which it contains so large a quantity as to lose two-thirds of its weight when calcined. From the argillaceous it differs in not burning hard when mixed with water, nor forming a tough ductile paste. It is easily soluble in all the acids, even the vitriolic; with which it forms the bitter purging salt commonly called Epsom salt, from its being first discovered in the waters of Epsom. With all the other acids it likewise forms purgative compounds, which are either very difficultly or not at all crystallizable.—Like other pure earths, it cannot be melted by itself; but, on proper additions, runs into a beautiful green glass.

The earth of burnt vegetables is thought by Dr Lewis to be the same with magnesia alba; but on trying the common wood ashes, they were found to be very different. This kind of earth is fusible, by reason of the alkaline salts contained in it. Animal earth is both very difficult of solution in acids, and impossible to be melted in the strongest fire. It dissolves, however, in acid liquors, though slowly; but the nature of the compounds formed by such an union are as yet unknown. The softer parts of animals, such as blood, flesh, &c. are said to yield a more soluble earth than the others.

Sect. III. Inflammable Substances

These comprehend all vegetable, animal, and some mineral substances. They are distinguished from all others, by emitting a gross thick smoke and flame, when a certain degree of heat is applied. To this, however, spirit of wine and all preparations from it are exceptions. They burn without the least smoke; and if a glass bell is held over the burning spirit, no foot is formed, only a quantity of water is found condensed on its sides. Even the groarser oils, if slowly burnt with a very small flame, will yield no foot; and an exceeding great quantity of water, fully equal in weight and bulk to the oil employed, may be obtained from them. We can scarcely, however, credit that such a quantity of water comes from the oil; as this would be a real transmutation; and we know, that, besides water, the oils contain also some quantity of fixed air, as well as earth. It is probable, therefore, that, as it is impossible to sustain flame without a decomposition of that part of the air which rushes in to support it, great part of the water in this case comes from the air, which always contains moisture in abundance.

Inflammable matters, on being burnt, generally leave behind a small quantity of earthy matter called ash; but to this, spirit of wine, camphor, the more volatile oils, and the mineral oil called naphtha, are exceptions. Vegetable substances when distilled in close vessels give out a quantity of air, some acid, and an empyreumatic oil, leaving behind a black spongy mass called charcoal. To this too there are a few exceptions, viz., spirit of wine, and the preparations from it, camphor, and perhaps some of the more volatile oils, or naphtha. Animal substances yield only a very fetid empyreumatic oil, and volatile alkali.

In general, all inflammable matters are acted upon treated with different acids.

There are two singularities observed among the inflammable substances. One is that bituminous matter called amber, which yields a volatile salt of an acid nature on distillation: When combined with alkalies, this acid is found to yield compounds similar to those made with the acetic acid and alkali. The other is, that gum called benzoin, which is used as a perfume, and yields by sublimation, a kind of volatile salt in fine shining crystals like small needles, and of a most grateful odour. These dissolve very readily in spirit of wine; but not at all in water, unless it is made very hot; so that they seem to contain more oily than saline matter. Neither the nature of these flowers, however, nor that of the salt of amber, is fully known.

**Sect. IV. Metallic Substances.**

These are distinguished from all other bodies by their great specific gravity, exceeding that of the most dense and compact stones. The heaviest of the latter do not exceed the specific gravity of water in a greater proportion than that of 4 to 1; but tin, the lightest of all the metals, exceeds the specific gravity of water in the proportion of 7 to 1. They are also the most opaque of all known bodies, and reflect the rays of light most powerfully.

Metallic bodies possess the quality of dissolving in and uniting with acid salts, in common with earths and alkalies; but, in general, their union is less perfect, and they are more easily separable. They effervesce with acids, as well as calcareous earths and alkalies; but their effervescence is attended with very different appearances. In the effervescence of acids with alkalies, or with calcareous earths, there is a discharge of the fluid called fixed air, which is far from being inflammable, that it will immediately extinguish a candle, or other small flame immersed in it. The mixture also is notably diminished in weight. When a metallic substance is dissolved in an acid, the weight of the mixture is never very much diminished, and sometimes it is increased. Thus, an ounce of quicksilver being slowly dropped into as much aqua fortis as was sufficient to dissolve it, and the solution managed so as to take up almost a whole day, the whole was found to have gained 7 grains. There is also a remarkable difference between the nature of the vapour discharged from metals, and that from alkalies; the former, in most cases, taking fire and exploding with violence; the latter, as already observed, extinguishing flame.

The metallic substances, at least such as we are able to decompose, are all composed of a certain kind of earth, and the inflammable principle called phlogiston. The earthy part by itself, in whatever way it is procured, goes by the name of calx. The other principle hath never yet been seen by itself. When these two principles are separated from one another, the metal is then said to be calcined. The calx being mixed with any inflammable substance, such as powdered charcoal, and urged with a strong fire, melts into metal again; and it is then said to be reduced, or revivificated; and this takes place whether the metal has been reduced to a calx by dissolution in an acid, or by being exposed to a violent fire. If, however, the calcination by fire has been very violent and long continued, the calx will not then readily unite with the phlogiston of the charcoal, and the reduction will be performed with more difficulty. Whether, by this means, viz. a long continued and violent calcination, metallic earths might entirely lose their property of combining with phlogiston, and be changed into those of another kind, deserves well to be inquired into.

When a metallic substance is dissolved in any kind of acid, and an alkali or calcareous earth not deprived of its fixed air is added, the alkali will immediately be attracted by the acid, at the same time that the fixed air contained in the alkali is disengaged, and the calx of the metal, having now no acid to keep it dissolved, immediately joins with the fixed air of the alkali, and falls to the bottom. Something similar to this happens when metals are calcined by fire. In this case, there is a continual decomposition of the air which enters the fire; and the fixed air contained in it, being, by this decomposition, set loose, combines with the calx; whence, in both cases, there is a considerable increase of weight. If the air is excluded from a metal, it cannot be calcined even by the most violent fire.

When a metal is precipitated by a mild alkali, or by an uncalcined calcareous earth, the reason of the increase of weight is very evident; namely, the adhesion of the fixed air to the metallic calx: but, though it is not so much increased when precipitated by caustic alkali, or by quicklime, there is nevertheless a very evident increase, which is not so easily accounted for. M. la Voilier, has mentioned some experiments made on mercury and iron dissolved in aqua fortis, which deserve to be taken notice of, as in a great measure accounting for the phenomenon already mentioned of the solution of metallic substances gaining an addition of weight; and likewise shew the proportion of increase of weight with the mild, or calcined calcareous earth.

"Exactly 12 ounces of quicksilver," says he, "were put into a matras, and 12 ounces of spirit of nitre set's expounded on it. Immediately a spontaneous effervescence ensued, attended with heat. The red vapours of the nitrous acid arose from the mixture, and the liquor assumed a greenish colour. I did not wait till the solution was entirely accomplished, before I weighed it; it had lost one drachm, 18 grains. Three hours after, the mercury was nearly all dissolved: but having again weighed the solution, I was much astonished to perceive, that it had increased instead of being diminished in weight; and that the loss, which was one drachm, 18 grains, at first, was now only 54 grains. The next day the solution of the mercury was entirely finished, and the loss of weight reduced to 18 grains; so that in 12 hours the solution, though confined in a narrow necked matras, had acquired an augmentation in weight of one drachm. I added some distilled water to my solution, to prevent it from crystallizing; the total weight of it was then found to be 48 ounces, one drachm, and 18 grains.

"I weighed separately, in two vessels, 8 ounces 15 grains of the above solution, each of which portions, according to the preceding experiment, ought to contain 2 ounces of nitrous acid, and 2 ounces of quicksilver. On the other side, I prepared 6 drachms 36 grains of chalk, and 4 drachms: 36 grains of lime these proportions having been found, by former experiments, just necessary to saturate two ounces of nitrous acid. I put the chalk in the one vessel, and the lime in the other.

"An effervescence attended the precipitation by chalk, but without heat; the mercury precipitated in a light yellow powder; at the same time the chalk was dissolved in the nitrous acid. The precipitation by the lime was effected without effervescence, but with heat; the mercury was precipitated in a brownish powder. When the precipitates were well subsided, I decanted off the liquors from them, and carefully edulcorated..." The precipitate by the chalk weighed 2 ounces, 2 drachms, 45 grains; that by the lime, weighed 2 ounces, 1 drachm, 45 grains.

Sixteen ounces of the nitrous acid, the same as employed in the former experiments, were placed in a matras, and some iron filings gradually added. The effervescence was brisk, attended with great heat, red vapours, and a very rapid discharge of elastic fluid; the quantity of iron necessary to attain the point of saturation, was two ounces, four drachms; after which, the loss of weight was found to be 4 drachms, 19 grains. As the solution was turbid, I added as much distilled water as made the whole weight of the solution to be exactly 6 pounds.

I took two portions, each weighing 12 ounces of the above solution, and containing 2 ounces of nitrous acid, and 2 drachms 36 grains of iron filings. I placed them in two separate vessels; to one were added 6 drachms 36 grains of chalk; and to the other, 4 drachms 36 grains of flaked lime, being the quantities necessary to saturate the acid.

The precipitation was effected by the chalk with effervescence and tumefaction; that by the lime, without either effervescence or heat. Each precipitate was a yellow brown ruff of iron. They were washed in several parcels of distilled water, and then dried in an heat somewhat superior to that used in the last experiment.

The precipitate by the chalk, when dried, was a greyish ruff of iron, inclining even to white by veins; it weighed 6 drachms, 35 grains; that by the lime was rather yellower; and weighed 4 drachms, 69 grains.

"The results of these experiments," says M. la Voilier, "are, 1. That iron and mercury dissolved in the nitrous acid, acquire a remarkable increase of weight, whether they be precipitated by chalk or by lime. 2. That this increase is greater in respect to iron than mercury. 3. That one reason for thinking that the elastic fluid contributes to this augmentation is, that it is constantly greater when an earth is employed, saturated with elastic fluid, such as chalk, than when an earth is used which has been deprived of it, as lime. 4. That it is probable that the increase of weight, which is experienced in the precipitation of lime, although not so great as that by chalk, proceeds in part from a portion of the elastic fluid which remains united to the lime, and which could not be separated by the calcination."

But though we are naturally enough inclined to think that the increase of weight in the precipitates formed by lime proceeded from some quantity of elastic fluid or fixed air which remained combined with the lime, it is by far too great to be accounted for in this way, even according to the experiments mentioned by M. la Voilier himself, and which, from the manner in which they are told, appear to have been performed with the greatest accuracy. He found, that 1 ounce 5 drachms and 36 grains of flaked lime contained 3 drachms and 3 quarters of a grain of water, and only 16 grains and an half of elastic fluid were separable from it. In the experiments above related, where only 4 drachms and 36 grains were employed, the quantity of elastic fluid could not exceed 6 or 8 grains. Yet the calx was increased in mercury, by no less than 105 grains; and in iron, by 203 grains; a quantity quite unaccountable from the elastic fluid, or fixed air, which we can suppose to be contained in the lime made use of.

It is much to be regretted, that the ingenious author of these experiments did not make use of the calces of metals obtained by lime, when trying to expel air from such substances by violent heat. This would have been the experimentum crucis in this case; and could an elastic fluid, similar to fixed air, be extracted from a metallic calx, precipitated by a substance which could communicate none to it, it would be as strong a proof of the generation of such air, as Dr Priestley's extraction of pure air from metallic calces and spirit of nitre is a proof of the original production or generation of air from these substances.

That the increase of weight in metallic calces prepared by fire is owing to an adhesion of air to them, is put beyond a doubt, because that air can be expelled from them. That the increase of weight in the calces prepared from metals dissolved in acids and precipitated by quicklime is owing to the same cause, hath not been proved, because nobody hath tried whether air can be expelled from them or not; at least we have not met with an account of any experiments where such calces were made use of. Hence there is as yet an uncertainty in this subject; and different theories have been invented to explain it. The most remarkable is the following.

Metals are found to be compounded of a kind of positive le-earth mixed with the inflammable principle or phlogiston. The latter is a substance so volatile, and which affords so much eludes our most accurate search, that it is thought to be a principle somewhat like that known in former ages by the name of positive levity. This principle is not only thought to have no tendency towards the earth, or not to be acted upon by the cause of gravitation, but to have a natural tendency upwards. Hence, in proportion as any body contains more of the phlogiston, and less of other principles, it is so much lighter, by reason of the tendency of the inflammable principle upwards, which forms some kind of counterpoise to the action of the cause of gravity on the other principles. The consequence of this is, that when any substance is deprived of its inflammable principle, it ought to be rendered heavier, and actually is so; (see FIRE). When a metal, therefore, is deprived of its phlogiston, we ought not to impute the increase of weight to any thing else than the want of the phlogiston, which formerly balanced in some measure the action of gravity upon the metallic calx.

In support of this theory, the increase of weight in a metallic solution of quicksilver, for instance, has been urged; but the experiments adduced in this way are now found to be fallacious.

Another argument made use of in support of this theory is, that metallic calces, though they are increased in absolute weight, are nevertheless very deficient in specific gravity. This, however, seems so far from the purpose, that it appears to us to prove the direct contrary of what what is intended. If the phlogiston is a principle of positive levity, and whatever substance contains the largest proportion of it is the lightest; then it follows, that the metallic calces, being deprived of this principle of levity, ought to be specifically heavier than after they are combined with it, and assume the metallic form; but as the metals themselves are always found specifically heavier than their calces, we are altogether at a loss for any solid argument in favour of the positive levity of the phlogiston. It is true, if two metals are mixed together, the compound sometimes turns out specifically heavier than either of them taken separately. This no doubt is a curious fact; but is no more extraordinary than that a quantity of salt should dissolve in water without increasing its bulk, (see Fluidity), and so render it specifically heavier than before, by more than the difference between the specific gravity of water and of salt. Thus, suppose the specific gravity of salt to water as 2 to 1. A cubic inch of salt then, dissolved in 5 cubic inches of water, ought to increase the bulk to 6 cubic inches, and render the water $\frac{1}{5}$ specifically heavier than before. But if we suppose the quantity of water capable of receiving a cubic inch of salt, without any addition to its bulk, the specific gravity of the fluid will then be increased by $\frac{1}{5}$. In like manner, if a cubic inch of silver, the specific gravity of which is as 11, receives half a cubic inch of mercury, the specific gravity of which is as 14, without increasing its bulk, the specific gravity of the mixture will be as 13; and mixtures of these two metals are found to be of considerably greater specific gravity than either the silver or mercury by themselves.

In this case, however, we mix two gravitating bodies together, and, by something resembling a penetration of dimensions, they become specifically heavier; but if we mix a gravitating substance with one which does not gravitate at all, we can never make a compound specifically heavier than before. If, instead of a substance which barely does not gravitate, we take one which is positively light, we will be far from making a compound specifically heavier than the original substance was, that it must necessarily be lighter, let us do as we will. The decrease of specific gravity, therefore, in metallic calces, undeniably proves, that along with the dissipation of the phlogiston, there is something added, which, by itself, is specifically lighter than the metal originally was, and to which the decrease of specific gravity in the whole is owing.

Though all metallic bodies, gold, silver, and platinum excepted, are capable of being reduced to a calx by the action of heat alone, yet very different degrees of it are required for calcining them. Lead and tin begin to calcine as soon as they are melted, long before they are made red-hot. The same happens to the feminimetal bismuth and zinc; the latter indeed, being combustible, cannot bear a greater heat in open vessels than that which is barely sufficient to melt it. Iron and copper require a red heat to calcine them; though the former may be made partly to calcine by being frequently wetted in a degree of heat considerably below that which is sufficient to make it red.

Most metals undergo a kind of spontaneous calcination in the open air, which is called their rusting; and which has given occasion to various conjectures. But M. la Voilier has shown, that this arises from the fixable part of the atmosphere attaching itself to their earthly part, and discharging the phlogiston. According to him, no metallic body can rust but where there is an absorption of air; and consequently metals can be but imperfectly rusted when kept under a receiver.

If two metals are mixed together, the compound fusibility generally turns out more fusible than either of them was before the mixture. There are indeed great differences in the degrees of heat requisite to melt them. Thus, lead and tin melt below that degree of heat which is required to make quicksilver or linseed-oil boil. Silver requires a full red heat, gold a low white heat, copper a full white, and iron an extreme white heat to make it melt. The feminmetal called bismuth melts at about $460^\circ$ Fahrenheit's thermometer; and tin at about $422^\circ$. When mixed in equal quantities, the compound melted at $283^\circ$. When the tin was double the bismuth, it required $334^\circ$ to melt it; with eight times more tin than bismuth, it did not melt under $392^\circ$. If to this compound lead is added, which by itself melts in about $540^\circ$, the fusibility is surprisingly increased. Mr Homberg proposed for an anatomical injection a compound of lead, tin, and bismuth, in equal parts; which he tells us keeps in fusion with a heat so moderate that it will notinge paper. Sir Isaac Newton contrived a mixture of the above mentioned metallic substances, in such proportions that it melted and kept fluid in a heat still smaller, not much exceeding that of boiling water. A compound of two parts of lead, three parts of tin, and five of bismuth, did not just stiffen at that very heat, and so would have melted with very little more; and when the lead, tin, and bismuth, were to one another in the proportions of 1, 4, and 5, the compound melted in $246^\circ$. We have seen, however, a piece of metal compounded of these three, the proportions unknown, which melted, and even underwent a slight degree of calcination, in boiling water, and barely stiffened in a degree of heat so gentle that the hand could almost bear it.

A slight degree of calcination seems to give the acids a greater power over metallic substances; a greater makes them less soluble; and if long and violently calcined, they are not acted upon by acids at all. Of all the acids the marine has the greatest attraction for metallic calces, and volatilizes almost every one of them.

Sulphur readily unites with most metals, destroys their malleability, and even entirely dissolves them. On gold and platinum, however, it has no effect, till metals united with a fixed alkaline salt, when it forms the compound called hiper sulphure; which is a very powerful solvent, and will make even gold and platinum themselves soluble in water, so as to pass the filter. This preparation is thought to be the means by which Moses dissolved and gave the Israelites to drink the golden calf which they had idolatrously set up.

When a metal is dissolved in an acid, it may be precipitated, not only by means of calcareous earths and alkalies, but also by some other metals; for acids do do not attract all metals with equal strength; and it is remarkable, that when a metal is precipitated by another, the precipitate is not found in a calcined state, but in a metallic one. The reason of this is, that the precipitating metal attracts the phlogiston which is expelled from that which is diffusing, and immediately unites with it, so as to appear in its proper form. The various degrees of attraction which acids have for the different metals is not yet fully determined. The best authenticated are mentioned in the Table of Affinities or Elective Attractions, (Sect.VII.)

Metalline substances are divided into metals and semimetals. The metals, which are distinguished from the semimetallic substances by their malleability or stretching under the hammer, are in number seven: gold, silver, copper, iron, lead, tin, and platinum. To these is added quicksilver; which Mr Brown's experiments have shown to be a real malleable metal, as well as others, but requiring so little heat to keep it in fusion, that it is always found in a liquid state. The semimetals are bismuth or tin-glass, zinc, regulus of antimony, and cobalt, nickel, and arsenic. This last substance is of a singular nature, and seems to possess a kind of middle nature between the metalline substances and salts. In common with the semimetals, it is capable of being united with the phlogiston in large quantity, when it assumes a splendid metalline form, but wants the ductility of a true metal; so can only be reckoned, even then, among the semimetals. It likewise unites with sulphur, with which it forms a compound of a red or yellow colour, according as more or less sulphur is used. This compound is easily fusible; though the arsenic, by itself, is so volatile as to go all off in vapour rather than melt. In common with the salts, it possesses the properties of dissolving in water, and uniting itself to alkalies. Water will dissolve about $\frac{1}{4}$ of its weight of pure arsenic; but if arsenic is boiled in a strong alkaline lixivium, a much greater proportion will be dissolved. Indeed strong alkaline lixivium will dissolve a part of almost every metalline substance, except gold, silver, and platinum; but, excepting copper, which may be formed into crystals by means of the volatile alkali, none of them will assume a crystalline form when united with alkalies. Arsenic, on the contrary, unites very readily with fixed alkalies, and shoots with them into a neutral salt. If it is mixed with nitre, it unites itself to the alkaline basis of that salt, and expels the acid in very volatile fumes, which are difficultly condensed into a blue liquor. The reason of this is probably the great attraction between the nitrous acid and phlogiston, which are always disposed to unite when a proper degree of heat is applied. Was the phlogiston contained in large quantity in the arsenic, and the heat sufficiently great, a violent deflagration would ensue; but as the arsenic attracts the alkaline part of the nitre, at the same time that the acid attracts the phlogiston, a double decomposition ensues, in a less degree of heat than would otherwise be necessary; and the nitrous acid arises in a very volatile state, as it always is when combined with phlogiston, which is the occasion of the blueness in aqua fortis so produced. The arsenic is also decomposed in part, by being deprived of its proper quantity of phlogiston; in consequence of which it attaches itself so strongly to the fixed alkali of the nitre, that the salt formed by their union cannot be decomposed by the strongest acids. The only method is to present to this salt a metallic substance, which the arsenic unites with in preference to the alkali. The common arsenical salts made with arsenic having its due proportion of phlogiston and alkali, may be decomposed by acids. For the extraction of metallic substances from their ores, and the various methods of refining them, see METALLURGY.

**Sect. V. Waters.**

The pure element of water, like that of fire, is so much an agent in most chemical operations, as to be itself very little the object of practical chemistry; no method being hitherto known of compounding or decomposing it. Waters, therefore, can only be the objects of chemistry in consequence of the impurities they contain: and, as these impurities are most commonly of the saline kind, it is impossible that any general theory can be given of waters, distinct from that of the salts contained in them; which all depend on the general properties belonging to salts, and which we have already mentioned. Any thing that can be said with regard to waters, then, must be postponed to the particular consideration of the properties of each of the saline bodies with which water is capable of being adulterated. We shall therefore refer entirely to the article WATER for what can be said on this subject.

**Sect. VI. Animal and Vegetable Substances.**

The general chemical properties of these have been already taken notice of under the name of inflammable substances. They agree in giving out a very thick fetid oil, when distilled by a strong fire; but in other respects they differ very considerably. Most kinds of vegetables give out an acid along with the oil; but all animal substances (ants, and perhaps some other insects, excepted) yield only a volatile alkali. Some kinds of vegetables, indeed, as mustard, afford a volatile alkali on distillation, similar to that from animal substances; but instances of this kind are very rare, as well as of animals affording an acid. Both animal and vegetable substances are susceptible of a kind of fermentation, called putrefaction, by which a volatile alkali is produced in great plenty: there is, however, this remarkable difference between them, that many vegetable substances undergo two kinds of fermentation before they arrive at the putrefactive stage. The first is called the vinous, when the ardent spirits are produced, which we have already mentioned when speaking of inflammable substances. This is succeeded by the acetic, wherein the vegetable acid called vinegar is produced in plenty: and lastly, the putrefactive stage succeeds when a volatile alkali is only produced; not the smallest vestige either of ardent spirits or of vinegar remaining. On the other hand, animal substances seem susceptible only of the putrefactive fermentation; no instance having ever occurred where there was the least drop, either of ardent spirit or of vinegar, produced from a purified animal substance. (See Fermentation and Putrefaction.) Sect. VII. Of the Chemical Characters, and Tables of Elective Attraction.

The different marks or characters by which the ancient chemists used to denote many different substances, were invented rather from a superstitious and fantastical principle than from any real necessity; or, perhaps, like the enigmatical language used by the alchemists, they have thereby sought to conceal their mysteries from the vulgar. In contriving these marks, they affected a great deal of ingenuity; intending them as symbols of the qualities possessed by each of the different substances. A circle being supposed the most perfect figure, was therefore used to represent the most perfect metal in nature, that is, gold. Silver being likewise a perfect and indestructible metal, is placed next to gold; but, on account of its inferiority, is expressed only by a crescent, as if half gold. A circle was likewise used to denote salt of any kind, as being something elaborate and perfect. A cross was used to denote acrimony of any kind, and consequently employed for the acrimonious salts of vitriol, alkali, &c. Hence, all the inferior metals have the cross somehow or other combined with the marks designed to represent them. Thus, the mark for quicksilver denotes, that it hath the splendor of silver, the weight of gold, but its perfection is hindered by an acrimony represented by the cross at bottom, &c. Fire is represented by an equilateral triangle, having one of its angles uppermost. This may be considered as a rude representation of flame, which is always pointed at top. Water, again, is represented by a triangle, with an angle downwards, shewing the way in which that element exerts its strength, &c. All these marks, however, as they were of no real use at first, so they are now becoming every day more and more neglected. Such of them, however, as may most readily occur in chemical books are represented and explained on Plate LXXVI.

Tables of affinities, or elective attractions, are but of late invention. They are consequences of an improved state of chemistry, when the different substances were found to act upon one another in most cases according to a fixed and settled rule. The most approved table of this kind for a long time was that composed by Mr Geoffrey. It was however found to be very incomplete, not only as to its extent, but likewise as heat and some other circumstances were found to vary the attractions considerably, and sometimes even to reverse them. Other tables have been constructed by Mr Gellert, &c., but none hath yet appeared so complete but that many additions may be made to it. The following is that at present exhibited by Dr Black in his course of chemistry:

1. Vitriolic Acid. Volatile alkali Phlogiston Fixed alkali Calcereous earth Zinc Iron Tin Copper Quicksilver Silver

2. Nitrous Acid. Phlogiston Fixed alkali Calcereous earth Zinc

3. Marine Acid. Fixed alkali Calcereous earth Zinc Iron Lead Tin Copper Regulus of antimony Quicksilver Silver Spirit of wine Volatile oils Gold.

4. Sulphur. Fixed alkali Calcereous earth Iron Nickel Copper Lead Tin Silver Regulus of antimony Quicksilver Arsenic.

5. Hepar Sulphuris is partially decomposed by Quicksilver Solution of fixed alkali Lime-water Volatile alkali.

6. Fixed Air. Calcereous earth Fixed alkali Magnesia Volatile alkali.

7. Alkaline Salts. Vitriolic acid Nitrous acid Marine acid Acetous acid Volatile vitriolic acid Sedative salt Fixed air Sulphur Exprefsed oils.

8. Calcereous Earth. Vitriolic acid Nitrous acid Marine acid

9. Metallic Substances, Lead and Regulus of Antimony excepted. Marine acid Vitriolic acid Nitrous acid Sulphur and acetous acid.

10. Lead. Vitriolic acid Marine acid Nitrous acid Acetous acid Exprefsed oils.

11. Regulus of Antimony. Vitriolic acid Nitrous acid Marine acid Acetous acid.

12. Arsenic. Zinc Iron Copper Tin Lead Silver Gold.

13. Regulus of Antimony with Metals. Iron Copper Tin Lead Silver Gold.

14. Quicksilver. Gold Lead and tin Copper Zinc, bismuth, and regulus of antimony.

15. Silver. Lead Copper Iron.

16. Water. Fixed alkali Spirit of wine Mild alkaline salts, and some neutrals.

17. Spirit of Wine. Water Oils and resins. Plate LXXVI

A Table of Chemical Characters.

| Symbol | Description | |--------|-------------| | △ | Fire | | ▲ | Air | | ▽ | Water | | ▼ | Earth | | £ | Flammable Air | | m | Mephatic Air | | ▽ | Clay | | ▽ | Gypsum | | ▽ | Calcareous Earth | | ΨCV | Quicklime | | ▽ | Vitrifiable or Siliceous Earths | | ▽ | Fluors or Fusible Earths | | X | Talk | | M | Magnesia | | AV | Earth of Alum | | ▽ | Sand | | ⊙ | Gold | | ▽ | Silver | | ⊙ | Copper | | ⊙ | Tin | | ⊙ | Lead | | ⊙ | Mercury | | ⊙ | Iron | | ⊙ | Zinc | | B | Bismuth | | ⊙ | Antimony | | ▽ | Regulus of Antimony | | ⊙ | Arsenic | | ⊙ | Regulus of Arsenic | | K | Cobalt | | N | Nickel | | S.M. | Metallic Substances | | C | Calcite | | ⊙ | Orpiment | | ⊙ | Cinnabar | | L.C. | Lapis Calaminaris | | ⊙ | Tully | | ⊙ | Vitriol | | ⊙ | Sea Salt | | ⊙ | Sal Gem | | ⊙ | Nitre | | S.S. | Sedative Salt | | ⊙ | Sal Ammoniac | | O | Allum | | ⊙ | Tartar | | ⊙ | Alkali | | ⊙ | Fixed Alkali | | ⊙ | Volatile Alkali | | m | Mild fixed Alkali | | c | Caustic fixed Alkali | | m | Mild vol. Alkali | | ⊙ | Potash | | ⊙ | Acids | | ⊙ | Vinegar | | ⊙ | Vitriolic Acid | | ⊙ | Nitrous Acid | | ⊙ | Marine Acid | | F.F. | Aqua Fortis | | R.R. | Aqua Regia | | ⊙ | Vol. Sulphurous Acid | | ⊙ | Phosphoric Acid | | V | Wine | | V | Spirit of Wine | | R | Rectified V | | ⊙ | Either | | ⊙ | Lime Water | | ⊙ | Urine | | ⊙ | Oil | | ⊙ | Essential Oil | | ⊙ | Fixed Oil | | ⊙ | Sulphur | | ⊙ | Hepar of Sulphur | | ⊙ | Phosphorus | | ⊙ | Phlogiston | | ⊙ | Soap | | ⊙ | Verdigrise | | ⊙ | Glass | | ⊙ | Caput Mortuum | | ⊙ | Powder | | E | Ashes | | B | Bath | | B.M.; VB; Water bath | | AB | Sand bath | | VB | Vapor bath | | ⊙ | An Hour | | ⊙ | A Day | | ⊙ | A Night | | ⊙ | A Month | | ⊙ | Amalgam | | ⊙ | To Distill | | ⊙ | To Sublime | | ⊙ | To Precipitate | | ⊙ | A Retort | | XX | An Alcumbic | | ⊙ | A Crucible | | SSS | Stratum Super | | ⊙ | Stratum | | C.C. | Cornu Cervi | | ⊙ | Hartshorn | | ⊙ | A Bottle | | gr. | A Grain | | ⊙ | A Scruple | | ℥ | A Dram | | ℥ | An Ounce | | ℥ | A Pound | | ℥ | A Penny-weight | In consequence of heat, sedative salt decomposes vitriolated tartar and sea-salt.—Phosphoric acid decomposes vitriolated tartar, nitre, and sea-salt.

Double Elective Attractions; which, in some cases, may be considered as exceptions to the foregoing table.

I. Those which happen in mixtures of watery substances.

1. Acids Volatile alkali Calc. earths, or metallic substances Fixed air.

2. Vitriolic or marine acids Mercury, silver, or lead Alkalies or earths Nitrous or acetous acids.

3. Lead Vitriol acid Nitrous, marine, or acetous acids Alkalies, earths, or M.S.

4. Silver Marine acid Vitriolic, nitrous, or acetous acids Alkaline salts, earths, or M.S.

5. Volatile alkali Fixed air Fixed alkali.

6. Nitrous, marine, or acetous acids Volatile alkali, magnesia, or earth of alum Calcareous earths Vitriolic acid.

II. Those which happen in distillations or sublimations, and require heat.

1. Vol. alkali Fixed air Calcaceous earths.

2. Acids Nitrous, marine, or acetous acids Fixed alkali.

3. Vitriol. acid Fixed alkali.

4. Vol. alkali Acetous acid Fixed alkali, or absorbent earths.

5. Reg. of antimon. Marine acid Sulphur Quicksilver.

III. Those which happen in mixtures by fusion.

1. Tin Iron

2. Silver Lead.

3. Copper Sulphur

4. Gold Lead.

5. M.S. Sulphur

6. Gold Reg. of ant.

The first of these tables requires very little explanation. The names printed in small capitals, are those of the substances which have the affinity with or attract those below them. Thus, vitriolic acid attracts most powerfully the phlogiston, or inflammable principle; next, fixed alkali; then, calcaceous earth; and so on, in the order in which they are marked.—The tables of double elective attractions cannot be made quite so distinct; though an explanation of one example will make this likewise easy to be understood. Thus in Table I., the first case is, If a combination of acids with calcaceous earths or metallic substances is mixed with a combination of volatile alkali and fixed air, the acids will unite themselves to the volatile alkali, and the fixed air to the calcareous earth or metallic substance.

Sect. VIII. Of the different Operations in Practical Chemistry, and the proper Instruments for performing each.

The most remarkable operations in chemistry, and by which the greatest changes are made upon those in bodies which are the objects of that science, may be comprehended under the following names. 1. Solution. 2. Filtration. 3. Precipitation, or coagulation. 4. Evaporation. 5. Crystalization. 6. Distillation. 7. Sublimation. 8. Delagration. 9. Calcination. 10. Fusion. 11. Maceration, or digestion. To which we may add, 12. Trituration, or levigation.

Before we proceed to a particular account of each of these operations, it is necessary to take notice, that how divided there are two different things proposed by those who enter on the practice of chemistry. Some have nothing farther in view than the enlargement of their knowledge, or making improvements in arts which are to be profited by others for their own advantage. Others design to follow chemistry as a trade, by which they hope to enrich themselves, or to get a comfortable livelihood. But the apparatus and utensils necessary for performing the very same operations are exceedingly different when experiments only are to be made, from what they must be when these operations are performed with a view to profit; and so great is this difference, that those who pursue chemistry with a view to advantage, will always find themselves very considerable losers if they follow the plan of an apparatus or a laboratory designed only for making experiments. Along with the apparatus, therefore, which is commonly described in chemical books, and proper only for experiments, we shall also give that which is necessary for preparing great quantities of any chemical article in the way of trade.

In general, those who practise chemistry merely with an experimental view, ought, as much as possible, to make use of glass vessels, as not being liable to be used, to be corroded by the most powerful solvents; and, by their transparency, giving an opportunity of observing what passes within them during the operation. But, by those who practise chemistry with a different view, these vessels ought, with equal care, to be avoided, on account of their expense, and brittleness. This last quality, indeed, is possessed by glass in so eminent a degree, that glass vessels will sometimes fly to pieces, and that with considerable violence, when standing by themselves, and nothing touching them. The principal objects which a chemist ought to have in view, in performing his operations, ought to be to save time and fuel, especially the first; and for this purpose, he would find himself a considerable gainer, though he should be at much greater expense in his apparatus than he would otherwise have occasion for.

We shall now proceed to a particular description of each of the operations above mentioned; and first of

Solution. By this is understood the dissolving a solid substance in a fluid, so as that the solid shall totally disappear, and become part of a transparent liquor. This operation applies particularly to salts, earths, and metals; as well as to several unctuous and inflammable substances. For performing this operation in a small way, common vials are in many cases sufficient. Where the solution is attended with effervescence and a discharge of vapors, the long-necked glasses called matraffet, or bolt-heads, (fig. 5), are necessary. Florence flasks are indeed exceedingly well adapted for this operation, as being of the proper shape, and capable of bearing heat so well, that they may be filled with any fluid, and set on a common fire like a metallic vessel. Solution is much promoted by agitating the vessel, and by heat. In some cases, indeed, it will not take place till the mixture becomes very hot; and in such cases it will be proper to make the fluid boiling hot by itself, and then slowly to add the substance to be dissolved.

When large quantities of saline matter are to be dissolved, metallic vessels must be used; but before any are made use of for this purpose, it will be necessary to make an experiment whether the salt receives any impregnation from the metal of which the vessel intended to be made use of is formed; and if this is found to be the case, it must not be used. The metals most liable to be corroded by saline bodies, are iron and copper; and indeed, unless it be for the single purpose of dissolving fixed alkaline salts, iron vessels are generally unfit for saline solutions of any kind. Copper vessels are also very liable to be corroded, and to communicate very deleterious qualities to the liquors which corrode them; for which reason, they ought never to be made use of for the purposes of solution. The metal least liable to be corroded, next to gold and silver, is lead; and therefore a chemist ought rather to provide himself with leaden vessels than those of any other metal. But though lead is not apt to be corroded by many kinds of salts, there are some which are found to act upon it, and to form therewith a very dangerous poison. The vegetable acid of vinegar is particularly apt to receive a dangerous impregnation from this metal, and therefore no solution of any salt containing this acid ought to be made in leaden vessels. It appears to be very little affected by the vitriolic or marine acids, and therefore any saline substance containing either of these acids may be safely enough dissolved in vessels made of lead.

In order to save time in making solutions, the vessels ought to be as large as possible; though even in this there must be a certain limit: for two small vessels filled with water will sooner acquire the necessary degree of heat than one large one; and in proportion as the vessel is made more capacious, the sides and bottom must be thicker, which considerably increases the expense. Fifteen or twenty English gallons is the utmost capacity of which they ever will be required; and is rather above what will on most occasions be necessary. They ought to be of a conical figure, round at the bottom; and to have a cover of thick plate-iron all around that part which is exposed to the action of the fire, that the lead may not bend on the application of heat, which it would otherwise be very apt to do. When the solution is to be made, the leaden vessel is first to be filled up with water so far as to have room for the quantity of salt intended to be dissolved; a fire is then to be applied so as to make it boil; and then the salt is to be added slowly, so as scarcely to hinder the boiling; for if a great quantity was thrown in at once, so as to cool the liquor very much, great part of the salt would concrete on the bottom, in such a manner as not only to be very difficultly fusible, but even to endanger the melting of the vessel. It is of some consequence also to avoid the hot steam which proceeds from the boiling water, and which issues with great force from a narrow-mouthed vessel such as we have been describing. That the operator may be out of the reach of this, and likewise dissolve the salt in a regular and gradual manner, without any danger of its concreting on the bottom, it will be proper to have a leaden, or even a wooden, vessel, with a long handle; which is to be filled with the substance to be dissolved, then immersed in the boiling liquor, and shaken about in it, till the salt is made into a kind of thick pap, which will be in no danger of concreting. It will also be proper not to saturate the water perfectly with salt; for it will in that case be impossible to hinder part of it from settling on the bottom, where it soon acquires such a degree of heat as to melt the lead. Before any saline substance is put into water for solution, it ought to be pounded and sifted through a hair sieve.

Where large quantities of metal are to be dissolved in acids, especially the nitrous acid, glass vessels are in a manner indispensible; although the common stoneware bottles, especially those made in Holland, will answer the purpose very well, as not being liable to corrosion, and not so apt to break as the glass vessels are. They may be got of such a size as to hold 3 or 4 gallons; but no vessel in which metallic solutions are made, ought ever to be above half full.

In solutions of oily and inflammable substances, cast iron vessels are perhaps the most proper of any; though copper ones are generally preferred. The copper is excessively soluble in oil, especially if it is left to cool in such a vessel; but iron is not soluble in any inflammable matter except sulphur. Copper has however this advantage over iron, that it is sooner cooled, as the vessels made of copper are thinner than they can be made of cast iron; so that if too great heat is applied to a copper vessel, it may be easily remedied by taking it off the fire; but in a cast iron vessel the heat continues so long as may sometimes produce dangerous consequences, even after the fire is removed.

2. Filtration. This operation is generally the attendant of solution: very few substances, of the saline kind especially, are capable of being dissolved without leaving some impurities, from which they must be freed; and the doing of this, so as to render the solution perfectly transparent, is what is understood by the word filtration.

For purposes merely experimental, a glass funnel and piece of paper are generally sufficient. The paper is formed into a conical cap, which being placed in the funnel with its point downwards, the funnel is then placed in the mouth of a vial; and the solution or other liquor to be filtered is poured into the paper cap, through which the liquor passes transparent, leaving its impurities on the paper. For the purpose of filtration, paper has come into such general use, that a particular kind kind of it is prepared under the name of filtering paper. This is of a reddish colour; but Dr Lewis prefers the whitish grey paper which comes from Holland about the pill boxes, as not giving any colour to the solutions which pass through it.

This operation, though apparently so simple and easy, is nevertheless attended with very troublesome circumstances, on account of the great time it takes up. Even where very small quantities of liquor are to be filtered, merely for experiment's sake, the impurities frequently settle on the paper so soon, and obstruct its pores to such a degree, that the operator is often quite wearied out; often too, the paper breaks; and thus the whole is spoiled, and the operation must be begun over again.

To avoid these inconveniences, another method of filtration hath been proposed; namely to use a number of cotton threads, the ends of which are to be immersed in the liquor, and the other ends are to hang over the side of the vessel which contains it, and to hang lower than the surface of the liquor. By this means they will act as to many capillary syphons, (see Syphon); the liquid will arise in them quite pure, and be discharged from their lower extremities into a vessel placed to receive it. That the liquor may flow freely into the cotton, it will be proper to wet the threads before they are used.

In point of efficacy, no doubt, this method excels every other; and where the operator has abundance of time and patience, may be proper for experiments; but, in the way of trade, such a contrivance is evidently useless. For filtering large quantities of liquor, therefore, recourse has been had to large funnels; earthen cullenders, or basins full of holes in the bottom, lined with filtering paper; and to conical bags of flannel or canvas.

The inconveniences attending funnels, when used only in the way of experiment, are much greater when they are employed for filtering large quantities of liquor; and therefore they are generally laid aside. The earthen cullenders, too, do not answer any good purpose; nor indeed does filtration through paper in general succeed well. The conical flannel or canvas bags are greatly preferable; but they have this inconvenience, that the pressure of the liquor is directed chiefly against one particular point, or a small part of the bottom, and therefore the impurities are forcibly driven into that place; and thus the operation becomes infinitely tedious.

The best method of obviating the inconveniences of filtration seems to be the following. Let a wooden frame of about three feet square be made, having four holes, one in each corner, about three quarters of an inch in diameter. This frame is to be supported by four feet, the ends of which must project an inch or two through the holes. Thus the whole may be occasionally set up and taken down, so as to go into very little compass; for if the feet are properly placed, each with a little projection outwards, there will be no danger of its falling. A square piece of canvas must also be procured, somewhat less than the wooden frame. On each corner of it there must be a very strong loop, which slips on one of the projecting ends of the feet, so that the canvas may hang a little slack in the middle of the frame. The liquor to be filtered is now poured into the canvas, and a vessel placed underneath to receive it. At first it will pass through very foul; but, being returned two or three times, will become perfectly transparent, and will continue to run with great velocity, if the filter is kept constantly full. A filter of the size just now mentioned will contain ten gallons of liquid; which is a very great advantage, as the heat of such a quantity of liquor is not soon dissipated, and every solution filters much faster when hot than when allowed to cool.

The advantages of a filter of this kind above others arise from the pressure of the liquor being more equally diffused over a large space, by which the impurities are not forced so strongly into the cloth as to stop it up entirely. Yet even here, where large quantities of liquor require filtration, the cloth is apt to be flopped up so as to make the operation not a little tedious and disagreeable. It will be proper therefore to have several cloths, that one may be applied as soon as another is taken off.

To promote the operation of filtration, it is very proper to let the liquors to be filtrated settle for some time; that to their groser feculencies may fall to the bottom, and thus there will be the fewer to retard the last part of the operation. Sometimes, however, these feculencies refuse to settle till after a very long time; and where this happens to be the case, a little powdered quicklime thrown into the boiling liquor remarkably promotes the separation. This, however, can only be used in certain cases.

In some cases, the discovery of a ready way of filtering a large quantity of liquor would be a matter of great consequence; as where a town is supplied with river-water, which is generally far from being clear, and often imparts a disagreeable colour to clothes washed with it. Some years ago, a scheme was proposed by a chemist for filtering muddy water in any quantity. His method was, to have a large cask covered over in the bottom with straw to the depth of some inches, and then filled up with sand. This cask was entirely open at one end, and had a hole in the other, which, by a means of a leaden pipe, communicated with a large reservoir of the water to be filtered, and which stood considerably higher than the cask. The water which descended through the pipe into the cask, having a tendency to rise up to the same level with that in the reservoir, would press violently against the sand, and, as he thought, run over the mouth of the cask perfectly filtrated, and free from its impurities. By this contrivance, indeed, a very violent pressure was occasioned; if the height of the reservoir was considerable; but the consequence was, not a filtration, but a greater of impurity in the water; for the sand was forced out of the cask along with it, and, however confined, the water always rose as muddy as it went in.

Where water is to be filtered in large quantity, as for the purposes of a family, a particular kind of soft spongy stones, called filtering stones, are employed. These, however, though the water percolates through them very fine, and in sufficient quantity at first, are liable to be obstructed in the same manner as paper, and are then rendered useless. A better method seems to be, to have a wooden vessel, lined with lead, three or four feet wide at top, but tapering so as to end in a small orifice at the bottom. The under part of the vessel is to be filled with very rough sand, or gravel, well freed from earth by washing. Over this, pretty fine sand may be laid to the depth of 12 or 14 inches, but which must likewise be well freed from earthy particles. The vessel may then be filled up to the top with water, pouring it gently at first, lest the sand should be too much displaced. It will soon filter through the sand, and run out at the lower orifice exceedingly transparent, and likewise in very considerable quantity. When the upper part of the sand begins to be flopped up, so as not to allow a free passage to the water, it may occasionally be taken off, and the earthy matter washed from it, when it will be equally serviceable before.

3. Precipitation, or Coagulation. This operation is the reverse of solution, and is the bringing a body suddenly from a fluid to a solid state. It differs from crystallization, in that it generally requires less time, and in crystallization the substance assumes regular figures, whereas precipitates are always in the form of powders.

Precipitation is generally preceded by solution and filtration; it is used for separating earths and metals from the acids which had kept them suspended. When a precipitation is made of the more valuable metals, glass vessels are to be used. When earths, or the imperfect metallic substances, are to be precipitated in large quantity, wooden ones answer every purpose. If a metal is to be precipitated by an alkali, this salt must first be dissolved in water, then filtered, and gradually added to the metallic solution. If particular circumstances do not forbid, the salt for precipitation should be chosen in its caustic state, or deprived of its fixed air, because then a very troublesome effervescence is avoided. To promote the operation also, the mixture, if contained in a glass, is to be shaken; or if in any other vessels, to be well stirred after every addition of alkali. If an earth is employed to precipitate a metal, the mixture must be in a manner constantly stirred or shaken, in order to promote the precipitation; and if one metal is to be precipitated by another, that which is used as a precipitant must be beaten into thin plates, that they may be frequently cleaned from the precipitating metal, which would otherwise very soon totally impede the operation.

Sometimes a precipitation ensues on the addition of water, or spirit of wine; but, in most cases, care must be taken not to add too much of the substance which is used to precipitate the other; because, in such a case, the precipitate may be dissolved after it has been thrown down. Thus, though volatile alkali will separate copper from aqua fortis, it will as effectually dissolve the precipitate if too much of it is used, as the acid itself. It is proper, therefore, to proceed cautiously, and examine a small quantity of the liquor from time to time. If an addition of the precipitant throws down any more, it will be proper to add some more to the whole solution.

It is seldom or never that precipitation can be performed so perfectly, but that one or other of the ingredients will prevail; and though they should not, a new compound, consisting of the acid united with the alkali, or other substance used for precipitation, is contained in the liquor through which the precipitate falls. It is proper, therefore, to wash all precipitates; otherwise they can never be obtained perfectly pure, or free from a mixture of saline substances. This is best done by pouring the whole into a filter, and letting the fluid part run off, as long as it will drop, without shaking the cloth. Some water is then to be cautiously poured all over the surface of the precipitate, so as to disturb it as little as possible. This water will push before it the saline liquor which is mixed with the powder, and render it much purer than before. A second, or third quantity of water may be used, in order to wash off all the saline matter. This is called edulcorating the precipitate.

4. Evaporation. This operation consists in evaporating the most fluid or volatile parts of any substance by means of heat. It most generally succeeds solution and filtration, being a preparatory for the operation of crystallization.

For the evaporation of saline solutions, which have been already filtered, and which it is of consequence to preserve from even the least impurities, distilling vessels are unquestionably the most proper; both as, by their means, the solution will be kept perfectly free from dust, and as the quantity of liquor evaporated can be known with certainty by measuring that which comes over. This also is probably the most expeditious method of evaporating, and which requires the least fuel. (See the detached articles Evaporation and Distillation.) With regard to vessels for evaporation, the same thing must be applicable which was mentioned above under Solution. No saline liquor must be evaporated in a vessel which would be corroded by it; and hence iron vessels are absolutely improper for evaporations of any kind of saline liquor whatever.—Lead is in this case the metal most generally useful. It must only be used, however, where the evaporation is not carried to dryness; for, on account of the great fusibility of this metal, nothing could be exsiccated in it without great danger of its melting. Where a saline liquor therefore is to be perfectly exsiccated, the evaporation, if performed in lead vessels, must be carried on so far only as to form a saline pellicle on the surface of the liquor. It is then to be drawn off; for which purpose, all evaporating vessels should have a cock near the bottom. The liquor must now be put into a number of stone-ware basins, set on warm sand, where the exsiccation may be finished.

5. Crystallization. This, though commonly accounted one of the processes in chemistry, is in reality only a natural one, and which the chemist can only prepare for, leaving the operation entirely in the hands of nature.—By crystallization is meant the separation of a salt from the water in which it has been dissolved, in transparent masses regularly figured, and differently formed, according to the different nature of the salts.

This process depends upon the constitution of the atmosphere more than any other; and therefore is difficult to be performed, nor does it always succeed equally well; neither have there yet been laid down any any rules whereby beautiful and regular crystals can with certainty be formed at all times.

As the different salts assume very different figures when crystallized, they are not subject to the same general rules in crystallization. Nitre, Glauber's salt, vitriol of iron, and many others, crystallize best on having their solutions set in a cold place after proper evaporation. Sal polycreft, and common salt, require the solution to be kept as hot as the hand can bear it during the time of crystallizing. Soluble tartar too, and other deliquescent salts, require to be kept warm while this operation is going on; and there are many saline substances, such as the combinations of calcareous earths and magnesia with acids, which can scarcely be crystallized at all.

Mr Beaumé has discovered, that when two or more salts are dissolved in the same quantity of water, when one crystallizes, the crystals of that salt will not contain the least quantity of any of the others; neither, although the liquor was acid or alkaline, will the crystals for that reason be either acid or alkaline, but will remain perfectly neutral; and the acid or alkaline liquor which adheres to the outside of the crystals may be absorbed by merely spreading them on filtering paper.—Hence we are furnished with a better method of shooting salts into large and well-formed crystals than merely by dissolving them in water; namely, by adding to the solutions when set to crystallize, a certain quantity of acid or alkaline liquor, according to the nature of the salts themselves. These additions, however, are not equally proper for all salts; and it is not yet determined what kinds of salts ought to be crystallized in alkaline, and what in acid, liquors.—Soluble tartar and Seignette's salt crystallize best when the liquor is alkaline. Sal sodatius, sal Glauberi, and sal polycreft, require an acid if crystallized in the cold; but sal polycreft forms into very fine and large crystals when the solution is alkaline, and kept as hot as the hand can easily bear.

The best general direction that can be given with regard to the regular crystallization of salts is, that they ought to be set to crystallize in as large a quantity at once as possible; and this, as far as we have observed, without any limit; for by this means, the crystals are formed much larger and better figured than they possibly can be by any other method hitherto known.—As to the form of the vessels in which salts are to be crystallized, little can be said with certainty. They are generally flat, and wider at top than at the bottom. The only proper material, in the large way, is lead.

6. Distillation. This is a kind of evaporation; only in such a manner, that the part of the liquor evaporated is not dissipated in the air, but preserved, by making the steam pass through a spiral pipe, which goes through a large vessel full of cold water, or into cold glass receivers.

This is one of the most common chemical operations; and as there are a variety of subjects which require to be distilled, there is consequently a considerable variety both in the form of the distilling vessels to be used on different occasions, and likewise in the materials of which they are made, as well as the management of the fire during the time of the operation.

The most simple and easily performed distillation is that by the common copper still, (fig. 3). It consists of two parts; one called the body, and the other the head. The body is a cylindrical vessel of copper, which is sometimes turned over in the inside; but where distillation is performed without any regard to the reticulum, the turning is useless. The upper part of the body terminates in a kind of arch, in the middle of which is a circular aperture, about one half, or something less, in diameter, of the breadth of the whole body.—Into this aperture, a round head, made likewise of copper, is fitted, so as to be removable at pleasure. In the top, or sometimes in the side of the head, is inserted a pewter pipe, which communicates with a spiral one of the same metal, that passes through a large wooden vessel, called the refrigerator, filled with cold water; each of its ends projecting a little above and below. The still is to be filled two thirds full of the substance to be distilled, the head put on, and the junctures well closed with a mixture of linseed meal and water, or common flour or chalk and water will answer the same purpose. This mixture is called the lining, or tube. A fire being kindled under the still, the vapours will arise; and, being condensed by the cold water through which the spiral pipe called the worm passes, will run in a stream more or less strong as the fire is more or less hastily urged, and is caught in a receiver set underneath.

This kind of distilling vessels is proper for procuring the essential oils of vegetables, vinous spirits from fermented liquors, and for the rectification of these after they are once distilled. Even the acetic acid may be very conveniently distilled in a copper vessel, provided the worm and all the defending parts of the pipe which communicates with it be of pewter; otherwise a milchious impregnation of copper would be communicated to the distilled vinegar. The reason of this is, that copper is not dissolved by vinegar, or in very small quantity, when that acid is boiled in it; but if the metal is exposed to the action of the acid when cold, or to its vapours, a considerable dissolution takes place. For this reason, too, the still must be washed out after the operation while it continues hot, and must be very carefully freed from the least remains of acid, otherwise it will be much corroded.

Copper-stills ought to be of as large a size as possible; but Dr Lewis very justly observes, that, in common ones, the width of the worm is by no means proportional to the capacity of the still; hence the vapour which issues from a large surface being violently forced through a small tube, meets with much resistance as sometimes to blow off the still-head. This inconvenience is ridiculously endeavoured to be prevented by strongly tying or otherwise forcing down the head; by which means, if the worm should happen to be choked up, a terrible explosion would ensue: for no ligature, or any other obstacle whatever, have yet been found strong enough to resist the elastic force of steam; and the greater the obstacle it has to overcome, the greater would the explosion be.—Dangers of this kind might be totally avoided by having the worm of a proper degree of wideness.

Sometimes, however, matters are to be distilled, minerals such as mineral acid spirits, which would corrode any kind of glass, how... kind of metallic vessels; and for these only earth, or the closest kind of stone-ware, can be used. These are more easily condensed than the steams of aqueous or vinous liquors, and therefore do not require to be passed through a pipe of such a length as is used for condensing the steams from the common still. In these cases, where a violent heat is not necessary, and the distillation is to be performed in glass vessels, the retort is used (fig. 4.) When a fluid is to be put into this vessel, the retort must be laid upon its back on sand, or any other soft matter that will support it without breaking. A funnel must also be procured with a long stem, and a little crooked at the extremity, that the liquor may pass at once into the belly of the retort, without touching any part of its neck; otherwise the quantity which adhered to the neck would pass into the receiver when the retort was placed in a proper situation for distilling, and foul the produce. When the vessel is properly filled, which ought never to be above two thirds, it is to be set in a sand bath; that is, in an iron pot, of a proper thickness, and covered over in the bottom, to the depth of one or two inches, with dry sand. When the retort is put in, so as to stand on its bottom, the pot is to be filled up with sand, as far as the neck of the retort. A glass receiver is then to be applied, which ought to be as large as possible, and likewise pretty strong, for which reason it will be proper not to let the capacity of it be above what is necessary to hold ten gallons. In the hinder part of it should be drilled a small hole, which may be occasionally shut by a small wooden peg. The mouth of the receiver ought to be so wide as to let the nose of the retort enter to the middle of it, or very near it; for if the vapours are discharged very near the luting, they will act upon it much more strongly than when at a distance. It is likewise proper to have the neck of the retort as wide as may be; for this has a very great effect in the condensation, by presenting a larger surface to the condensing vapour.

The luting for acid spirits ought to be very different from that used in other distillations; for these will penetrate the common lutes so as to make them liquid and fall down into the receiver. Some have used retorts, the necks of which were ground to the receivers with emery; but these are very difficult to be procured, and are expensive, and consequently have never come into general use. Various kinds of lutes have been proposed, but the preference seems due to a mixture of clay and sand. We are not to understand, however, that every kind of clay is fit for this purpose: it must only be such as is not at all, or very little, affected by acids; and this quality is only possessed by that kind of which tobacco-pipes is made. Trial ought to be made of this before the distillation is begun, by pouring a little nitrous acid on the clay intended to be made use of. If a violent effervescence is raised, we may be sure that the clay is unfit for the purpose. Finely powdered alabaster would answer extremely well, had it the ductility of clay. As this kind of lute remains soft for a considerable time, it ought to be farther secured by a bit of rag spread with some strong cement, such as quicklime mixed with the white of an egg, &c. Matters, however, ought to be managed in such a manner, that the luting may give way, rather than the vessels burst; which would not only occasion a certain loss of the materials, but might endanger the persons who were standing by.

The iron pots commonly used for distillations by the sand-bath, or balneum mariae, are commonly made very thick; and are to be sold at large foundries, under the name of sand-pots. The shape of these, however, is by no means eligible: for, as they are of a figure nearly cylindrical, if the retort is of such a size as almost to fill their cavity, it cannot be put into them when full, and often pretty heavy, without great danger of touching the sides of the pot; and in this case, touching and breaking are synonymous expressions. It is much better, therefore, to have them in the figure of a punch-bowl; and the common cast-iron kettles, which may be had much cheaper than the sand-pots usually sold, answer extremely well. If the distilling vessel is placed in a pot filled with water, the distillation is said to be performed in a water-bath, or balneum mariae.

When the matter to be condensed is very volatile, a number of open receivers with two necks, called adopters, (fig. 7,) may be used, with a close receiver at the end. Each of these adopters must be fitted with as much care as when only a single receiver is made use of. Vessels of a similar kind were formerly much used by chemists for particular sublimations, under the name of aludels.

Formerly, instead of retorts, a vessel called a cucurbit, (fig. 5, and 6,) with a head like the common still, called an alembic, were used; but the more simple figure of the retort gives it greatly the preference. It is but seldom that vessels of this kind are useful, which will take notice of when describing the particular operations; and if at any time an alembic head should be necessary, its use may be superceded by a crooked glass tube, which will answer the purpose equally well.

Sometimes a very violent fire is required in distillations by the retort. Here, where it is possible, glass or earthen vessels should be avoided, and iron pots substituted in their stead. The hardest and best cast iron, however, will at last melt by a vehement heat; and therefore there is a necessity for using earthen ware, or coated glass. This last is better than most kinds of earthen ware, as being less porous; for when the vessel is urged by a very intense heat, the glass melts, and forms a kind of semivitreous compound with the inside of the coating, so that its figure is still preserved, and the accidental cracks in the luting are filled up.

For coating of glasses, mixtures of colochothar of vitriol, sand, iron filings, blood, chopped hair, &c., have been recommended. We cannot help thinking, however, that the simple mixture of tobacco-pipe clay and sand is preferable to any other; especially if, as Dr Black directs, that part next the glass is mixed with charcoal dust.

The proportions recommended by the Doctor for luting the joints of vessels, are, four parts of sand, and one of clay; but, for lining the insides of furnaces, and we should think, likewise for coating glass vessels, he directs 6 or 7 of sand to 1 of clay; that the contraction tion of the clay in drying may thereby be the more effectually prevented. Besides this, he directs a mixture of three parts of charcoal-dust with one of clay, to be put next the furnace itself, as being more apt to confine the heat; but possibly the first composition might be sufficient for glazes.

The coating of large glazes must be a very troublesome and tedious operation; and, therefore, coated glass is never used but in experiments. When large distillations are to be performed in the way of trade, recourse must be had either to iron pots, or to earthenware. Of the most proper kinds of earthenware for resisting violent heats, we shall take notice under the article Fusion.

In all distillations by the retort, a considerable quantity of air, or other incondensible vapour, is extricated; and to this it is absolutely necessary to give vent, or the vessels would be burst, or the receiver thrown off. For this purpose, Dr Lewis recommends an open pipe to be inserted at the lattering, of such an height as will not allow any of the vapour to escape; but this we cannot approve of, as by that means a constant communication is formed between the external atmosphere, and the matters contained in the retort and receiver, which is at all times to be avoided as much as possible, and in some cases, as the distillation of phosphorus, would be very dangerous. The having a small hole drilled in the receiver, which is to be now and then opened, must answer the purpose much better, although it takes more attendance; but if the operator is obliged to leave the vessels for some time, it will be convenient either to leave the little hole open, or to contrive it so that the wooden peg may be pushed out with less force than is sufficient to break the line.

7. Sublimation. This, properly speaking, is only the distillation of a dry substance; and therefore, when volatile matters, such as salt of hartshorn, are to be sublimed, the operation is performed in a glass retort set in a sand bath, and the salt passes over into the receiver. The cucurbit and alembic were formerly much in use for this purpose; and a blind head, without any spout, was applied. A much simpler apparatus, however, is now made use of. A globe made of very thin glass, or an oblong vessel of the same kind, answers the more common purposes of sublimation. For experiments, Florence flasks are excellent; as being both very cheap, and having the necessary shape and channels requisite for bearing the heat without cracking. The matter to be sublimed must not, on almost any occasion, take up more than a third part of the subliming vessel. It is to be set in a sand-bath, that the heat may be more equally applied than it could otherwise be. The heat must be no greater, or very little, than is necessary for sublimation, or it will be in danger of flying out at the mouth of the subliming vessel, or of choking it up so as to burst. The upper part of the vessel, too, must by no means be kept cool, but slightly covered with sand, that the matter may settle in a kind of half-melted state, and will thus form a compact, hard cake, which is the appearance sublimes are expected to have. Hence this operation requires a good deal of caution, and is not very easily performed.

8. Deflagration. This operation is always performed by means of nitre, except in making the flowers of zinc. It requires open vessels of earth, or Deflagration; the latter are very apt to be corroded, and the former to imbibe part of the matter. To perform this process with safety, and without loss, the nitre ought to be mixed with whatever matter is to be deflagrated with it, and thrown by little and little into the vessel previously made red-hot. If much is put in at once, a great deal will be thrown out by the violent commotion; and to perform this operation in close vessels is in a manner impossible, from the prodigious quantity of elastic vapour generated by the nitre. Care must also be taken to remove the whole mixture to some distance from the fire, and not to bring back any spark from the quantity deflagrating, with the spoon which puts it in; otherwise the whole would irremediably be consumed at once.

9. Calcination. This is the subjecting any matter to a heat so violent, as to dissipate some part of it, without melting what remains. It is often practised on metallic substances, particularly lead, for obtaining the calx of that metal called salammoniac, or red lead.

This operation, as indeed all other chemical ones, is best performed in large quantities, where a particular furnace is constructed for purpose, and a fire kept on day and night without interruption. The flame is made to play over the surface of the metal, and it is continually stirred so as to expose different parcels of it to the action of the heat.

10. Fusion. This is when a solid body is exposed to such a degree of heat as makes it pass from a solid to a fluid state; and as different substances are affected by very different degrees of fusibility, the degrees of melting heat are very various.

Besides the true fusion, there are some kinds of salts which retain to a large proportion of water in their crystals, as to become entirely fluid upon being exposed to a very small degree of heat. This is commonly called the watery fusion; but is really a solution of the salt in that quantity of water retained by it in its crystalline form: for such salts afterwards become solid by the evaporation of the water they contained; and then require a strong red heat to melt them thoroughly, or perhaps are absolutely unfusible.

Of all known substances, metallic and inflammable ones become fluid with the least heat; then come the more fusible metals, lead, tin, and antimony; then some of the more fusible salts; and then the harder metals, silver, gold, copper, and iron; then the mixtures for making glazes; and last of all, the metal called platinum, which has hitherto been incapable of fusion, except by the violent action of the sun-beams in the focus of a large burning glass. This substance seems to be the most refractory of all others, even the hardest flints melting into glaze long before it. (See Platinum.)

Fusion of small quantities of matter is usually performed in pots called crucibles, which, as they are required to stand a very violent heat, must be made of the most refractory materials possible.

The making of crucibles belongs properly to the potter; but as a chemist ought to be the judge of their proper composition, we shall here give some account of the different different attempts to make these vessels of the necessary strength.

All earthen vessels are composed, at least partly, of that kind which is called the argillaceous earth or clay, because they only have the necessary ductility, and can be formed into vessels of the proper form. Pure clay is, by itself, absolutely unfusible; but is exceedingly apt to crack when exposed to sudden changes of heat and cold. It is also very apt to melt when mixed with other substances, such as calcareous earths, etc. When mixed in a certain proportion with other materials, they are changed by violent heat into a kind of half-melted substance, such as our stone-bottles. They cannot be melted completely, however, by almost any fire; they are also very compact, and will contain the most fusible substances, even glaas of lead itself; but as they are very apt to crack from sudden changes of heat and cold, they are not so much used; yet, on particular occasions, they are the only ones which can be made use of.

The more dense any kind of vessels are, the more apt they are, in general, to break, by a sudden application of heat, or cold; hence crucibles are not, in general, made of the greatest density possible; which is not at all times required. Those made at Hesse, in Germany, have had the best reputation for a long time. Mr Pott, member of the academy of sciences at Berlin, hath determined the composition of these crucibles to be, one part of good refractory clay, mixed with two parts of sand, of a middling fineness, from which the finest part has been sifted. By sifting the finer particles from the sand, too great compactness is avoided; but at the same time this mixture renders them apt to be corroded by vitrifying matters kept a long time in fusion; for they do not fail to act upon the sand contained in the composition of the crucible, and, forming a vitreous mass, at last run through it.

This inconvenience is prevented, by mixing, instead of sand, a good baked clay in grofs powder. Of a composition of this kind are made the glaas-houfe pots, which sometimes sustain the violent heat employed in making glaas, for several months. They are, however, gradually consumed by the glaas, and become constantly more and more thin.

As the containing vessel, however, must always be exposed to a more violent heat than what is contained in it, crucibles ought to be formed of such materials as are not vitrifiable by any heat whatever. But, from the attempts made to melt platina, it appears, that of all known substances it would be the most desirable for a melting-vessel. Hessian crucibles, glaashoufe pots, Sturbridge-clay, in short every substance which could be thought of to resist the most violent heat, were melted in such a manner as even to stop up the pipes of large bellows, while platina was not altered in the least; and Meffrs Macquer and Beaumé have flown, that though platina cannot be melted so as to cast vessels of it, it may nevertheless be cupelled with lead so as to become malleable, and thus vessels might otherwise be made from that substance.

The extreme scarcity of this mineral, however, leaves no room to hope anything from it; and Mr Pott has made so many experiments upon clays mixed with different substances, that he has in a manner exhausted the subject. The basis of all his compositions was clay. This he mixed in different proportions with metallic calces, calcined bones, calcareous earths, talcs, amianthus, asbestos, pumice-stones, tripoli, and many others; but he did not obtain a perfect composition from any of them. The best crucibles, according to Scheffer, cannot easily contain metals dissolved by sulphur, in the operation of parting by means of sulphur. They may be made much more durable and solid, by steeping them a few days in linseed-oil, and fuming powdered borax upon them before they are dried.

The results of Mr Pott's experiments are:

1. Crucibles made of fat clays are more apt to crack when exposed to sudden heat, than those which are made of lean or meagre clays. Meagre clays are those which contain a considerable quantity of sand along with the pure argillaceous earth; and fat clays are those which contain but little.

2. Some crucibles become porous by long exposure to the fire, and imbibe part of the contained metals. This may be prevented, by glazing the internal and external surfaces; which is done, by moistening these with oil of tartar, or by fuming upon them, when wetted with water, powdered glaas of borax. These glazings are not capable of containing glaas of lead.

3. Crucibles made of burnt clay grofsly powdered, together with unburnt clay, were much less liable to crack by heat than crucibles made of the same materials where the burnt clay was finely powdered, or than crucibles made entirely of unburnt clay.

4. If the quantity of unburnt clay be too great, the crucible will be apt to crack in the fire. Crucibles made of 10 ounces of unburnt clay, 10 ounces of grofsly powdered burnt clay, and three drachms of calcined vitriol, are capable of retaining melted metals, but are pervaded by glaas of lead. The following composition is better than the preceding: Seven ounces of unburnt clay, 14 ounces of grofsly powdered burnt clay, and one drachm of calx of vitriol. These crucibles may be rendered more capable of containing glaas of lead, by lining their internal surfaces, before they are baked, with unburnt clay diluted with water. They may be further strengthened by making them thicker than is usually done; or by covering their external surfaces with some unburnt clay, which is called arming them.

5. The composition of crucibles most capable of containing the glaas of lead, was 18 parts of grofsly powdered burnt clay, as much unburnt clay, and one part of fusible spar. These crucibles must not, however, be exposed too suddenly to a violent heat.

6. Crucibles capable of containing glaas of lead very well, were made of 24 parts of unburnt clay, four parts of burnt clay, and one part of chalk. These require to be armed.

7. Plume alum powdered, and mixed with whites of eggs and water, being applied to the internal surface of a Hessian crucible, enabled it to retain for a long time glaas of lead in fusion.

8. One part of clay, and two parts of Spanish chalk, made very good crucibles. The substance called Spanish chalk is not a calcareous earth, but appears to be a kind of floaties.

9. Two parts of Spanish chalk, and one part of powdered tobacco-pipes, made good lining for common crucibles.

10. Eight parts of Spanish chalk, as much burnt clay, and one part of litharge, made solid crucibles. Crucibles made of black lead are fitter than Hessian crucibles for melting metals; but they are porous, that fused salts pass entirely through them. They are more tenacious than Hessian crucibles, are not apt to burst in pieces, and are more durable.

Crucibles placed with their bottoms upwards, are less apt to be cracked during the baking, than when placed differently.

The paste of which crucibles are made, ought not to be too moist; else, when dried and baked, they will not be sufficiently compact; hence they ought not to be so moist as to be capable of being turned on a potter's lathe; but they must be formed in brass or wooden moulds.

On this subject Dr Lewis hath also made several observations, the principal of which are,

1. Pure clay softened to a due consistence for being worked, not only coheres together, but sticks to the hands. In drying, it contracts 1 inch or more in 12; and hence it is very apt to crack, unless it is dried exceeding slowly. In burning, it is subject to the same inconvenience, unless very slowly and gradually heated. When thoroughly burnt, if it has escaped those imperfections, it proves solid and compact; and so hard as to strike fire with steel. Vessels made of it are not penetrated by any kind of liquid; and refuse salts and glazes brought into the thinnest fusion, excepting those which by degrees corrode and dissolve the earth itself, as glaze of lead; and even this penetrating glaze, is resisted by it better than by almost any other earth; but, in counterbalance to these good qualities, they cannot be heated or cooled, but with such precautions as can rarely be complied with in the way of utensils, without cracking, or flying in pieces.

2. Clay that has been once exposed to any considerable degrees of heat, and then powdered, has no longer any tenacity. Fresh clay, divided by a due proportion of this powder, proves less tenacious than by itself; not sticking to the hands, though cohering sufficiently together. It shrinks less in drying, is less apt to crack, and less susceptible of injury from alterations of heat and cold; but at the same time is less solid and compact. Considerable differences are observed in these respects; not only according to the quantity of dividing matter, but according as it is in finer or coarser powder.

3. Vessels made with a moderate proportion of fine powder, as half the weight of the clay, are compact and solid, but still very apt to crack, from sudden heat or cold; those with a larger proportion, as twice or thrice the quantity of the clay, are free from that imperfection, but so friable as to crumble between the fingers. Nor does there appear to be any medium between a disposition to crack, and to crumble; all the compounds made of clay and fine powders having the one or the other, or both imperfections. Coarser powders of the size of middling sand, form, with an equal weight of clay, compounds sufficiently solid, and much less apt to crack than the mixtures with fine powders. Two parts of coarse powder, and one of clay, prove moderately solid, and but little disposed to crack: a mixture of three parts and one, though heated and cooled suddenly, does not crack at all, but suffers very fluid substances to transmute through it; solidity, and resistance to quick vicissitudes of heat and cold, seeming here also to be incompatible.

4. Pure clay, mixed with pure clay that has been burnt, is no other than one simple earth; and is neither to be melted, nor softened, nor made in any degree transparent, with the most intense fires.

5. Mixtures of clay with gypseous earths burn whiter than clay alone; in certain proportions, as two parts of clay to three of gypsum, they become, in a moderate fire, semi-transparent, and in a strong one they melt.

6. Calcareous earths in small proportion bake tolerably compact and white; and added to other compositions, seem to improve their compactness. If the quantity of the calcareous earth nearly equals that of the clay, the mixture melts into a yellow glass; if it considerably exceeds, the product acquires the qualities of quicklime.

7. Vessels made from clay and sand, in whatever proportion, do not melt in the strongest fire; but they sometimes bend or soften, so as to yield to the tongs. Glazes in thin fusion penetrate them by dissolving the sand. If gypseous or calcareous earths are urged in such crucibles with a vehement heat, the vessels and their contents run all into one mass. In moderate fires, these vessels prove tolerably compact, and retain most kinds of salts in fusion; but they are liable to crack, especially when large; and do not long sustain melted metals, being burst by their weight. Such are the Hessian crucibles.

8. Mixtures of clay and black-lead, which seems a species of talc, are not liable to crack from alternations of heat and cold; but are extremely porous. Hence black-lead crucibles answer excellently for the melting of metals, and stand repeated fusions; whilst salts flowing thin, transmute through them almost as water through a sieve: sulphureous bodies, as antimony, corrode them.

9. Pure clay, softened with water, and incrusted on earthen vessels that have been burnt, does not adhere to them, or scales off again upon exposure to the fire; applied to unburnt vessels, it adheres and incorporates. Divided clay unites with them in both states. Vitreous matters, melted in vessels of pure clay, adhere so firmly as not to be separated; from vessels of divided clay they may be knocked off by a hammer.

10. The saline fluxes which promote the fusion of clay, besides the common ones of all earths, alkali and borax, are chiefly arsenic fixed by nitre, and the fusible salt of urine, both which have little effect on the other earths though mixed in a large proportion. Nitre which readily brings the crystalline earths into fusion, and sal mirabile and saldiver, powerful fluxes for the calcareous earths, do not perfectly vitrify with clay. Burnt clay does not differ in these respects from such as has not been burnt; nor in that singular property of vitrifying with gypseous or calcareous earths, without any saline or metallic addition; the utmost vehemence of fire seeming to destroy only its ductility, or that power by which it coheres when its parts are moistened with water.

But, though it seems impossible to make perfect vessels from mixtures of clay in its two different states, of burnt and unburnt, more is to be hoped from the mixtures which are employed in making porcelain. Manufacturers of this kind of ware have been attempted in different countries, (see Porcelain); and in some places the qualities requisite for chemical vessels have been given to it in a very surprising degree. The count de Lauragnais, a French nobleman, and member of the academy of sciences, has distinguished himself in a very eminent manner by attempts of this kind. The translator of the chemical dictionary affirms us, that he had it from a gentleman of undoubted veracity, that this nobleman having heated a piece of his porcelain red hot, threw it into cold water, without breaking or cracking it.

The most useful attempt, however, for the purposes of chemistry, seems to be the discovery by Mr Reaumur of converting common green glass into porcelain. This was published so long ago as the year 1739; yet we have not heard of any chemist, nor Dr Lewis himself, who has made trial of chemical vessels formed of this sort of porcelain, although the very use to which Mr Reaumur thought this preparation could be applicable was that of bringing chemical vessels to a degree of perfection which could not otherwise be done. The following is the result of Mr Reaumur's experiments:

Green glass, surrounded with white earthy matters, as white sand, gypsum, or plaster of Paris, &c., and exposed to a considerable heat not strong enough to alter its figure, as that of a potter's furnace, acquires different shades of blue, and by degrees begins to grow white. On breaking the glass, the white coat appears to be composed of fine, white, glossy, satin-like fibres, running transversely, and parallel to one another; the glass in the middle being scarcely altered. On continuing the cementation, the change proceeds further and further, till at length the white fibrous parts from both sides meet in the middle, and no appearance of glass remains. By this means, entire vessels of glass may be changed into porcelain.

The substance into which glass is thus converted, is opaque, compact; internally of great whiteness, equal to that of the finest china-ware; but, externally, of a much duller hue. It is considerably harder than glass, much less fusible in the fire, and suffers alterations of heat and cold without injury. Vessels of it, cold, bear boiling liquids; and may be placed on the fire at once, without danger of their cracking. "I have put a vessel of this porcelain (says the author) into a forge, surrounded it with coals, and kept vehemently blowing for near a quarter of an hour; I have melted glass in this vessel, without its having suffered any injury in its figure." If means could be found of giving the outside a whiteness equal to the internal part, glass vessels might thus be converted into a valuable kind of porcelain, superior to all that have hitherto been made. Chemistry, says he, may receive from this discovery, in its present state, such vessels as have been long wanted; vessels which, with the compactness and impenetrability of glass, are also free from its inconveniences.

The common green glass bottles yield a porcelain of tolerable beauty; window-glasses, and drinking-glasses, a much inferior one; while the finer kinds of crystalline glasses afforded none at all. With regard to the cementing materials, he found white sand and gypsum, or rather a mixture of both, to answer best. Coloured earths generally make the external surface of a deeper or lighter brown colour; foot and charcoal, of a deep black; the internal part being always white.

The account of this kind of porcelain given by Mr Dr Lewis's Reaumur, induced Dr Lewis, who had also observed the same changes on the bottom of glass-retorts exposed to violent heat in a fand-bath, to make further experiments on this matter, an account of which he has published in his Philosophical Commentaries on Arts. The results of his experiments were, 1. Green glass, cemented with white sand, received no change in a heat below ignition. 2. In a low red heat, the change proceeded exceeding slowly; and in a strong red heat, approaching to white, the thickest pieces of glass bottles were thoroughly converted in the space of three hours. 3. By continued heat, the glass suffered the following progressive changes: first, its surface became blue, its transparency was diminished, and a yellowish hue was observable when it was held between the eye and the light. Afterwards it was changed a little way on both sides into a white substance, externally still bluish; and, as this change advanced still further and further within the glass, the colour of the vitreous part in the middle approached nearer to yellow: the white coat was of a fine fibrous texture, and the fibres were disposed nearly parallel to one another, and transverse to the thicknesses of the piece: by degrees the glass became white and fibrous throughout, the external bluishness at the same time going off, and being succeeded by a dull whitish or tan colour. By a still longer continuance in the fire, the fibres were changed gradually from the external to the internal part, and converted into grains; and the texture was then not unlike that of common porcelain. The grains, at first fine and somewhat glossy, became by degrees larger and duller; and at last the substance of the glass became porous and friable, like a mass of white sand slightly cohering. 4. Concerning the qualities of this kind of porcelain, Dr Lewis observes, that, while it remained in the fibrous state, it was harder than common glass, and more able to resist the changes of heat and cold than glass, or even porcelain; but, in a moderate white heat, was fusible into a substance not fibrous, but vitreous and smooth, like white enamel: that when its texture had become coarsely granulated, it was now much softer and unfusible: and lastly, that when some coarsely granulated unfusible pieces, which, with the continuance of a moderate heat, would have become porous and friable, were suddenly exposed to an intense fire, they were rendered remarkably more compact than before; the solidity of some of them being superior to that of any other ware.

It seems surprising that this able chemist, who, on other occasions, has the improvements of the arts so jealously at heart, did not put some vessels of this kind imperfect of porcelain to other severe trials, besides attempting to fuse it by itself with a violent fire: for though pieces of it were absolutely unfusible, we are not sure but they might have been corroded by alkaline salts, acids, acids, calcareous earths, or glaas of lead; nay, it should seem very probable that they would have been so, in which case they would not be much superior to the vessels made from earthly materials. When a fritter chemist publishes any thing in an imperfect state, inferior ones are discouraged from attempting to finish what he has begun; and thus, notwithstanding that these experiments have been to long published, nobody has yet attempted to investigate the properties of this kind of porcelain, by getting chemical vessels made of it, and trying how they answer for crucibles, or retorts.

All that has been said concerning the proper materials for crucibles, must likewise be applicable to the materials for retorts, which are required to stand a very violent heat. Mr Reaumur's porcelain bids fairest for answering the purpose of retorts, as well as crucibles. The great disadvantage of the common earthen ones, is, that they suffer a quantity of volatile and penetrating vapours to pass through them. This is very observable in the distillation of phosphorus; and though this substance has not hitherto been used for any purpose in medicine, and very little in the arts, its acid only being sometimes used as a flux, if vessels could be made capable of containing all the steam, and at the same time bearing the heat necessary for its distillation, phosphorus, perhaps, might be obtained in such quantity, as to show that it is a preparation not altogether useless.

With regard to stone-ware vessels, and all those in which the composition of land or flint enters, we shall only further observe, that they will be corroded by fixed alkaline salts, especially of the caustic kind, in a very moderate heat. Dr Black, having evaporated some caustic ley in a stone-ware basin, and then melted the dry salt in the same vessel, found it so corroded, as afterwards to be full of small holes; and he found nothing to resist the action of this salt so well as silver.

11. Maceration, or Digestion. This is the mixing two bodies, generally a solid and a fluid, together, and then exposing them to a moderate degree of heat for a considerable length of time, so that they may have the better opportunity of acting upon one another. Digestion is usually performed in the glaases already mentioned, called macerates or bolt-heads; and is done in a land heat. When any of the substances are very volatile, as spirit of wine, or the matter requires to be heated so considerably that a quantity of vapour will be raised, the necks of the bolt-heads ought to be pretty long; or a tin pipe may be inserted, of sufficient length to prevent the escape of any part of the steam.

12. Levigation. This is the reducing any body to a very fine powder, which shall feel quite soft between the fingers or when put into the mouth. It is performed by grinding the substance upon a flat marble stone, with some water, or by rubbing it in a marble mortar. In the large way, levigation is performed by mills drawn by horses, or driven by water; some of them are so small as to be turned by the hand. They consist of two smooth stones, generally of black marble, or some other stone equally hard, having several grooves in each, but made to run in contrary directions to one another when the mill is set in motion. The matter being mixed with water, is put in by a funnel, which is fixed into a hole in the upper stone, and turns along with it. The under millstone has round it a wooden ledge, whereby the levigating matter is confined for some time, and at length discharged, by an opening made for that purpose, when it has accumulated in a certain quantity.

In this operation, when the matters to be levigated are very hard, they wear off a part of the mortar, or stones on which they are levigated; so that a substance perfectly hard, and which could not be worn by any attrition, is as great a defideratum for the purposes of levigation, as one which could not be melted is for those of fusion. Dr Lewis proposes the porcelain of Mr Reaumur as an improvement for levigating planes, mortars, &c. because, while in its fibrous state, it is considerably harder than glass, and consequently much less liable to abrasion by the harder powders.

In many cases levigation is very much accelerated by what is called elutriation. This is the method by which many of the painters colours are prepared of the requisite fineness; and is performed by mixing any substance, not totally reduced to the necessary degree of fineness, with a sufficient quantity of water, and stirring them well together. The finer parts of the powder remain some time suspended in the water, while the coarser particles fall to the bottom. The separation is then easily made, by pouring off the water impregnated with these fine parts, and committing the rest to the levigating mill, when it may again be washed; and this may be repeated till all the powder is reduced to the utmost fineness. Substances soluble in water cannot be levigated in this manner.

Of Chemical Furnaces.

The two general divisions we have already mentioned of those who practise chemistry, namely, those who have no other view than mere experiment, and those who wish to profit by it, render very different kinds of furnaces necessary. For the first, those furnaces are necessary which are capable of acting upon a small quantity of matter, yet sufficient for all the changes which fire can produce, from simple digestion, to the most perfect vitrification. For the others, those are to be chosen which can produce the same changes upon very large quantities of matter, that as much may be done at once as possible.

To avoid the trouble and expense of a number of portable furnaces, a portable one hath long been a defideratum furnace among those chemists who are fond of making experiments. One of the best of those, if not the very best, that hath yet appeared, is that described in Shaw's edition of Boerhaave's chemistry, and represented fig. 1.

This furnace is made of earth; and, as the workmanship of a furnace requires none of the neatness or elegance which is required in making potters vessels, any person may easily make a furnace of this kind for himself, who has time and patience for so doing. With regard to the most proper materials, all that we have laid concerning crucibles and retorts must be applicable. cable to the materials for constructing a furnace; only here we need not care so much for the porosity, or disposition to crumble, as when crucibles or other distilling vessels are to be made.

Plate-iron is commonly directed for the outside of portable furnaces; but we cannot help thinking this is a very needless expense, seeing the coating which it necessarily requires on the inside may be supposed to harden to such a degree as soon to support itself, without any affluence from the plate-iron. This will be the less necessary, if we consider, that, for the thickness of the walls of any furnace where a considerable heat is wanted, two or three inches are by no means sufficient. When the inside of a furnace is heated, the walls, if very thin, are soon penetrated by the heat, and great part of it by this means diffused in the air. If they are of a sufficient thickness, the heat cannot penetrate so easily; and thus the inner part of the furnace preserves the heat of the fuel, and communicates it to the contained vessel. In the construction of a portable furnace, therefore, it will be convenient to have all parts of it fix inches thick at least. This will also give it a sufficient degree of strength; and, as it is formed of several different pieces, no inconvenience can follow from the weight of each of them taken separately.

In Boerhaave's chemistry, this furnace is represented as narrower at bottom than at the top; but we cannot suppose any good reason for such a form, seeing a cylindrical one behoved to answer every purpose much better, as allowing a larger quantity of air to pass through the fuel, and likewise not being so apt to be overturned as it necessarily must be where the upper part is considerably heavier than the lower. We have, therefore, given a representation of it as of a cylindrical form.

The furnace consists of five, or more parts. C represents the dome, or top of the furnace, with a short earthen funnel E for transmitting the smoke. B, B, B, are moveable cylinders of earth, each provided with a door D, D, D. In Boerhaave's chemistry these doors are represented as having iron hinges and latches; but they may be formed to more advantage of square pieces of earth, having two holes in the middle, by which they may be occasionally taken out, by introducing an iron fork. In like manner, the domes and cylinders, in Boerhaave's chemistry, are represented with iron handles; but they may be almost as easily taken off by the cheaper contrivance of having four holes in each, two directly opposite to one another, into which two short forks may be introduced when the parts are to be separated.

In the lowermost cylinder is to be placed an iron-grate, a little below the door, for supporting the fire. In the under part is a small hole, big enough for introducing the pipe of a pair of good perpetual bellows, when the fire is to be violently excited. Dr Lewis prefers the organ-bellows to any other kind.

When the bellows is used, the whole must stand upon a close cylinder A, that the air may be confined, and made to pass through the fuel. By having more bellows, the fire may be excited to a most intense degree. In this case, the pipe of every one of them must enter the cylinder B.

Each of the cylinders should have, in its upper part, a round hole, opposite to its door, for carrying off the smoke, by means of a pipe inserted into it, when the furnace is used for distillations by the sand-bath. Each cylinder ought likewise to have a semicircular cut in the opposite sides, both above and below, that when the under cut of the upper cylinder is brought directly above the upper cut of the lower one, a perfect circle may be formed. There are for giving a passage to the necks of retorts, when distillation by the retort is to be performed. The holes may be occasionally filled with stopples made of the same materials with the body of the furnace.

The most convenient situation for a furnace of this kind would be under a chimney; the vent of which might be easily stopped up by a broad plate of iron, in which a hole ought to be cut for the reception of the earthen tube of the dome. By this means the use of a long tube, which at any rate must be very troublesome, might be easily avoided, and a very strong blast of air would pass through the fuel. If it is found convenient to place the furnace at some distance from the chimney, a plate-iron pipe, must be procured to fit the earthen pipe of the dome, and carry the smoke into the chimney. This pipe will also be of use, when the furnace is used for distillations by the sand-bath; it must then be inserted into the hole opposite to the door of any of the cylinders, and will convey away the smoke, while the mouth of the cylinder is totally covered with a sand-pot.

For portable furnaces, Dr Lewis greatly recommends the large black crucibles, marked No. 60, on portable account of their resisting a violent heat, and being very easily cut by a knife or saw, so that doors, &c., may be formed in them at pleasure. The bottom of one of these large ones being cut out, a grate is to be put into the narrow part of it. For grates, the doctor recommends cast-iron rings, having each three knobs around them. These knobs go into corresponding cavities of the outer rings, and the knobs of the outermost rest on the crucible, which is to be indented a little to receive them, so that the grate may rest the more firmly, and the furnace not be endangered from the swelling of the iron by heat. When this is to be made use of as a melting-furnace, and a violent heat be to be excited, another crucible must be inverted on that which contains the fuel, which serves instead of the dome of the last mentioned furnace; and as whatever is said of it must likewise be applicable to the two crucibles when placed above one another, we need give no farther description of the doctor's portable furnace.

No doubt, the great experience of Dr Lewis in chemical matters must give very considerable weight to his advice; and the warmth with which he recommends these furnaces must convince us, that cases have been found them abundantly answer the purposes of experiments. We cannot help thinking, however, that where a very great and lasting heat is to be given, the thinness, and even the form, of these crucibles, is some objection to their use. It is certain that such a permanent, or, as the workmen call it, a solid heat, can never be given where the walls of a furnace are thin, thin, as when they are of sufficient thickness. They are also very apt to burst with great heat; and, for this reason, Dr Lewis desires his furnace to be strengthened with copper hoops. This disposition to burst proceeds from the inner parts, which are more intensely heated than the outer, expanding more than they do, and consequently bursting them. Hence the doctor desires his furnace to be strengthened also by putting it within another crucible of a larger size, and the intermediate space to be filled up with a mixture of sifted ashes and water. For most chemical processes, where only a small degree of heat is requisite, these furnaces answer beyond anything that has hitherto been attempted. The whole is to be supported by an iron ring with three feet.

When furnaces are used in the large way they are always built of brick, and each particular operation has a furnace allotted for itself. The melting-furnace, where very large quantities of matter are not to be melted at once, requires only to be built of brick in such a form as we have already described; only, as it would perhaps be troublesome to procure a dome of the proper figure, the forepart of it may be left entirely open for the admission of melting vessels. The opening may be closed up with bricks and earth, during the operation. There is no necessity for having the inside of a circular form; a square one will answer the purpose equally well. According to the author of the Chemical Dictionary, when the internal diameter D C of such a furnace is 12 or 15 inches, the diameter of the tube G I, 8 or 9 inches, and its height 18 or 20 feet, and when the furnace is well supplied with fuel, an extreme heat is produced; in less than an hour the furnace will be white and dazzling like the sun; its heat will be equal to the strongest glass-house furnace; and in less than two hours will be melted whatever is fusible in furnaces. The hottest part is at H F, 4 or 6 inches above the grate. A plate-iron tube may be advantageously supplied by a short chimney of bricks, built under a pretty high vent, so as the whole may easily be stopped, except that passage which transmits the smoke of the furnace. By this means a very strong current of air will be made to pass through the fuel.

Chemists have generally believed that a wide and high air-hole greatly increases the power of a melting furnace; but this advantage is found to be merely imaginary, as well as that of introducing the air through a long tube to the air-hole; unless where the furnace is placed in a close room, so that it is necessary to furnish a greater blast of air than can otherwise have access.

For the form of the furnaces necessary in assaying and melting of ores, or making glass, see Essaying, Glass, and Smelting.

When large stills, sand-pots, &c., are to be fixed, with a view to daily use, it is a matter of no small consequence to have them put up in a proper manner. The requisites here are, 1. That the whole force of the fire should be spent on the distilling vessel or sand-pot, except what is necessarily imbibed by the walls of the furnace. 2. That the vessel should be set in such a manner as that they may receive heat even from the furnace walls; for a still which contains any liquid, can never be made so hot as a piece of dry brick.

3. It is absolutely necessary that the force of the fire be not allowed to collect itself upon one particular part of the vessel; otherwise that part will soon be destroyed.

4. The draught of air into furnaces of this kind ought to be moderate; only so much as will prevent smoke. If a strong blast of air enters, not only a great part of the heat will be wasted by going up the chimney, but the outside of the vessel will be calcined every time the fire is kindled, and thus must be soon rendered unfit for use.

There are few of the common workmen that are capable of building furnaces properly; and it is very necessary for a chemist to know when they are properly done, and to make the workmen act according to his directions. As the still, or whatever vessel is to be fixed, must have a support from the furnace on which it is built, it is evident the whole of its surface cannot be exposed to the fire. For this reason many of these vessels have had only their bottom exposed to the fire; no more space being left for the action of the heat, than the mere circular area of the still bottom; and the fire, passing directly through a hole in the back part of the building, which communicated with a chimney, and consequently had a strong draught, scarce spent any of its force on the still, but went furiously up the chimney. By this means an extraordinary waste of fuel was occasioned; and that part of the still-bottom which was next the chimney receiving the whole force of the flame, was soon destroyed. Attempts were made to remedy this inconvenience, by putting the fire something forward, that it might be at a greater distance from the chimney, and consequently might not spend its force in the air. This too was found to avail very little. A contrivance was then fallen upon to make the vent pass round the body of the still in a spiral form. This was a considerable improvement; but had the inconvenience of making the fire spend itself uselessly on the walls of the furnace, and besides wasted that part of the still which touched the under part of the vent. A much better method is to build the back part of the furnace entirely close, and make the fire come out through a long narrow opening before, after which it passes out through a flue in the back and upper part of the furnace, into the chimney.

The only inconvenience of this form is, that the vent must either be very wide, or it is apt to choke up with dust, which last is a very troublesome circumstance. If the vent is made very wide, a prodigious draught of air rushes through the fuel, and increases the heat to such a degree as to calcine the metal of which the still is made; and, on the other hand, nothing can be more disagreeable than to have the vent of a furnace stopped up with dust. These inconveniences, however, are totally avoided by making two small vents, one on each side of the distilling vessel, which may communicate with a chimney, by means of two tubes either of plate-iron, or formed with clay or bricks, which may be occasionally taken off if they happen to be choked up. The vessel is to be suspended by three trunions, so that the whole surface may be exposed to the fire, excepting a ring the thickness of a brick all round; so that a very strong heat will The two small vents on each side will draw the flame equally; and by this means the most equable heat can be preserved, and may be pushed so far, as to make the whole bottom and sides of the vessel intensely red. Such a construction as this is more especially useful for sand-pots, and those which are used for distilling alkaline spirits from bones.

In the use of the furnaces hitherto described, the attendance of the operator is necessary, both for inspecting the processes, and for supplying and animating the fuel. There are some operations of a slower kind, that require a gentle heat to be continued for a length of time; while demand little attendance in regard to the operations themselves; and in which, of consequence, it is extremely convenient to have the attendance in regard to the fire as much as possible dispensed with. This end has been answered by the furnace called athanor; but the use of it has been found attended with some inconveniences, and it is now generally laid aside.

Sundry attempts have been made for keeping up a continued heat, with as little trouble as in the athanor, by the flame of a lamp; but the common lamp-furnaces have not answered so well as could be wished. The lamps require frequent fiddling, and smoke much; and the foot accumulated on the bottom of the vessel placed over them, is apt, at times, to fall down and put out the flame. The largeness of the wick, the irregular supply of oil from the reservoir by jets, and the oil being suffered to sink considerably in the lamp, so that the upper part of the wick burns to a coal, appeared to be the principal causes of these inconveniences; which, accordingly, were found to be in great measure remedied by the following construction.

The lamp consists of a brass pipe, 10 or 12 inches long, and about a quarter of an inch wide, inserted at one end into the reservoir of the oil, and turned up at the other to an elbow, like the hole of a tobacco-pipe, the aperture, which is extended to the width of near two inches. On this aperture is fitted a round plate, having 5, 6, or 7 small holes, at equal distances, round its outer part, into which are inserted as many pipes about an inch long; into these pipes are drawn threads of cotton, all together not exceeding what in the common lamps form one wick: by this division of the wick, the flame exposes a larger surface to the action of the air, the fuliginous matter is consumed and carried off, and the lamp burns clear and vivid.

The reservoir is a cylindric vessel, eight or ten inches wide, composed of three parts, with a cover on the top. The middle partition communicates, by the lateral pipe, with the wicks; and has an upright open pipe soldered into its bottom, whose top reaches as high as the level of the wicks; so that, when this part is charged with oil, till the oil rises up to the wicks in the other end of the lamp, any further addition of oil will run down through the upright pipe into the lower division of the reservoir. The upper division is designed for supplying oil to the middle one; and, for that purpose, is furnished with a cock in the bottom, which is turned more or less, by a key on the outside, that the oil may drop fast enough to supply the consumption, or rather faster, for the surplus is of no inconvenience, being carried off by the upright pipe; so that the oil is always, by this means, kept exactly at the same height in the lamp. For common uses, the middle division alone may be made to suffice; for, on account of its width, the sinking of the oil will not be considerable in several hours burning. In either case, however, it is expedient to renew the wicks every two or three days; oftener or seldom, according as the oil is more or less foul; for its impure matter, gradually left in the wicks, occasions the flame to become more and more dull. For the more convenient renewing of them, there should be two of the perforated plates; that, when one is removed, another, with wicks fitted to it, may be ready to supply its place.

One of the black lead-pots, recommended by Dr Lewis for his portable furnace, makes a proper furnace for the lamp. If one is to be fitted up on purpose for this use, it requires no other aperture than one in the bottom for admitting air, and one in the side for the introduction of the elbow of the lamp. The reservoir stands on any convenient support without the furnace. The stopper of the side aperture consists of two pieces, that it may be conveniently put in after the lamp is introduced; and has a round hole at its bottom fitting the pipe of the lamp. By these means, the furnace being set upon a trever or open foot, the air enters only underneath, and spreads equally all round, without coming in streams, whence the flame burns steady. It is not advisable to attempt raising the heat higher than about the 450th degree of Fahrenheit's thermometer; a heat somewhat more than sufficient for keeping tin in perfect fusion. Some have proposed giving a much greater degree of heat in lamp-furnaces, by using a number of large wicks; but when the furnace is so heated, the oil emits copious fumes, and its whole quantity takes fire. The bain-marie, or other vessel including the subject-matters, is supported over the flame by an iron ring, as already described in the sand-bath and still: a bath is here particularly necessary, as the subject would otherwise be very unequally heated, only a small part of the vessel being exposed to the flame.

**Part II. Practice of Chemistry.**

**Sect. I. Salts.**

1. Vitriolic Acid, and its Combinations.

The vitriolic acid is never found pure, but always united with some proportion, either of phlogiston, or metallic and earthy substances. Indeed there is scarcely any kind of earth which does not contain some portion of this acid, and from which it may always come away or other be separable. When pure, the vitriolic acid appears in the form of a transparent colourless liquor. By distilling in a glass retort, the aqueous part arises, and the liquor which is left becomes gradually more and more acid. This operation is generally Generally called the rectification, or dephlegmation, of the acid. After the distillation has gone on for some time, the water adheres more strongly to what remains in the retort, and cannot be forced over without elevating part of the acid along with it. The remaining acid, being also exceedingly concentrated, begins to lose its fluidity, and puts on the appearance of a clear oil. This is the state in which it is usually sold, and then goes by the name of oil of vitriol. If the distillation is still further continued, with a heat below 600° Fahrenheit's thermometer, the acid gradually loses more and more of its fluidity, till at last it congeals in the cold, and becomes like ice. In this state it is called the icy oil of vitriol. Such exceeding great concentration, however, is only practised on this acid for curiosity. If the heat be suddenly raised to 600°, the whole of the acid rises, and generally cracks the receiver. Clear oil of vitriol is immediately turned black by an admixture of the smallest portion of inflammable matter.

The icy oil of vitriol, and even that commonly sold, attracts the moisture of the air with very great force. Newman relates, that having exposed an ounce of this acid to the air, from September 1736 to September 1737, at the end of the twelvemonth it weighed seven ounces and two drachms; and thus had attracted from the air above six times its own weight of moisture. This quantity, however, seems extraordinary; and it is probable, that in so long a time some water had been accidentally mixed with it; for Dr Gould, professor at Oxford, who seems to have tried this matter fully, relates that three drachms of oil of vitriol acquired, in 57 days, an increase only of six drachms and an half. The acid was exposed in a glass of three inches diameter; the increase of weight the first day was upwards of one drachm; in the following days less and less, till, on the fifty-sixth, it scarce amounted to half a grain. The liquor, when saturated with humidity, retained or lost part of its acquired weight, according as the atmosphere was in a moist or dry state; and this difference was so sensible as to afford an accurate hygrometer. Hoffman having exposed an ounce and two scruples in an open glass-dish, it gained seven drachms and a scruple in 14 days.

This acid, when mixed with a large quantity of water, makes the temperature something colder than before; but if the acid bears any considerable proportion to the water, a great heat is produced, so as to make the vessel uninhabitable to the hand; and therefore such mixtures ought very cautiously, or rather not at all, to be made in glass vessels, but in the common stone-bottles, or leaden vessels, which are not apt to be corroded by this acid. The greatest heat is produced by equal parts of acid and water.

Though the vitriolic acid unites itself very strongly with alkalies, both fixed and volatile, it does not saturate near so much of the latter as of the former. A pound of oil of vitriol will saturate two of the common fixed alkalies, but scarcely one of volatile alkalies. The specific gravity of good oil of vitriol is to water as 17 to 8.

If the concentrated acid is applied slightly and superficially to the skin of a living animal, it raises a violent burning heat and pain; but a larger quantity pressed on, so as to prevent the ingress of aerial moisture, occasions little pain or erosion. If diluted with a little water, it proves corrosive in either case. Largely diluted with water, this acid is employed medicinally for checking putrefaction, abating heat, and quenching thirst; in debilities of the stomach, and heartburn. To persons of weak and unfound lungs, to women who give suck, to hydroptic or emaciated persons, it is injurious. Some recommend it as a collyrium for sore eyes; but as it coagulates the animal juices, corroding and indurating the follicles, it seems very unfit for being applied to that tender organ.

The vitriolic acid is so much used in different arts and manufactures, that the making of it has become a profitable trade by itself; and the procuring it in plenty, and at a cheap rate, would be a very advantageous piece of knowledge to any person who could put it in practice. This, however, is very far from being easily done; for though it exists in almost every mineral substance, the attraction between this acid and the bases with which it unites, is found to be so strong, that we can only decompose such combinations by presenting another substance to the acid, to which it has a greater attraction than that one wherewith it is joined. Thus the first combination is indeed dissolved; but we have another from which it is equally difficult to extricate the acid by itself. Thus, if we want to disengage the vitriolic acid from any metallic substance, suppose iron, this may be easily done by throwing a calcareous earth into a solution of green vitriol. We have now a compound of vitriolic acid with the calcareous earth, which is known by the name of gypsum or selenite. If we want to decompose this, we must apply a volatile or a fixed alkali; and the result of this will certainly be a new combination, which we are as unable to decompose, and indeed more so, than the first. There are two general methods which have been in use for procuring the vitriolic acid in such quantity as to supply the demands of trade. The one is from pyrites, and the other from sulphur.

The extraction of Vitriolic Acid from Pyrites, the making of Copperas, and obtaining the pure Oil of Vitriol from it.

Pyrites are found in large quantity in the coal-mines of England, where most of the copperas is made. They are very hard and heavy substances, having a kind of brassy appearance, as if they contained that metal; and hence they are called brasses by the workmen. A very large quantity of these is collected, and spread out upon a bed of stiff clay to the depth of three feet. After being some time exposed to the air, the uppermost ones lose their metallic appearance, split, and fall to powder. The heaps are then turned, the under part uppermost, so as to expose fresh pyrites to the air. When they are all reduced to powder, which generally requires three years, the liquor, which is formed by the rain-water running from such a large mass, becomes very acid, and has likewise a styptic vitriolic taste. It is now conveyed into large cisterns lined with clay, whence it is pumped into a very large flat vessel made of lead. This vessel, which contains about 15 or 20 tons of liquor, is supported by cast-iron plates. plates about an inch thick, between which and the lead a bed of clay is interposed. The whole rests upon narrow arches of brick, under which the fire is placed. Along with the liquor, about half a ton or more of old iron is put into the evaporating vessel. The liquor, which is very far from being saturated with acid, acts upon the iron, and, by repeated filling up as it evaporates, dissolves the whole quantity. By the time this quantity is dissolved, a pellicle is formed on the surface. The fire is then put out; and as such a prodigious quantity of liquor does not admit of filtration, it is left to settle for a whole day, and then is let off by a cock placed a little above the bottom of the evaporating vessel, so as to allow the impurities to remain behind. It is conveyed by wooden spouts to a large leaden cistern, five or six feet deep, sunk in the ground, and which is capable of containing the whole quantity of liquor. Here the copperas crystallizes on the sides, and on sticks put into the liquor. The crystallization usually takes up three weeks. The liquor is then pumped back into the evaporating vessel; more iron, and fresh liquor from the pyrites, are added; and a new solution takes place. See no. 41.

Copperas is used, in dyeing, for procuring a black colour; and is an ingredient in making common ink. It is also used in medicine as a corrosive, under the name of salt of steel; but before it is used with this intention, it is redissolved in water, and crystallized, with the addition of a little pure oil of vitriol. Whether it is at all mended by this supposed purification, either in appearance or quality, is very doubtful.

This process furnishes us first with a very impure vitriolic acid, which could not be applied to any useful purpose; afterwards with an imperfect neutral salt, called green vitriol, which is applicable to several purposes where the pure acid itself could not be used; but still the acid by itself is not to be had, without a very troublesome operation.

Though this acid adheres very strongly to iron, it is capable of being expelled from it by fire; yet not without a very violent and long-continued one. If we attempt to distill green vitriol in a retort, it swells and boils in such a manner by the great quantity of water contained in its crystals, that the retort will almost certainly crack; and though it should not, the salt would be changed into an hard flinty mass, which the fire could never sufficiently penetrate so as to extract the acid. It must therefore be calcined, previous to the distillation. This is best done in flat iron-pans, set over a moderate fire. The salt undergoes the watery fusion, (see Fusion); after which it becomes opaque and white. By a continuance of the fire, it becomes brown, yellow, and at last red. For the purposes of distillation, it may be taken out as soon as it has recovered its solidity.

The dry vitriol, being now reduced to powder, is to be put into an earthen retort, or rather long neck, (a kind of retort where the neck issues laterally, that the vapours may have little way to ascend), which it may nearly fill. This retort must be placed in a furnace capable of giving a very strong heat, such as the melting furnace we have already described. A large receiver is to be fitted on; and a small fire made in the furnace, to heat the vessels gradually. White fumes will soon come over into the receiver, which will make the upper part warm. The fire is to be kept of an equal degree of strength, till the fumes begin to disappear, and the receiver grows cool. It is then to be increased by degrees; and the acid will become gradually more and more difficult to be raised, till at last it requires an extreme red, or even white heat. When nothing more will come over, the fire must be suffered to go out, the receiver be unluted, and its contents poured into a bottle fixed with a glass stopper. A fulphureous and suffocating fume will come from the liquor, which must be carefully avoided. In the retort, a fine red powder will remain, which is used in painting, and is called celestite of vitriol. It is useful on account of its durability; and, when mixed with tar, has been employed as a preservative of wood from rotting; but Dr Lewis prefers finely powdered pitch-coal. As a preservative for masts of ships, he recommends a mixture of tar and lamp-black, concerning which he relates the following anecdote.

"I have been favoured by a gentleman on board of a vessel in the East Indies, with an account of a violent thunder-storm, by which the main-mast was greatly damaged, and whose effects on the different parts of the mast were pretty remarkable. All the parts which were greased or covered with turpentine were burst in pieces; those above, between and below the greased parts, as also the yard-arms, the round top or scaffolding, coated with tar and lamp-black, remained unhurt."

Oil of vitriol, when distilled in this manner, is always of a black colour, and must therefore be rectified by distillation in a glass retort. When the acid has attained a proper degree of strength, the blackness either flies off, or separates and falls to the bottom, and the liquor becomes clear. The distillation is then to be discontinued, and the clear acid which is left in the retort kept for use.

This was the first method by which the vitriolic acid was obtained; and from its being distilled from vitriol has ever since retained the name of oil of vitriol. Green vitriol is the only substance from which it is practicable to draw this acid by distillation; when combined with calcareous earths, or even copper, (though to this last it has a weaker attraction than to iron,) it refills the fire most obstinately. When distillation from vitriol was practised, large furnaces were erected for that purpose, capable of containing an hundred long necks at once: but as it has been discovered to be more easily procurable from sulphur, this method has been laid aside, and it is now needless to describe these furnaces.

To procure the Vitriolic Acid from Sulphur.

This substance contains the vitriolic acid in such quantity of plenty, that every pound of sulphur is reckoned to contain 15 ounces of pure acid; which being in a state perfectly dry, is consequently of a strength far beyond that of the most highly rectified oil of vitriol. Common oil of vitriol requires to be distilled to one fourth of its quantity before it will coagulate when cold; and even in this state, it undoubtedly contains some water. Making allowances, therefore, for the acid which rises in distillation, we may reckon, that in a pound Every pound of sulphur therefore, if all the acid it contains could be preserved, ought to yield two pounds and an half of highly concentrated acid. No method, however, has as yet been fallen upon to condense all the fumes of burning sulphur; nor is any other profitable way of decomposing sulphur known, than that by burning; and in this way the most successful operators have never obtained more than 14 ounces of oil from a pound of sulphur.

The difficulties here are, that sulphur cannot be burnt but in an open vessel; and the stream of air, which is admitted to make it burn, also carries off the acid which is emitted in the form of smoke. To avoid this, a method was contrived of burning sulphur in large glass globes, capable of containing an hog's head or more. The fume of the burning sulphur was then allowed to circulate till it condensed into an acid liquor. A greater difficulty, however, occurs here; for though the sulphur burns very well, its fumes will never condense. It has been said, that the condensation is promoted by keeping some warm water continually smoking in the bottom of the globe; and even Dr Lewis has affected this; but the steam of warm water immediately extinguishes sulphur, as we have often experienced; neither does the flame of burning sulphur seem at all inclined to join with water, even when forced into contact with it. As it arises from the sulphur, it contains a quantity of phlogiston, which in a great measure keeps it from uniting with water; and the deflagration is not something to make the sulphur burn freely, but to deprive the fumes of the phlogiston they contain, and render them miscible with water. For this purpose nitre has been advantageously used. This consumes a very large quantity of the phlogiston contained in sulphur, and renders the acid easily condensable; but it is plain that few of the fumes, comparatively speaking, are thus deprived of the inflammable principle; for the vessel in which the sulphur and nitre are burnt, remains filled with a volatile and most suffocating fume, which extinguishes flame, and inflates in such quantity as to render it highly dangerous to stay near the place. It has been thought that nitre contributes to the burning of the sulphur in close vessels; but this too is a mistake. More sulphur may be burnt in an oil of vitriol globe without nitre than with it, as we have often experienced; for the acid of the sulphur unites with the alkaline basis of the nitre, and forms therewith an uninflammable compound, which soon extinguishes the flame, and even prevents a part of the sulphur from being burnt either at that time or any other.

In the condensation of the fumes of sulphur by means of nitre, a remarkable effervescence happens, which naturally leads us to think that the condensation is produced by some struggle between the vitriolic and nitrous acids.—Dr Lewis is of opinion that the acid thus obtained is perfectly free from an admixture of the nitrous acid: but in this he is certainly mistaken; for, on rectifying the acid produced by sulphur and nitre, the first fumes that come over are red, after which they change their colour to white. How the nitrous acid should exist in the liquor, indeed, does not appear; for this acid is totally destructible by deflagration with charcoal: but it does not follow, that because the nitrous acid is destroyed when deflagrated with charcoal, that it must likewise be so if deflagrated with sulphur. Indeed, it certainly is not; for the effluvium of nitre made with sulphur, is very different from that made with charcoal. (See Nitrous Acid decomposed by Charcoal, below). This is not the only instance in which we must not reason too close from analogy.

The proportions of nitre to the sulphur, used in the large oil of vitriol works are not known, every thing being kept as secret as possible by the proprietors. Dr Lewis reckons about six pounds of nitre to a hundred weight of sulphur; but from such experiments as we have made, this appears by far too little. An ounce and an half, or two ounces, may be advantageously used to a pound of sulphur. In greater proportions, nitre seems prejudicial.

A very great improvement in the apparatus for Lead vitriol making oil of vitriol, lies in the using lead vessels instead of glass globes. The globes are so apt to be broken by accident, or by the action of the acid upon them, that common prudence would suggest the use of lead to those who intend to prepare any quantity of vitriolic acid, as it is known to have so little effect upon the metal. The leaden vessels, according to the best accounts we have been able to procure, are cubes of about three feet, having on one side a door about six inches wide. The mixture of sulphur and nitre is placed in the hollow of the cube, in an earthen faucer, set on a stand made of the same materials. The quantity which can be consumed at once in such a vessel is about two ounces. To prevent the remains from sticking to the faucer, it is laid on a square bit of brown paper. The sulphur being kindled, the door is to be close shut, and the whole let alone for two hours. In that time the fumes will be condensed. The door is then to be opened; and the operator must immediately retire, to escape the suffocating fumes which issue from the vessel. It will be an hour before he can safely return, and introduce another quantity of materials, which are to be treated precisely in the same manner.

Where oil of vitriol is made in large quantities, the slovenly of the operation requires a great number of globes, and constant attendance day and night. Hence the making of this acid is very expensive; and none but men of fortune need attempt it. The apparatus usually costs L.1500.

Vitriolic Acid combined with fixed Alkali.

Dilute a pound of oil of vitriol with ten times its Vitriolated quantity of water; dissolve also two pounds of fixed tartar alkaline salt in ten pounds of water, and filter the solution. Drop the alkali into the acid, as long as any effervescence arises; managing matters so, that the acid may prevail. The liquor will now be a solution of the neutral salt called vitriolated tartar, which may be procured in a dry form, either by evaporation or crystallization. In case the latter method is made use of, some more alkali must be added when it is set to evaporate, for this salt crystallizes best in an alkaline liquor. See no 74.

Concerning the preparation of this salt, Neuman relates relates several experiments which seem to have been very inaccurately made; and many circumstances which we are assured, from later discoveries, cannot possibly be true. "I prepared," says he, "a caustic alkali, by mixing one ounce of pure alkaline salt with two ounces of quicklime; boiling the mixture in fresh parcels of water till the liquor no longer acquired any saline taste; then filtering the several decoctions, and evaporating them to dryness. The dry salt weighed two scruples and an half more than the alkali employed; and the remaining lime weighed a drachm less than at first."

Here, we are sure, that he must have been mistaken; for as a pure alkali loses weight by being deprived of its fixed air, so the quicklime gains by being combined with the fixed air which the alkali loses; but Neuman's account would cause us believe, that the contrary took place.—He further takes notice, that having added by degrees two drachms of spirit (probably the oil) of vitriol, to one of the caustic alkali, a perfect saturation took place. During the effervescence a bright brownish earth fell to the bottom, which weighed, when dry, three grains. The filtered liquor deposited in evaporation first five grains and an half of white earthy creme, and afterwards five grains of a yellowish one. The dried salt weighed only two scruples and eight grains. During the saturation an abominable urinous smell arose, from which the exsiccated salt was not wholly free.

On this experiment we may observe, in the first place, that if the alkali had been perfectly caustic, little or no effervescence would have taken place. (See Air.) If the spirit of vitriol was highly rectified, the quantity was by far too great for the alkali. If it was not rectified, there can be no judgment formed concerning the experiment. The quantity of salt left, too, is much less than it ought to have been; for the vitriolic acid adds greatly to the weight of those substances which it unites with. The urinous smell is totally unaccountable. Several other strange appearances are mentioned by this author, on trial of mild alkalies, and differences which happened on mixing the acid with the alkali, or the alkali with the acid; but all of them have too much the appearance of inaccuracy, that they can by no means be depended on.

Other methods, besides that above described, have been recommended for preparing vitriolated tartar; particularly that of using green vitriol instead of the pure vitriolic acid. In this case the vitriol is decomposed by the fixed alkali; but as the alkali itself dissolves the calx of iron after it is precipitated, it is next to impossible to procure a pure salt by such a process; neither is there occasion to be solicitous about the preparation of this salt by itself, as the materials for it are left in greater quantity than will ever be demanded, after the distillation of spirit of nitre.

Vitriolated tartar is employed in medicine as a purgative; but is not at all superior to other salts which are more easily prepared in a crystalline form. It is very difficultly soluble in water, from which proceeds the difficulty of preparing it in a crystalline form; for if the acid and alkali are not very much diluted, the salt will be precipitated in powder, during the time of saturation.—It is very difficult of fusion, requiring a strong red heat; but, notwithstanding its fixedness in a violent fire, it arises with the steam of boiling water in such a manner as to be almost totally dissipated along with it by strong boiling.—This salt has been used in making glas; but with little success, as the glass wherein it is an ingredient always proves very brittle and apt to crack of itself.

If, instead of the vegetable fixed alkali, the vitriolic acid is saturated with the fusible one called the salt of Soda, a kind of neutral salt will be produced, having very different properties from the vitriolated tartar. This compound is called Glauber's salt. It dissolves easily in water, shoots into long and beautiful crystals, which contain a large quantity of water, in consequence of which they undergo the aqueous fusion when exposed to heat. They are also more easily fusible than vitriolated tartar.—This kind of salt was formerly much recommended as a purgative, and from its manifold virtues was intitled by its inventor sal mirabile. It is, however, found to possess no virtue different from that of other purgative salts; and its use is, in many places, entirely superseded by a salt prepared from the bittern, or liquor which remains after the crystallization of sea-salt, which shall be afterwards described.

Vitriolic Acid combined with Volatile Alkali.

Take any quantity of volatile alkaline spirit; that prepared with quicklime is preferable to the other, on secret account of its raising no effervescence. Drop into this liquor, contained in a bottle, diluted oil of vitriol, shaking the bottle after every addition. The saturation is known to be complete by the volatile smell of the alkali being entirely destroyed. When this happens, some more of the spirit must be added, that the alkali may predominate a little, because the excess will fly off during the evaporation. The liquor, on being filtered and evaporated, will shoot into fine fibrous plates like feathers. This salt, when newly prepared, has a fulphurous smell, and a penetrating pungent taste. It readily dissolves in water, and increases the coldness of the liquor; on standing for a little time, it begins to separate from the water, and vegetate, or arise in efflorescences up the sides of the glass. It easily melts in the fire; penetrates the common crucibles; and if sublimed in glass vessels, which requires a very considerable heat, it always becomes acid, however exactly the saturation was performed.

This salt has been dignified with the names of Glauber's secret salt ammoniac, or philosophic sal ammoniac, from the high opinion which some chemists have entertained of its activity upon metals: but from Mr Pott's experiments, it appears, that its effects have been greatly exaggerated. It dissolves or corrodes in some degree all those metals which oil of vitriol dissolves, but has no effect upon those on which that acid does not act by itself.

Gold is not touched in the least, either by the salt in fusion, or by a solution of it: the salt added to a solution of gold in aqua regia occasions no precipitation or change of colour. On melting the salts with inflammable matters, it forms a fulphurous compound, which dissolves gold in fusion, in the same manner as compositions of sulphur and fixed alkaline salt. Melt- Vitriolic Acid combined with Calcaceous Earth.

This combination may be made by saturating diluted oil of vitriol with chalk in fine powder. The mixture ought to be made in a glass; the chalk must be mixed with a pretty large quantity of water, and the acid dropped into it. The glass must be well shaken after every addition, and the mixture ought rather to be over saturated with acid; because the superfluous quantity may afterwards be washed off; the felsite, as it is called, or gypsum, having very little solubility in water.

This combination of vitriolic acid with chalk or calcareous earth, is found naturally in such plenty, that it is seldom or never made, unless for experiments sake, or by accident. Mr Poit indeed says, that he found some slight differences between the natural and artificial gypsum, but that the former had all the essential properties of the latter.

The natural gypsums are found in hard, semitransparent masses, commonly called alabastrum, or plaster of Paris. (See Alabaster, Gypsum, and Plaster.) By exposure to a moderate heat, they become opake, and very friable. If they are now reduced to fine powder, and mixed with water, they may be cast into moulds of any shape; they very soon harden without shrinking; and are the materials whereof the common white images are made. This property belongs likewise to the artificial gypsum, if moderately calcined.

Mr Beaume has observed, that gypsum may be dissolved in some measure by acids; but is afterwards separable by crystallization in the same state in which it was before solution, without retaining any part of the acids. This compound, if long exposed to a pretty strong heat, loses great part of its acid, and is converted into quicklime. In close vessels, it gives over no acid with the most violent heat. It may be fused by suddenly applying a very violent heat. With clay it soon melts, as we have observed when speaking of the materials for making crucibles. A like fusion takes place when pure calcareous earth is mixed with clay; but gypsum bubbles and swells much more in fusion with clay than calcareous earth.

From natural gypsum we see that vitriolated tartar may be made, in a manner similar to its preparation from green vitriol. If fixed alkaline fat is boiled with any quantity of gypsum, the earth of the latter will be precipitated, and the acid united with the alkali. If a mild volatile alkali is poured on gypsum contained in a glass, and the mixture frequently shaken, the gypsum will in like manner be decomposed, and a phlogistic sal ammoniac will be formed. With the caustic volatile alkali, or that made with quicklime, no decomposition ensues.

Vitriolic Acid combined with Argillaceous Earth.

The produce of this combination is alum; the nature of which was long unknown, but has been discovered by Messrs Geoffroy and Boudin, to be the acid of vitriol imperfectly saturated with argillaceous earth. Whether the earth of alum pre-existed in the clay, having the same nature as afterwards in the salt of alum, the above-mentioned gentlemen did not determine. Dr Lewis has made some experiments on this subject, and is of opinion that some change is made upon the earth during the operation. His process is as follows. "Powdered tobacco-pipe clay being boiled in a considerable quantity of oil of vitriol, and the fire continued to dryness, the matter, examined when grown cold, discovers very little taste, or only a slight acidulous one. On exposure to the air for a few days, the greatest part of it was changed into lanuginous efflorescences, in taste exactly like alum. The remainder treated with fresh oil of vitriol in the same manner, exhibits the same phenomena, and this repeatedly, till nearly the whole of the clay is converted into an affrangent salt."

Alum is never prepared for the purposes of trade by the above process; the materials for it are found in different places, and the method of extracting the alum differs according to the nature of these materials. Different kinds of it are known, under the names of rock-alum, plum-alum, &c.

That called rock-alum is usually of a reddish colour; different and consequently seems impure, as containing a little kind of vitriolic matter. It is nevertheless preferred, and sold at a higher price than the poorer kinds. This seems to be the kind called by the author of the Chemical Dictionary Roman alum. He says, that on trial it was found perfectly free from any admixture of vitriolic matter; though it is difficult to account for its redness on this supposition.

This kind of alum is prepared in the territory of Civita Vecchia, about 14 leagues from Rome. It is produced from a hard stone which is found there, and Italy is neither pyritous nor calcareous. It is calcined for 12 or 14 hours, after being broken in pieces. When thus calcined, it is laid in heaps upon places surrounded by ditches filled with water. It is sprinkled with this water three or four times each day, for 40 days, or till the calcined stone is covered with a reddish efflorescence.

Then the stones are boiled with water in caldrons to dissolve all the alum which is formed, and the water is evaporated to the point of crystallization. This water is made to flow quite hot into oaken vessels; where, by cooling, a great quantity of irregular crystals are formed, having a pale reddish tinge.

In Sweden is found a kind of mineral which yields sulphur, vitriol, and alum. This mineral appears to be a kind of pyrites. The sulphur is first extracted by distillation; the redissolved, strongly calcined, is boiled in water, and the vitriol crystallized. What remains, being treated with urine, and a ley drawn from ashes, yields alum.

In England, alum is prepared from certain black laminated argillaceous irata. Sometimes these require calcination, and at others only to be exposed to the open air, when they fall into powder in the same manner as the pyrites from which copperas is made.

The mineral, when sufficiently impregnated with alum, is boiled with water; the liquor boiled down, commonly with an addition of urine or alkaline ley, or both together. The clear part is poured off and set to float; the crystals, if not sufficiently pure, are dissolved again, boiled down with a little more alkali, and crystallized afresh.

Some earths have a manifestly aluminous taste when at newly dug; and hence are directly boiled, without any previous preparation. Of this kind is the earth found at Solfatara in Italy, where large quantities of sulphur are also made. The author of the Chemical Dictionary says that this earth very much resembles the marle found in the same plain, but differs essentially from it in not effervescing with the nitrous acid.

Caldrons of lead, two feet and an half in diameter, and as much in depth, are filled with this earth or stone, to three quarters of their contents. These caldrons are sunk so as to be almost on a level with the ground, under a great shed, at the distance of about 400 paces from the sulphur furnaces. Water is thrown into each caldron till it rises three or four inches above the earth. The natural warmth of the ground is here sufficient to heat the matter, being upwards of 100° of Fahrenheit's thermometer. By this means fuel is spared; and the salt floats in large crystals on the surface, as the water evaporates. The alum in this state, being mixed with many impurities, is carried to a building at the entry into Solfatara, where it is dissolved in a great stone-vessel shaped like a funnel. The alum is there crystallized again by the heat only of the ground, and becomes purer.

The alum flats, near York in England, are considerably sulphurous, and therefore require calcination to make them become aluminous. The reason of this is, that, during the calcination, the phlogiston is separated from the sulphur, and its acid combines with the aluminous earth. Long exposure to the air produces the same effect.

Alum is usually crystallized in large, strong, wooden casks; whose flaves and hoops are all marked with numbers, that they may readily be put together. The liquor is either boiled down greatly beyond the crystallizing point, that so it may all congeal into one lump when cold, or the crystals are taken out of the casks; and as they are capable of undergoing the aqueous fusion, they are by this means all cut into lumps. It is absolutely certain, that the huge masses in which alum is sold, can never be the effect of regular crystallization.

Plume alum is sometimes found native, and crystallized like feathers, in grottos through which aluminous waters pass. From Dr Lewis's experiment with tobacco-pipe clay, it appears that this kind is easily prepared. It is rarely found native, and is not used in commerce. The name of plume alum has been improperly given to other matters, such as a kind of albus; and by some alchemists to a compound formed of arsenic and vitriolic acid.

One remarkable circumstance attends the crystallization of alum, namely, that good crystals of it cannot be formed without the addition of an alkaline lixivium, or urine, to the liquor when set to crystallize. It was supposed, that, by adding these matters, some metallic or impure earthy substance, which prevented the crystallization, was precipitated; but Mr Margraaf found by experiments, that he could not form good crystals by combining vitriolic acid with earth of alum, with calcined alum, or with clay, unless he added a lixivium of fixed or volatile alkali, or urine. As the Roman alum is the only kind which is not prepared with these additions, it may possibly derive its superiority over other kinds of this salt from its want of such substances.

Alum has an austere, sweetish, and strongly astringent taste. It is soluble, according to Neuman, in 10 times, according to others in 14 times its quantity of water. It dissolves in much greater quantity in hot than cold water. When evaporated to the crystallizing point, and slowly cooled, the greatest part of its crystals are found to be triangular pyramids, whose four angles seem cut off. It retains half its weight of water in crystallizing. When exposed to a moderate fire, it melts, bubbles, and swells up; and is gradually changed into a light, white, and spongy mass, called calcined or burnt alum. After evaporation, it may be again dissolved and crystallized as before.

This salt is very easily decomposed; and, according to the different substances made use of for this purpose, we may produce different compounds. From alum may be prepared a vitriolated tartar; a philoforphic sal ammoniac, or gypsum, according as we use a fixed alkali, a volatile one, or a calcareous earth, for its decomposition. The last will as effectually and readily decompose alum, as fixed alkali itself. Nor is there any difference between the mild and caustic alkalies with regard to this salt, the latter decomposing it as readily as the first. Neither are these the only substances capable of decomposing alum. Iron itself, which has generally much less attraction for acids than earthy substances, will decompose it, and thus form a green vitriol or copperas.

Though the vitriolic acid, however, has so little attraction for the earthy basis of alum when in a moist state, it is obstinately retained by it when heated, neither is it possible to distil the acid from alum, as from vitriol. Mr Geoffroy put five pounds of calcined alum into an earthen retort, and exposed it to a most violent fire for six days and six nights, during which time he obtained only 3 ounces of vitriolic acid. Alum is very much used in dyeing, and the preparations of some colours. It is likewise used in medicine as a styptic.

Vitriolic Acid combined with Magnesia.

The earthy substance called magnesia alba is never found by itself, and consequently this combination cannot originally take place by art. The vitriolic acid, however, is found combined with magnesia in great plenty in the bitter liquor which remains after the crystallization of common salt, from whence the magnesia is procured by precipitating with a fixed alkali. If this liquor, which, when the common salt is extracted, appears like clear oil of vitriol, is let by for some time in a leaden vessel, a large quantity of salt floats, very much resembling Glauber's sal mirabile. This salt is in many places sold instead of the true Glauber's salt; and is preferred to it, because the true sal mirabile calcines in dry air, which the spurious kind does not. If after the first crystallization of the bittern, the remainder is gently evaporated farther, a fresh quantity of Glauber's salt will float; and if the liquor is then hastily evaporated, a salt will still be crystallized; but, instead of large regular crystals, it will concreted into very small ones, having something of the appearance of snow, when taken out of the liquid. These salts are essentially the same, and are all used in medicine as purgatives. The salt that floats into small crystals is termed Epsom salt, from its being first produced from the purging waters at Epsom in England. The bitter afforded this kind of salt in such great plenty, these waters were soon neglected, as they yielded it but very sparingly, and the quantity prepared from them was insufficient for the demand. Neuman says, that having infused two quarts of Epsom water, he scarce obtained half an ounce of saline matter.

Combinations of Vitriolic Acid with Metals.

1. Silver. Oil of vitriol boiled on half its weight of silver filings, corrodes them into a saline mass. This substance is not used in medicine, nor in the arts. The only remarkable property of it is, that it has a very strong attraction for mercury; coagulating and hardening as much quicksilver as the acid weighed at first. If the hard concrete be diluted with fresh acid, it melts easily in the fire, and does not part with the mercury in the greatest heat that glass vessels can sustain. The vitriolic acid, by itself, strongly retains mercury, but not near so much as when combined with silver.

Silver thus corroded by the vitriolic acid, or precipitated by it from the nitrous, may in great part be dissolved by cautiously applying a very little water at a time; and more effectually by boiling in fresh oil of vitriol.

2. Copper. With this metal the vitriolic acid cannot be combined, unless in its concentrated state, and strongly heated. If pure oil of vitriol is boiled on copper filings, or small pieces of the metal, it dissolves it into a liquor of a deep blue colour, which easily crystallizes. The crystals are of a beautiful blue colour, and are sold under the name of blue vitriol, or Roman vitriol.

Where sulphur is found in great plenty, however, Roman vitriol is made by stratifying thin plates of copper with sulphur; and upon slowly burning the sulphur, its acid corrodes the copper. The metal is then to be boiled in water, that the saline part may be distilled. The operation is to be repeated, till all the copper is consumed; and all the saline liquors are to be evaporated together to the crystallizing point. By this method, however, a great part of the acid is lost; and in Britain, where the sulphur must be imported, we should think the pure acid preferable for those who prepare blue vitriol.

This salt, on being exposed to the fire, first turns phenomena white, then of a yellowish red colour. On urging it on distillation with a strong fire, the acid slowly exhales, and a dark red calx of copper remains. The whole of the vitriolic acid cannot be expelled from copper by heat: as much of it still remains as to render a part of the metal soluble in water. After this soluble part has been extracted, a little acid is still retained, amounting to about \( \frac{1}{2} \) of the calx.

Vitriol of copper is employed in medicine as a caustic, in which respect it is very useful; but when used internally, is dangerous, as indeed all the preparations of copper are found to be. It has, nevertheless, according to Neuman, been recommended in all kinds of intermittents, and the lepra. The smallest portion, he says, occasions sickness and nausea; a somewhat larger, reaching and violent vomitings, accompanied often with convulsions. If the quantity taken has been considerable, and is not soon discharged by vomiting, the stomach and intestines are corroded, intense pains, inflammations, and death, succeed.

3. Iron. The vitriolic acid does not act upon this metal till considerably diluted. Common oil of vitriol requires to be mixed with ten or twelve times its quantity of water, before it will act briskly on the metal. In this state it effervesces violently with iron filings, or small bits of the metal, and a great quantity of inflammable vapour is discharged. (See Art.) The liquor assumes a fine green colour; and, by evaporation and slow cooling, very beautiful rhomboidal crystals are formed. These are named salt of steel, and are used in medicine; but, for the salt made with the pure acid and iron, the common copperas, made with the impure acid extracted from pyrites, is commonly substituted. (See Vitriolic Acid extracted from Pyrites, above, no. 110.) This is generally esteemed a venial fraud, and no doubt is so in medicinal respects; but when it is considered, that, by this substitution, common copperas is imposed on the ignorant, at the price of 2s. per pound, the affair appears in a different light.

Pure vitriol of iron is originally of a much more beautiful appearance than common copperas, and retains its colour much better; the reason of which seems to be, that the salt thus prepared is more free from phlogistic matters than the copperas. If either of the kinds, however, are exposed to the air for a sufficient length of time, part of the acid is dissipated, and the vitriol becomes yellowish or brownish. If the salt is now dissolved in water, a brown precipitate falls, which is part of the iron in a calcined state. If the liquor is separated from this precipitate by filtration, a similar one forms in a short time, and by long standing a considerable quantity subsides. According to Dr Lewis, the precipitation is greatly expedited by... Practice by a boiling heat; by which more of the metal separates in a few minutes, than by standing without heat for a twelvemonth. This change takes place in no other metallic solutions.

The calx of iron, precipitated by quicklime from green vitriol, appears, when dry, of a yellow colour; and is recommended, in the Swedish transactions, instead of yellow ochre, as a colour for house-painting. Solutions of green vitriol are also recommended for preserving wood, particularly the wheels of carriages, from decay. When all the pieces are fit for being joined together, they are directed to be boiled in a solution of vitriol for three or four hours; and then kept in a warm place for some days to dry. By this preparation, it is said, wood becomes so hard, that moisture cannot penetrate it; and that iron nails are not so apt to rust in this vitriolated wood as might be expected, but last as long as the wood itself.

4. Tin. This metal cannot be dissolved in the vitriolic acid, but in the same manner as silver; namely, by boiling concentrated oil of vitriol to dryness upon fillings of the metal. The saline mass may then be dissolved in water, and the solution will crystallize. The salt, however, formed by this union, is not applied to any useful purpose. A salt of tin, indeed, formed by the union of vitriolic acid with this metal, has been recommended for some medical purposes, and processes are given for it in the dispensaries; but they have never come much into practice.

5. Lead. While lead is in its metallic state, the vitriolic acid acts very little upon it, either in a diluted, or concentrated state; but if the metal is dissolved in any other acid, and oil of vitriol added, a precipitation immediately ensues, which is occasioned by the combination of vitriolic acid with the lead. This precipitate will be more or less white, as the metal is more or less deprived of its phlogiston by calcination, before solution. If a little strong spirit of nitre is poured upon litharge, which is lead calcined to the greatest degree possible without vitrification, the acid unites itself to the metal with considerable effervescence and heat. Some water being now poured on, and the vial containing the mixture shaken, a turbid solution of the litharge is made. If a little oil of vitriol is then added, it throws down a beautifully white precipitate; and the acid of nitre, being left at liberty to act upon the remaining part of the litharge, begins anew to dissolve it with effervescence. When it is again saturated, more oil of vitriol is to be dropped in, and a white precipitate is again thrown down. If any of the litharge is still undissolved, the nitrous acid, being set at liberty a second time, attacks it as at first; and, by continuing to add oil of vitriol, the whole of the litharge may be converted into a most beautiful and durable white. Unfortunately this colour cannot be used in oil, tho' in water it seems superior to any. If the process is well managed, an ounce of spirit of nitre may be made to convert several pounds of litharge into a white of this kind.

6. Quicksilver. The dissolution of quicksilver in vitriolic acid cannot be performed but by a concentrated oil and strong boiling heat. The metal is first corroded into a white calx, which may afterwards be easily dissolved by an addition of fresh acid. Every time it is dissolved, the mercury becomes more and more fixed and more difficult to dry. If the evaporation and distillation has been repeated several times, the matter becomes at last so fixed as to bear a degree of red heat. This combination is the basis of a medicine formerly of some repute, under the name of turbit mineral. The process for making turbit mineral is given by the author of the Chemical Dictionary as follows:

"Some mercury is poured into a glass retort, and upon it an equal quantity of concentrated oil of vitriol, mineral, or more, according to the strength of the acid. These matters are to be distilled together, in the heat of a sand-bath, till nothing remains in the retort but a dry saline mass, which is a combination of the vitriolic acid and mercury. The acid which passes into the receiver is very suffocating and sulphureous; which qualities it receives from the phlogiston of the mercury. The white saline mass which is left at the bottom of the retort is to be put into a large vessel; and upon it are to be poured large quantities of hot water, at several different times. This water weakens the acid, and takes it from the mercury; which is then precipitated towards the bottom of the vessel, in form of a very shining yellow powder. The water with which it is washed, contains the acid that was united with the mercury; and likewise a little mercury, rendered soluble by means of the very large quantity of acid.

Most chemists have believed, that a portion of vitriolic acid remains united with the turbit mineral, only too little to render it soluble in water. But Mr Beaumé, having examined this matter, affirms, that turbit mineral contains no acid, when it has been sufficiently washed; and that, by frequently boiling this preparation in a large quantity of distilled water, not a vestige of acid will adhere to it."

Dr Lewis, who is of opinion that the whole of this Dr Lewis's mercurial calx is soluble in a very large quantity of directions, water, desires the water with which it is washed to be impregnated with some alkaline salt; which makes the yield of turbit greater than when pure water is used. The author of the Chemical Dictionary also observes, that the precipitate remains white till well freed from the acid; and the more perfectly it is washed, the deeper yellow colour it acquires.

7. Zinc. This metal is not acted upon by the vitriolic acid in its concentrated state; but, when diluted, is dissolved by it with effervescence, and with the extrication of an inflammable vapour in the same manner as iron. Neuman observes, that, during the distillation, a grey and blackish spongy matter fell to the bottom; but, on standing for some days, was taken up, and dissolved in the liquor, nothing being left but a little yellowish dust scarcely worth mentioning. Six parts of oil of vitriol, diluted with an equal quantity of water, dissolves one part of zinc.

The product of this combination is white vitriol; which is used in medicine as an ophthalmic, and in painting for making oil-colours dry quickly: what is used for this purpose, however, is not made in Britain, but comes from Germany. It is made at Goslar by the following process. An ore containing lead and silver, having been previously roasted for the obtaining... Part II.

Practicing of sulphur, (see Metallurgy), is lixiviated with water, and afterwards evaporated in leaden boilers, as for the preparation of green vitriol; but here a regular crystallization is prevented; for when the salt has assumed any kind of crystalline form, the crystals are made to undergo the watery fusion in copper caldrons. It is then kept constantly stirring till a considerable part of the moisture is evaporated, and the matter has acquired the consistence of fine sugar. White vitriol generally contains some ferruginous matter, from which it may be entirely freed, by some fresh zinc; for this semimetal precipitates from the vitriolic acid all other metallic substances; but notwithstanding this strong attraction, the vitriolic acid is more easily expelled by distillation from white, than green, or blue vitriol. Towards the end of the distillation of white vitriol, the acid arises exceedingly concentrated, though sulphureous; so that, if mixed with common oil of vitriol, it will heat it almost as much as oil of vitriol heats water.

8. Regulus of Antimony. To combine vitriolic acid with regulus of antimony, the same method must be used, as directed for uniting it with quicksilver, for making turbith mineral, viz., to employ a very concentrated acid, and to distil in close vessels. The same phenomena also occur in this case as in making turbith mineral; a very inflammable sulphureous acid rises; and, as Mr Geoffrey observes, a true sulphur sublimes into the neck of the retort; a white, saline, tanned mass remains in the vessel; and when the vessels are undulated, a white fume issues, as in the smoking spirit of liquor. See Combinations of marine acid with tin, infra.

9. Regulus of Cobalt. The only accounts we have of the combination of vitriolic acid with this semimetal are, that it is slowly dissolved into a rose-coloured liquor, which, in evaporation, throws off to the sides a blue powder, that on cooling grows white.

10. Arsenic. Neuman relates, that powdered white arsenic being distilled in a retort with oil of vitriol, a transparent sublimate like glass arose, which in a few days lost its transparency, and became opake like the arsenic itself. The arsenic remaining in the retort, sustained an open fire without any sensible alteration. The author of the Chemical Dictionary says, that if a concentrated vitriolic acid is distilled from arsenic, the acid which comes over smells exactly like marine acid. When the solution is distilled till no more acid rises, the retort is then almost red-hot, and no arsenic is sublimed; but it remains fixed at the bottom of the retort; and, when cold, is found to be an heavy, compact mass, brittle and transparent as crystal-glass. This kind of arsenical glass, exposed to the air, soon loses its transparency from the moisture it attracts, which dissolves and partly deliquesces it. This deliquium is extremely acid.—None of the three last mentioned combinations have been found applicable to any useful purpose.

Vitriolic Acid combined with Inflammable Substances.

1. Oil. The product of this combination is a thick black substance, very much resembling balsam of sulphur in colour and consistence; to which it is sometimes substituted. If this substance is distilled with a gentle heat, great part of the acid becomes volatile, and evaporates in white fumes, having a pungent smell resembling that of burning sulphur. This gives by the volatile name of volatile or sulphureous vitriolic acid; and a salt was formerly prepared from it by saturation with fixed acid alkali, which was thought to possess great virtues. From its inventor it was called the sulphureous salt of Stahl. The most singular property of this volatile acid is, that though the vitriolic in its fixed state is capable of expelling any other acid from its basis, the volatile one is expelled by every acid, even that of vinegar. It is very difficultly condensible, as we have already taken notice; and, when mixed with water, seems scarcely at all acid, but rather to have a bitterish taste.

Several methods have been proposed for procuring this acid from burning sulphur, which yields it in its red by Dr Priestley's greatest degree of volatility, as well as concentration; but the produce is so exceedingly small, that none of them are worth mentioning. Dr Priestley has given very good directions for obtaining the volatile vitriolic acid in the form of air. His method was, to pour, on some oil of vitriol contained in a vial, a very small quantity of oil olive; as much as was sufficient to cover it. He then applied the proper apparatus for the reception of air in quicksilver, (see Air); and, holding a candle to the vial, the volatile vitriolic acid rushed out in great quantity. Had he received this air in water, instead of quicksilver, the consequence would have been, that some part of it, at least, would have been absorbed by the water, and a sulphureous acid liquor produced. This seems indeed almost the only method of procuring the sulphureous vitriolic acid of any tolerable strength; but it is never required in the form of a liquor except for experimental purposes. The only useful property hitherto discovered about this kind of acid is, that it is remarkably destructive of colours of all kinds, and hence the fumes of sulphur are employed to whiten wool, &c.

2. Phlogiston of Charcoal. If charcoal is mixed with concentrated vitriolic acid, and the mixture distilled, the same kind of acid is at first obtained, which comes over when oil is used; and towards the end, when the matter begins to grow dry, a true sulphur sublimes. The best way, however, of producing sulphur from the vitriolic acid is by combining it, when in a perfectly dry state, with the phlogiston. By this means sulphur may very readily be made at any time. The process is generally directed to be performed in the following manner.

Reduce to fine powder any quantity of vitriolated sulphur tartar. Mingle it carefully with a 16th part of the prepared weight of charcoal-dust. Put the whole into a covered crucible set in a melting furnace. Give a heat sufficient to melt the salt; and when thoroughly melted, pour it out on a flat stone. The vitriolated tartar and charcoal will now be converted into a sulphureous mass similar to a combination of alkaline salts with sulphur. See Alkaline Salts, below.

3. Spirit of Wine. The result of this combination is one of the extraordinary phenomena in chemistry; being that fluid, which, for its extreme degree of volatility, was first distinguished by the name of ether; and now, since a liquor of the like kind is discovered Practice to be preparable from spirit of wine by means of other acids, this species is distinguished by the name of vitriolic ether. The method of preparing this subtle liquor recommended by Mr Beaumé, seems to be the best of any hitherto discovered.

"Mix together equal parts by weight, of highly rectified spirit of wine and concentrated oil of vitriol, or somewhat more than two measures of spirit of wine with one of the acid. The mixture is to be made in a flint glass-retort, the bottom and sides of which are very thin, so that it may not break from the heat which is suddenly generated by the union of these two substances. The spirit of wine is first put into the retort, and then the acid is poured in by a glass funnel, so that the stream may be directed against the side of the glass; in which case it will not exert much of its force on the spirit, but will lie quietly below at the bottom. The retort is now to be very gently shaken, that the acid may mingle with it by little and little. When the mixture is completed, very little more heat will be necessary to make the liquor boil.

This mixture is to be distilled with as brisk and quick a heat as possible; for which reason, immediately after the acid and spirit are mixed, the retort should be put into a lead furnace heated as much as the mixture is. The distillation should be continued only till about one-third of the liquor is come over; if it is continued farther, part of the vitriolic acid rises in a sulphurous state. In the retort a thick, black, acid matter remains, which is similar to a combination of oil of vitriol with any inflammable matter, and from which a little sulphur may be obtained. Along with the sulphurous acid, a greenish oil, called oleum vitrioli dulcis, arises, which has a smell compounded of that of the ether and sulphurous acid; and Mr Beaumé has shown that it is compounded of these two; for if it is rectified with an alkali, to attract the acid, it is changed into ether. If, after the distillation of the ether, some water be poured into the retort, the liquor by distillation may be brought back to the state of a pure vitriolic acid.

As the streams of the ethereal liquor are exceedingly volatile, and at the same time a quick fire is necessary to the success of the operation, the receiver must be carefully kept cool with very cold water, or with snow. Care must also be taken to prevent any of the sulphurous acid steams from coming over; but as it is impossible to prevent this totally, the liquor requires rectification. This is the more necessary, as a part of the spirit of wine always rises unchanged. From the acid the liquor is easily set free, by adding a small quantity of alkaline salt, and re-distilling with a very gentle heat; but as spirit of wine is likewise very volatile, the distillation must be performed in a very tall glass. Dr Black recommends a matras, or bolt-head, with a tin pipe adapted to the head, so as to convey the steams at a right angle, to be condensed in the receiver.

Ether is the lightest of all known fluids, except air; and is so volatile, that in vacuo its boiling point is 20° below 0° of Fahrenheit's thermometer. If a small quantity is poured out on the ground, it instantly evaporates, diffusing its fragrance all through the room, and scarcely perceptibly moistening the place on which it fell. It difficulty mixes with water, as being of an oily nature: ten parts of water, however, will take up one part of ether. Its great volatility renders it serviceable in nervous diseases, and removing pains, when rubbed on with the hand, and kept from evaporating immediately. By spontaneous evaporation, it produces a great degree of cold. (See Evaporation and Congelation). The most extraordinary property, however, is, that if gold is dissolved in aqua regia, (see Metallic Substances, below), and ether added to the solution, the gold will leave the acid and permanently unite with the ether. The exceeding great volatility of ether renders it very easily inflammable even on the approach of flame; and therefore it ought never to be distilled, or even poured from one vessel to another, by candle-light. If a less quantity of the vitriolic acid is added to the spirit of wine than what is sufficient to produce ether, the product is called spiritus vitrioli dulcis.

II. Of the Nitrous Acid and its Combinations.

This acid is far from being so plentiful as the vitriolic. It has been thought to exist in the air; and Dr Priestley's experiments have indeed shown, that a part of it enters into the composition of our atmosphere: but it is greatly to be doubted whether it can be recovered from it in its proper form; for no method of doing this has hitherto been found. Fixed alkaline salts, indeed, when exposed to the air, imbibe its fixable part, and by that means become susceptible of crystallization; which probably has given occasion to the notion that they imbibe vitriolic acid from it; but we have no experiments from which it can be inferred, either that the least portion of nitrous acid exists ready formed in the air, or that the materials for its formation are to be found there.

What has given rise to the opinion of the nitrous acid being absorbed from the atmosphere, seems to be, that some kinds of earth, the rubbish of old houses, &c., are found impregnated with this acid, on being some time exposed to the air; but we might as well make this an argument for the vitriolic acid being absorbed from the air by pyrites, when exposed to it for making green vitriol. In this last case, however, we know that there is no attraction of new acid; but only a decomposition of sulphur which contains the vitriolic acid already formed, and which then fully manifests itself. In like manner, the nitrous acid may originally lie hid in those earths which are found to contain it plentifully in exposure to the air, and only wants such an exposure to become visible.

The most probable opinion is, that the nitrous acid is only the vitriolic altered by a mixture of inflammable matter; and, in support of this, the similarity between the nitrous and sulphurous vitriolic acids is urged. Experiments are also brought in proof of this; none of them, however, seem to be decisive. In the Berlin memoirs, we have an account of some experiments made by Dr Pictsch on this subject; who says, that, having soaked a calcareous stone with vitriolic acid and urine, he found it, after being some time exposed to the air, to abound greatly in nitre. Neuman says, that if two ounces of good spirit of nitre are mixed with half an ounce of oil of turpentine, Practice the mixture will be a true balsam of sulphur. The most remarkable experiment, however, is one by Wallerius, mentioned by Dr Lewis in his notes on Neuman's chemistry.

"Some salt of tartar," says he, "being mixed with the dulcified spirit of vitriol, or perhaps with the ether, (for the author expresses himself a little ambiguously,) the full bottle stood with a cork, tied over with bladder, and laid on its side; on standing for four months, the greatest part of the spirit was found to have escaped, and the salt was shot into hexangular prismatic crystals resembling nitre. It tasted strongly of the spirit, but had no other particular taste. Laid on a burning coal, it crackled, exploded with a bright flash, and flew into the air. He afterwards found, that by adding to the spirit a drop or two of any acid, the salt crystallizes the sooner; that in this case it has a fourth taste, but in other respects it is the same with that made without acid. This salt-petre, says the author, promises, from the violence of its explosion, to make the strongest gun-powder in the world, but a very dear one. Though the experiment should not be applicable to any use in this way, it will probably contribute to illustrate the generation of nitre; as it palpably shows nitre, that is, the acid or characteristic part of nitre, produced from the vitriolic acid and phlogiston."

We cannot here help again regretting that chemists of superior abilities should sometimes leave very important discoveries only half finished, so that chemists of an inferior rank know not what to make of them. Had Wallerius, who seems more than once to have been in possession of this salt, only poured on it a few drops of oil of vitriol, the peculiar colour and smell of the fumes must have been a much more convincing proof of the reality of this transmutation, than that of the mere deflagration; because the latter can be otherwise accounted for.

It is certain, that many substances, water itself not excepted, will explode with great violence if suddenly heated beyond what they are able to bear. If spirit of wine is confined in a close vessel, it will also by means of heat burn it as effectually as water; and as the vapours of this substance are inflammable, the explosion will be attended with a flash, if any flame is near. In like manner, ether, on the approach of a candle, takes fire, and goes off in a flash like lightning; but this happens, not from any thing nitrous, but from its great volatility and inflammability. If therefore the vapours of the ethereal liquor are confined, and heat is applied suddenly to the containing vessel, their great volatility will cause them make an instantaneous effort against the sides of it, which, increasing with a swiftness far beyond that of aqueous or spirituous vapours, will make a much quicker as well as a much stronger explosion than either of them; and if a flaming substance is near, the explosion will be attended with a bright flash like that of the ether itself.

In the experiment now before us, the salt tasted strongly of the spirit, or ether, from which it was made. The spirit was therefore confined in the crystals of salt; and his volatile liquor, which, even under the pressure of the atmosphere, boils with the heat of 100° of Fahrenheit, was, in a confined state, subjected to the heat of a burning coal; that is, to more than ten times the degree of heat necessary to convert it into vapour. The consequence of this could be no other, than that the particles of salt, or perhaps the air itself, not being capable of giving way soon enough to the forcible expansion of the ether, a violent explosion would happen, and the salt be thrown about; which accordingly came to pass, and might very reasonably be expected, without anything nitrous contained in the salt.

The accounts we have of the production of nitre itself, which is a combination of the nitrous acid and fixed alkali, generally agree pretty much in its being extracted from particular kinds of earth, or from the rubbish of old houses; but the impossibility of procuring the least quantity of nitrous salt from these substances in this country, renders the whole of such accounts justly suspected. The warmth of particular climates indeed might be very justly conjectured to be the reason why nitre should be made to advantage in some countries, and not in others; but it can scarce account for the impossibility of making a single grain in colder climates. It seems very strange, that the same means which produce great quantities of nitre in France shall produce none in Britain, though used with the utmost exactness and care.

The requisites for the production of nitre, according to Neuman, are, 1. Putrid, or putrefied matters, either of the animal or vegetable kingdom. 2. Certain earths; as lean clays, limestone, gypsum, the rubbish of old buildings, &c. And, 3. Air. The greatest quantities of salt-petre, says he, are produced from urine and dung, whose strong tendency to putrefaction renders them preferable to other matters. Human urine, the urine of sheep, goats, and horses, and the dung of pigeons, are found to answer best. The leaves, flowers, and stalks of vegetables, hay and straw, saw-dust, &c. are mixed in the composition, and contribute more or less to the effect, according to their degree of putrefiability. On these principles, says he, nitre may be produced in all countries; and in many places very advantageous works might be established, if we were better acquainted with the manner of its production.

According to this author, likewise, certain earths Nitrous may be improved and improved, as to afford more ferment, nitre by the admixture of different substances, which may properly be called nitrous ferment. The following compositions are proper. 1. Lime, sheep's dung, sheep's urine, and common salt. 2. Lime, salt, rasped horns and hoofs, cuttings of leather, and other refuse of animal substances. 3. Human urine and lime. 4. Human urine, lime, salt, and pigeons' dung; and other like mixtures of lime and salt with animal matters. 5. Compositions of animals and vegetables together; as bitter plants boiled in the urine, and poured on the earth. 6. Tartar, lime, and urine. 7. Lime, prelavings of grapes, and the liquor of dung-hills. 8. Wine lees, dung-hill liquor, lime, and salt. 9. Tartar and lime. 10. Lime, salt, dung-hill liquor, and the matter remaining after the distillation of spirit of wine. 11. Lime, salt, urine, dung, and martial liquor.

The author of the Chemical Dictionary acquaints us, that this salt is found naturally crystallized in India, nitre. Practice dia, and is swept from the earths and stones which produce it; for which reason it is called salt-petre sweepings. A nitre may also be obtained from several plants. These, says he, are the only two kinds of natural nitre; all other nitre is only begun by nature, and is found in the walls of old buildings. The most favourable places for the production of nitre, are the habitations of men and animals, particularly such as are low and moist, as cellars, kitchens, stables, houses of office, and others of that kind, which are apt to be impregnated with vegetable and animal matters, and also to have an habitual moisture, which is favourable to putrefaction; and lastly, which are sheltered from rain, which might dissolve and carry off the nitre as soon as it is formed.

In the East Indies, the nitrous acid is said to be found in great plenty, mixed with some kinds of marly earths; from whence it is extracted by lixiviation with water, and precipitating the earth with a fixed alkali. An account of the method of making salt-petre in Virginia has been published by order of the society for encouraging arts, &c. This, however, gives no light as to the origin of the acid; only informing us that nitre is found in tobacco-houses, stables, cowhouses, hen and pigeon houses, and in any covered place where the influence of the sun seldom reaches. A sixty feet tobacco-house will yield 16 hundred weight yearly; and so in proportion for other houses. The directions for preparing the floors of these houses for yielding nitre, follow:

"In order to prepare the floors for attracting nitre, all dung and other trash must be removed; and if the floors are not level, they must be made so by laying on marl, or any soil not too stiff, which must be lightly trod down with the feet.

"The floor being thus prepared, sprinkle strong amber over it, made from tobacco trash; and cover it with wet ground leaves, or other tobacco trash, for a fortnight; then clean out the trash; and in any cool dry morning that succeeds, you will find on the floor, the nitre attracted and condensed like hoar-frost, which must be carefully swept off as often as that appearance is observed."

In the annual register for 1763, we meet with a paper signed J. R., the author of which relates some attempts of his own to make salt-petre, in so sensible a manner, that we cannot help giving an account of them in his own words.

"Having perused," says he, "what Hoffman, Stahl, Boerhaave, and others, have delivered on the formation of nitre; and being furnished with an account of the nitre-works near Paris, and with the method of making this salt at Calcutta, I entered upon the subject with as much affluency and attention, as a man can apply to one he is either pleased with, or interested in. The writers above-mentioned differ so little in their accounts of the constitution of nitre, and the materials which supply it, that I shall, for brevity's sake, confine myself to what is delivered by Hoffman. He says, in the first place, that nitre has two principles, or elements; one the universal, simple, and primogenial acid which inhabits the air, good ventre fue portat; the other an alkaline, sulphurous, fat earth; and that this last is a matrix, which, by attracting to itself and imbibing the former from the air, constitutes nitre. He further observes, that the substances which supply nitre in greatest plenty, are the rubbish of demolished houses, all kinds of earth, clay, and loam, lime, ashes, and soap-boilers drags; and that these always produce the most nitre, in proportion as they are combined with the urine and excrements of animals, and with corrupted vegetables. All these materials I soon furnished myself with, and for greater certainty procured some of them from different places; but after frequent trials, by drenching and boiling them in water, could not procure anything at all like nitre from them. I then provided a great number of flat glazed earthen pans; and in these exposed the same substances for several months, in a dry state, to the air; but found myself equally disappointed. I likewise placed in the same situation a quantity of the vegetable alkaline salt called pearl-ash, some of it alone, and some mixed with the fore-mentioned earthy substances; but to no better purpose: for which I am induced to believe, notwithstanding the authority of Hoffman, and the opinion of many concerning the residence of the nitrous acid in the air, that it is not to be found therein; and this I am the better authorized to deliver, as I never could procure, after proper trials, any vestiges of nitre from hail, snow, rain-water, or dew."

These experiments having proved totally fruitless, our author was obliged to have recourse to other sources for the nitrous acid; and the only one he could find was hard spring-water. Many of these waters do indeed contain a combination of the nitrous acid with calcareous earths; but in too small quantity to be worth extracting. The operation is very easy, consisting only in dropping into the water a solution of fixed alkaline salt, filtering the liquor, and evaporating to a sufficient degree; when crystals of true nitre will be formed. From three pounds of such water as we have tried, only 20 grains of nitre were procurable; it is probable, therefore, that in all places where this salt is found in plenty, there are some circumstances which contribute to its formation, that are either overlooked, or designedly concealed.

The distinguishing characteristic of the nitrous acid is its great disposition to unite with the phlogiston; and, when so united, first to become exceedingly volatile, and at last to be dissipated in a very white bright flame: this is called its detonation or deflagration. In the strongest state in which this acid is procurable in a liquid form, it is of a reddish yellow colour, and continually exhales in dense, red, and very noxious fumes; and in this state is called smoking, or, from its inventor, Glauber's spirit of nitre.

To extract the Nitrous Acid by means of the Vitriolic.

Into a glass-retort put two pounds of good salt-spirit of petre, and pour upon it 18 ounces of concentrated oil nitre, of vitriol; set the retort in a fond heat, and lute on a large receiver with the composition recommended no 75, for resisting acid fumes; the mixture will grow very warm, and the retort and receiver will be filled with red vapours. A small fire is then to be kindled, and cautiously raised till no more drops will fall from the hole of the retort. What comes over will be a very strong and smoking spirit of nitre. In this process, the nitrous acid is generally mixed with part of the vitriolic which comes over along with it, and from which it must be freed if designed for nice purposes. This is most effectually done by dissolving in it a small quantity of nitre, and re-distilling the mixture. The vitriolic acid which came over in the first distillation is kept back by the nitre in the second, combining with its alkaline basis, and expelling a proportionable quantity of the nitrous acid.

We have here directed the pure vitriolic acid to be used, in order to expel the nitrous one; but for this purpose any combination of the vitriolic acid with a metallic or earthy basis may be used, though not with equal advantage. If calcined vitriol is made use of, as much phlogiston is communicated by the calx of iron contained in that salt, as makes the nitrous acid exceedingly volatile, so that great part of it is lost. If calcined alum, or felsparite, is made use of, the vitriolic acid in these substances immediately leaves the earth with which it was combined, in order to unite with the alkaline basis of the nitre, and expels its acid: but the moment the nitrous acid is expelled from the alkali, it combines with the earth which the vitriolic acid had left; from which it cannot be driven without a violent fire; and part of it remains obstinately fixed, so as not to be expelled by any degree of heat. Hence the produce of spirit, when nitre is distilled with such substances, always turns out considerably less than when the pure vitriolic acid is used. Alum is preferable to felsparite, for the purposes of distilling spirit of nitre; because the acid does not adhere so strongly to argillaceous, as to calcareous earth.

Spirit of nitre is very useful in the arts of dyeing and refining, where it is known by the name of aqua fortis; and therefore an easy and cheap method of procuring it is a valuable piece of knowledge. Many difficulties, however, occur in this process, as well as that for the vitriolic acid. Oil of vitriol, indeed, always expels the nitrous acid with certainty; and on distilling the mixture, a spirit of nitre arises: but if a glas-retort is used for the purpose of distilling this acid, the quantity of residuum left in distillation is so great, and so insoluble in water, being no other than vitriolated tartar, that the retort must always be broke in order to get it out; and the produce of spirit will scarce afford the expense of breaking a retort. If earthen retorts are made use of, they must certainly be of that kind called stone-ware, and the price of them will be very little if at all inferior to that of glass. Iron pots are said to be made use of in the distillation of common aqua fortis in large quantities; but they have the great inconvenience of making a quantity of the acid so volatile, that it not only will not condense, but spreads its suffocating vapours all around in such a manner as to prove very dangerous to those who are near it. If an iron vessel, therefore, is thought of for the purpose of distilling aqua fortis, it will be proper at least to attempt lining over the inside with a mixture of gypseous earth and sand, to prevent as much as possible the acid from attacking the metal.

To procure the Nitrous Acid by means of Arsenic. Pulverise equal quantities of dried nitre and white crystalline arsenic; mix them well together, and distil in a glas-retort with a fire very cautiously applied; for the arsenic acts on the nitre with such violence, and the fumes are here so volatile, that, unless great care is taken, a most dangerous explosion will almost certainly happen. As, in this case, the nitrous fumes arise in a perfectly dry state, some water must be put into the receiver, with which they may unite and condense. The aqua fortis so produced will have a blue colour, owing to the inflammable principle separated from the arsenic, by which its extreme volatility is likewise occasioned. If this blue aqua fortis is exposed to the air, its colour soon flies off.

Nitrous Acid combined with Alkaline Salts.

1. Vegetable fixed alkali. This salt, combined with saltpetre, the nitrous acid to the point of saturation, regenerates nitre. It is observable, however, according to Neuman, that there is always some difference between the original, and regenerated nitre, unless quicklime is added. The regenerated salt, he says, always corrodes tin, which the original nitre does not. Boiling with quicklime deprives it of this quality, and makes it exactly the same with original nitre.

2. Felspar alkali. The neutral salt arising from a cubicature combination of the nitrous acid and felspar alkali is somewhat different from common nitre; being more difficult to crystallize, inclining to deliquesce in the air, and shooting into crystals of a cubic form, whence it gets the name of cubic nitre. Its qualities are found somewhat inferior to the common nitre; and therefore it is never made, unless by accident, or for experiments.

Nitre is one of the most fusible salts. It is liquefied fusibility, in a heat much less than what is necessary to make it red; and thus remain in tranquillisation, without swelling. If nitre thus melted be left to cool and fix, whether it has been made red hot or not in the fusion, it coagulates into a white, semi-transparent, solid mass, called mineral crystal, having all the properties of nitre itself. By this fusion, Mr Beaumé observes that nitre loses very little, if any, of the water contained in its crystals, since the weight of mineral crystal is nearly the same with that of the nitre employed.

When nitre is kept in fusion with a moderate heat, and at the same time does not touch any inflammable matter, nor even flame, it remains in that state without suffering any very sensible alteration; but if it is long kept in fusion with a strong fire, part of the acid is destroyed by the phlogiston which penetrates the crucible; and hence the nitre becomes more and more alkaline. (See no 220.)

Nitre is of very extensive use in different arts; being the principal ingredient in gun-powder; and serving as an excellent flux to other matters; whence its use in glas making. (See Glass.) It is also possessed of a considerable antiseptic power; whence its use in preserving meat, to which it communicates a red colour. In medicine, nitre is used as a diuretic, sedative, and cooler; but very often fits uneasily on the stomach. The resemblance of the crystals of nitre to those of Glauber's salt has sometimes been the occasion of dangerous mistakes. Dr Alexander mentions a swelling over the whole body of a woman, occasioned Practice by her taking a solution of nitre instead of Glauber's salt. Two mistakes of the same kind we have also known. In one an ounce, and in the other upwards of two ounces, of nitre, were swallowed. The symptoms occasioned were universal coldness and shivering, extreme debility and sickness at stomach, cold sweats, and faintings. Neither of the cases proved mortal.

The cure was effected by cordials and corroborants.

A process has obtained a place in the dispensaries for a supposed purification of nitre, by means of flowers of brimstone. A pound of salt-petre is to be melted in a crucible; or small iron vessel; and an ounce of flowers of sulphur thrown upon it, by small quantities at a time: a violent deflagration ensues on each addition; and after the whole is put in, the salt is poured out in moulds, and then called sal prunella. It has been disputed whether the nitre was at all depurated by this process; Dr Lewis thinks it is not. From our own experience, however, we can affirm, that by this means a sediment falls to the bottom, which carries with it any impurities that may have been in the nitre, and leaves the fluid salt clear and transparent as water. This precipitate is probably no other than a vitriolated tartar formed by the union of the sulphurous acid and alkali of the nitre, which being less fusible than the nitre, subsides in a solid form and clarifies it.

3. Volatile alkali. The nitrous acid seems peculiarly adapted to an union with volatile alkali; saturating as much, or rather more of it than the strongest vitriolic acid is capable of doing. The product is a very beautiful salt, called volatile nitre, or nitrous sal ammoniac. It very readily dissolves, not only in water, but in spirit of wine, which distinguishes it from the vitriolic, and common kind of sal ammoniac. It also requires less heat for its sublimation: indeed care must be taken not to apply too great a heat for this purpose, as the nitrous sal ammoniac has the property of deflating by itself without any addition of inflammable matter; and this it does more or less readily, as the volatile alkali with which it was made was more or less impure and oily.

The medical virtues of this kind of nitre have not been inquired into. It seems to have made the principal ingredient in the famous Dr Ward's white drop, which was celebrated as an anticoagulant; with what justice, those who have tried it must determine. The first step towards the preparation of this medicine, was the distillation of a spirit of nitre from equal parts of nitre and green vitriol calcined to whiteness by a very gentle heat. The product, we know, must have been an aqua fortis of a quality nowise extraordinary. To fifteen ounces of this aqua fortis rectified, seven ounces of volatile sal ammoniac are to be added. The quantity of volatile alkali is insufficient to saturate so much acid; but, as far as it goes, will form nitrous sal ammoniac. To the acid, thus partly saturated, four ounces of quicksilver is to be added, of which it is to be allowed to dissolve as much as it can. The liquor now contains a solution of nitrous sal ammoniac, and of quicksilver. It is then to be evaporated to a pellicle; and as the nitrous sal ammoniac must be in greater quantity than the mercurial salt, it will at any rate crystallize first. Mr Beaumé has shown that the crystals of one kind of salt contain none of any other, though they are both dissolved in the same liquid. The salt produced could therefore contain no mercury, except what adhered to the outside of its crystals; and this was rendered as little as possible, by carefully draining the corrosive oil (as it is called in the receipts) from them. The nitrous sal ammoniac now rendered almost pure, is to be dissolved in twice its quantity of rose-water: and this solution is the famous white drop; a most excellent, and perhaps the greatest, anticoagulant and purifier of the blood. (Annual Register for 1765). The dose of this powerful medicine was two drops in the 24 hours. Two drops of any liquid weigh about a grain. A fourth part of this, or a quarter of a grain, was nitrous sal ammoniac impregnated with an uncertain, but inconceivably little portion of mercury dissolved in the nitrous acid; and we agree with the chemist who signed the receipt, that, if proper care was taken to drain the salt, such a medicine was not dangerous.

Nitrous Acid combined with Earths.

1. Calcaceous. The nitrous acid dissolves into calcareous a transparent colourless liquor; but for this purpose it nitre, must be very much diluted, or the solution will have a gelatinous consistence. This compound is not applicable to any useful purpose. It has a very acrid taste; and, if inflamed, attracts moisture from the air. If it is totally dried, it then resembles an earthy matter, which deflages very weakly. By distillation in a retort, almost all the acid may be expelled, and what little remains flies off in an open fire.

Mr Pott, who has particularly examined the combination of nitrous acid with quicklime, says that the decomposed acid suffered remarkable alterations by distillation fed from quicklime, and repeated cohabitations upon it. By these experiments he obtained a salt more sensibly susceptible of crystallization and detonation, than what can be obtained by a single combination. From his experiments it would seem, that nitrous acid, by this treatment with quicklime, was capable of being entirely decomposed.

If a solution of chalk in the nitrous acid be evaporated to dryness, and then gently calcined, it acquires the property of shining in the dark, after having been exposed to the sun's rays, or even to the light of a candle. This substance, from its inventor, is called Baldwin's phosphorus; or, from its being necessary to keep it in a glass hermetically sealed, phosphorus hermeticus. (See Earths.)

2. Argillaceous earths and magnesia.—All that is known concerning the combinations of nitrous acid with these earths is, that the first produce astringent, and the second purgative, compounds, similar to alum and Epsom salt, and which are not susceptible of crystallization.

Nitrous Acid combined with Metallic Substances.

1. Gold.—Till very lately, it has been the opinion of chemists, that the nitrous acid by itself was incapable of acting upon this metal.—Dr Brandt, however, produced before the Swedish academy of sciences, a solution of gold in the nitrous acid, obtained in parting, by that acid, a mixture of gold and silver.— The mixed metal was boiled with aqua fortis, in a glass body fitted with a head and receiver, the liquor poured off; and the cotion repeated with fresh parcels of stronger and stronger nitrous spirits, till all the silver was judged to be extracted. The last parcel was boiled down till the matter at the bottom looked like a dry salt; on boiling this in fresh aqua fortis in close vessels, as before, a part of the gold was dissolved, and the liquor tinged yellow. But though gold is by this means truly soluble in the nitrous acid, the union is extremely slight; the gold being not only precipitated on the addition of silver, but likewise spontaneously on exposure to the air.—Dr Lewis very justly observes, that this solution may have been often made unknown to the chemists who did so; and probably occasioned the mistakes which some have fallen into, who thought that they were in possession of aqua fortis capable of transmuting silver into gold.

2. Silver.—Pure spirit of nitre will dissolve its own weight of silver; and shoots with it into fine white crystals of a triangular form, consisting of very thin plates joined closely one upon another. These crystals are somewhat deliquescent; of an extremely bitter, pungent, and nauseous taste; and, if taken internally, are highly corrosive and poisonous. They melt in a small heat; and form, on cooling, a dark-coloured mass still more corrosive, called lunar caustic or lapis infernalis. They readily dissolve in water; and, by the affluence of warmth, in spirit of wine. In the Allae Naturae Curiosorum, tom. vii., there is a remarkable history of silver being volatilized by its combination with the nitrous acid. Four ounces of silver being dissolved in aqua fortis, and the solution set to distil in an earthen retort, a white transparent butter arose into the neck, and nothing remaining behind; by degrees the butter liquefied, and passed down into the phlegm in the receiver. The whole being now poured back into the retort, the silver arose again along with the acid. The volatilization being attributed to the liquor having stood in a laboratory where charcoal was burning in, the experiment was repeated with a fresh solution of silver, and a little powdered charcoal, with the same event.

Solution of silver in the nitrous acid stains hair, bones, and other solid parts of animals, and different kinds of wood, of all the intermediate shades from a light brown to a deep and lasting black. The liquors commonly sold for staining hair brown or black, are no other than solutions of silver in aqua fortis, so far diluted in water as not sensibly to corrode the hair.

It gives a permanent stain likewise to sundry stones; not only to those of the softer kind, as marble; but to some of considerable hardness, as agates and jaspers. The solution for this purpose should be fully saturated with the metal; and the stone, after the liquor has been applied, exposed for some time to the sun. M. du Fay observes, (in a paper on this subject in the French memoirs for 1728), that if the solution be repeatedly applied, it will penetrate in the whitish agate, or chalcedony, about one-twelfth of an inch; that the tincture does not prove uniform, on account of the veins in the stone; that the colours, thus communicated by art, are readily distinguishable from the natural, by disappearing on laying the stone for a night in aqua fortis; that, on exposing it to the sun afterwards for some days, the colour returns: that the solution gave somewhat different tinctures to different stones; to oriental agate, a deeper black than to the common chalcedony; to an agate spotted with yellow, a purple; to the jade stone, a pale brownish; to the common emerald, an opake black; to common granate, a violet unequally deep; to serpentine stone, an olive; to marble, a reddish, which changed to purple, and fixed in a brown; that on flutes, talcs, and amianthus, it had no effect.

If solution of silver be diluted with pure water, a considerable quantity of pure mercury added, and the whole set by in a cold place; there will form by degrees a precipitation and crystallization resembling a little tree, with its root, trunk, and branches, called arbor Diana or the philosophic silver tree. Another kind of artificial vegetation may be produced by spreading a few drops of solution of silver upon a glass plate, and placing in the middle a small bit of any of the metals that precipitate silver, particularly iron. The silver quickly concretes into curious ramifications all over the plate.

Like other metallic solutions, this combination of solution of the nitrous acid with silver is decomposed by fixed and volatile alkalies, calcareous earths, and several metals, (see the Table of Affinities); but with several peculiar circumstances attending the precipitation. With metals, the silver is readily and copiously thrown down at first, but slowly and difficulty towards the end. The menstruum generally retains some portion of the silver, as the silver almost always does of the metal which precipitated it. For recovering the silver from aqua fortis after parting, the refiners employ copper. The solution, diluted with water, is put into a copper vessel, or into a glass one with thin plates of copper, and set in a gentle warmth. The silver begins immediately to separate from the liquor in form of fine grey scales, or powder; a part of the copper being dissolved in its place, so as to tinge the fluid more or less of a bluish green colour. The plates are now and then shaken, that such part of the silver as is deposited upon them may fall off, and settle to the bottom. The digestion is continued till a fresh bright plate, kept for some time in the warm liquor, is no longer observed to contract any powdery matter on the surface; when the liquor is poured off, and the precipitate washed with fresh parcels of boiling water. It is observable, that though the acid in this process saturates itself with the copper, in proportion as it lets go the silver, yet the quantity of copper which it takes up is not near so great as that of the silver which it deposits. One drachm of copper will precipitate three of silver, and saturate all the acid that held the three drachms dissolved.

Calcareous earths, as chalk, or quicklime, throw down a part of the silver, but leave a very considerable part suspended in the liquor. If the earth be moistened with the solution into the consistence of a paste, and exposed to the sun, it changes its white colour to a dark purplish black; distinct characters may be exhibited on the matter, by intercepting a part of the sun's light by threads, slit paper, &c., placed on the outside of the glass. Culinary fire does not affect Practice feet its colour; after the mass has been exsiccated by this, it changes as before, on exposure to the sun.

Mild volatile alkaline spirits, added to a solution of silver precipitate, but little, and caustic volatile alkalies, none. Pure fixed alkalies, and alkalies rendered caustic by quicklime, throw down the whole. Fixed alkalies impregnated with inflammable matter by calcination with animal coals, occasion at first a confusable precipitation; but, if added to a larger quantity, take up great part of the metal again. Mr Margraf relates, that educolated calces of silver totally dissolve, both in a lixivium of these alkalies, and in volatile spirits; and that the marine acid precipitates the silver from the volatile, but not from the fixed, alkaline solution. Kunckel reports, that the calx precipitated by volatile spirits made with quicklime, fulminates or explodes in the fire; and that by infusorating a solution of pure silver, melting the dry redhium, pouring it on spirit of urine supersaturated with salt, and setting the mixture in a gentle warmth, a blood-red mass is produced, so tough as to admit of being wound about the fingers.

3. Copper. The nitrous acid very readily dissolves this metal into a green-coloured and very caustic liquor. The solution, if properly evaporated, will crystallize; but the crystals are deliquescent, and therefore difficult to be preserved. The only use of this combination is for the preparation of the pigment called verditer. Of this there are two kinds, the blue and green. The blue is by far the brightest colour, and consequently the most valuable. It is said that this is obtained by precipitating a solution of copper by any calcareous earth; and therefore is sold by the refiners, who have large quantities of solution of copper accidentally made. The solution is said to be precipitated by chalk, or whiting; and that the precipitate is the beautiful blue colour called verditer.

Though this process has obtained so much credit as to be printed, and implicitly copied from one book into another, it is certain that nothing can be more false. We have dissolved copper in the nitrous, the vitriolic, the marine, and the vegetable acids. These solutions we have precipitated with fixed alkalies, with volatile alkalies, with absorbent earths, without being able to produce any other colour than the dirty green verditer. We have combined the precipitates with different blues, rubbing them long with Prussian blue, with saltna, bice, &c., in different proportions, without producing the desired colour. Of these, final mixed with green verditer was found to come nearest to the colour of blue verditer. We have also dissolved copper in alkalies, both fixed and volatile; precipitating the solutions with acids, and endeavouring to combine them, when precipitated, with a small quantity of fixed alkali; but with no success. Blue verditer itself dissolves in the nitrous acid with a strong effervescence; and if precipitated from it by a fixed alkali, a volatile alkali, or an absorbent earth, is always changed into green verditer. This last, when pure, always dissolves without effervescence, and is precipitated unchanged.

Dr Merrit likewise takes notice of the fallacy of the receipts given for making verditer; and seems inclined to believe that silver is necessary to its formation, as this colour is prepared only by the refiners; but in whatever proportion we combined solution of copper with silver, we never could produce the desired colour.

4. Iron. On this metal the concentrated nitrous acid acts very violently; and plentifully corrodes, but does not dissolve it; the calx falling almost as fast as dissolved; and when it is once let fall, fresh acid will not take it up again. If the acid was diluted at first, it takes up a considerable proportion, provided the metal be leisurely added. If the solution is performed with extreme slowness, the colour will be green; but, if otherwise, of a dark red. It does not crystallize; and, if infusorated to dryness, deliquesces in the air.

5. Tin. Concentrated nitrous acid acts upon tin with great force, but only corrodes the metal into a white indissoluble mass; in order to obtain a perfect solution of tin in the nitrous acid, the metal must be put in by very little at a time, and a diluted aqua fortis made use of. This solution has been considerably used in dyeing, and is remarkable for heightening red colours of all kinds; but the solution made with aqua regis is preferable. See Tin, Sect. III.

6. Lead. Proof aqua fortis, lowered with an equal quantity of water, dissolves about half its weight of lead. On diluting the solution with a large quantity of water, it turns milky, and deposits great part of the metal. The solution floats, upon exhaling part of the menstruum, into small pyramidal crystals with square bases, of an agreeable sweet taste.

In the memoirs of the French academy for 1733, there is a particular account of an experiment, in which mercury is said to have been extracted from lead by dissolving it in the nitrous acid. During the dissolution, there fell a precipitate, which is plainly proved to be mercury, and was looked upon to be one of the constituent parts of the lead separated by this simple process; it seems probable, however, that the mercury in this case had been contained in the aqua fortis; for pure lead dissolved in pure aqua fortis gives no such precipitate.

The crystals of lead in the nitrous acid, when thrown into the fire, do not deliquesce as other combinations of this acid with metallic or saline bases; but crackle violently, and fly around, with great danger to the bystanders. If they are rubbed into very fine powder, they may then be melted without any danger. By repeated dissolutions in fresh aqua fortis, they at last form a thick fluid like oil, which cannot be dried without great difficulty. This composition is not adapted to any particular use, and is a violent poison.

7. Quicksilver. Aqua fortis, of such a degree of quicksilver strength as to take up half its weight of silver, dissolves with ease above equal its weight of mercury into a limpid liquor, intensely corrosive and poisonous, which spontaneously floes into white crystals. These crystals, or the solution exsiccated, and moderately calcined, assume a sparkling red colour; and are used in medicine as an elixirotic, under the name of red precipitate. This precipitate has sometimes been given internally, it is said, in very large quantities; even a whole drachm at one dose. But this would seem incredible; and the present practice does not countenance countenance the taking of red precipitate inwardly. This solution seems to have been what gave the efficacy to Ward's white drop. See no. 190.

When red precipitate is prepared in quantity, it is proper to distil the mercurial solution; because most of the aqua fortis may then be saved. It is exceedingly pure, if by purity we mean its being free of any admixture of vitriolic or marine acids; but is considerably tainted with the inflammable principle of the mercury extracted during the distillation. In consequence of this, it is very volatile and smoking; which has generally, though improperly, been taken as a sign of strength in the nitrous acid.

8. Bismuth. This semimetal is very readily acted upon by the nitrous acid. Proof aqua fortis dissolves about half its weight of bismuth. If the metal was halflly added, the solution proves of a greenish colour; if otherwise, it is colourless and transparent. Unless the acid was diluted with about an equal quantity of water, a part of the bismuth crystallizes almost as fast as it dissolves. The metal is totally precipitated both by fixed and volatile alkalies. The last, added in greater quantities than are sufficient for precipitation, take it up again. The liquor generally appears greenish; by alternate additions of the alkaline spirit and solution, it becomes bluish, or purple. Fixed alkalies calcined with inflammable matter, likewise dissolve the bismuth after they have precipitated it.

The only use of this compound is for the precipitation, which is used as a cosmetic, under the name of magistery of bismuth. The common way of preparing this is by diluting the solution very largely with water, upon which it turns milky, and a fine white precipitate falls, which is to be well edulcorated with water, and is then employed as a cosmetic, both in washes and pomatums.

Concerning the preparation of this cosmetic, Neuman observes, that there are sundry variations. "Some," says he, "take aqua regia for the menstruum; and for the precipitant a solution of sea-salt, alkalies, spirit of wine, &c. Some mix, with the solution of bismuth, a solution of benzoin in spirit of wine, and thus obtain a magistery compounded of bismuth and benzoin. Others add a solution of chalk to the metallic solution, and precipitate both together by alkalies. I have made trial with a good number of different precipitants; and found, that with common fixed alkali and caustic alkali, with watery and vinous alkaline spirits, the magistery was white, and in considerable quantity; the liquor, after the precipitation with volatile spirits, appearing blue. That oil of vitriol threw down a white precipitate very copiously; but that with spirit of salt, or spirit of vitriol, the precipitate was in very small quantity, in colour like the foregoing; distilled vinegar making no precipitation at all. Common rectified spirit of wine, and tartarized spirit, common water, and lime-water, gave white precipitates. Solutions of nitre, vitriolated tartar, sal mirabile, alum, borax, common salt, sal ammoniac, the combination of marine acid with calcareous earth, and terra foliata tartari, all precipitated the bismuth white. With a solution of gold in aqua regia the magistery proved grey; with a solution of the same metal in aqua regia made with spirit of salt, the precipitate was likewise grey, and in small quantity; with solution of copper in aqua fortis, white, and in very small quantity, the liquor continuing blue; with solution of vitriol of copper, white; with solution of mercury sublimate, white and plentiful; with solution of iron in aqua fortis, yellowish; with solution of lead in aqua fortis, and of sugar of lead, white; with solution of zinc in aqua fortis, there was little precipitate; and with solutions of silver, tin, regulus of antimony, and of mercury, in the same acid, none at all."

9. Zinc.—Upon this semimetal, the nitrous acid acts with greater violence than any other, and will forfeit any other metallic substance for it. The whole is very soon dissolved into a transparent colourless liquor. The calces or flowers of zinc (See Zinc, Sect. III.) are likewise soluble in the nitrous acid; but neither the solution of the flowers, nor of the metal itself, has been yet found applicable to any useful purpose. Neuman remarks, that on extracting with nitrous acid the soluble parts of calamine, which is an ore of zinc, the solution, infusoried to dryness, left a reddish brown mass, which, on digestion with spirit of wine, exploded, and burnt the vessel.

10. Regulus of Antimony. The nitrous acid rather corrodes, than dissolves, this semimetal. The corroded powder forms a medicine formerly used under the name of bezzar mineral, but now disregarded. See Antimony, Sect. III.

11. Regulus of Cobalt. This semimetal dissolves readily in the nitrous acid, both in its metallic form, cobalt, and when reduced to a calx. The solution is of a red colour. Hence the nitrous acid furnishes means of discovering this semimetal in ores, after strong calcination; very few other calces being soluble in the nitrous acid; and those that are, not influencing the colour.

12. Nickel. This semimetal is easily dissolved by the nitrous acid into a deep green liquor; but neither this solution, nor indeed the semimetal of which it is made, has hitherto been found of any use.

13. Arsenic. This substance is readily dissolved by the nitrous acid, but the nature of the compound is not known.

Nitrous Acid combined with Inflammable Substances.

1. Exprefsed oils. These, as well as all other fatty or unctuous substances, are considerably thickened and hardened by their union with the nitrous acid. There is only one preparation where this combination is applied to any use. It is the unguentum citrinum of Unguentum flores. This is made by adding to some quantity of melted hog's-lard, a solution of quicksilver in the nitrous acid. The acid, though in a diluted state, and combined with mercury, nevertheless acts with such force on the lard, as to render the ointment almost of the consistence of tallow.

1. Vinous spirits. If highly rectified spirit of wine and strong spirit of nitre are suddenly mixed together, the acid instantly becomes volatile, and is diffused with great heat and effervescence, in highly noxious red fumes. If the acid is cautiously poured into the spirit, in the proportion of five, six, or even ten parts of spirit to one of acid, and the mixture distilled in a glass- Nitrous Acid decomposed by Phlogiston.

1. Essential oils. If equal quantities of strong nitrous acid and oil of cloves are poured into the same vessel, the mixture instantly takes fire; both acid and oil burning with great fury till only a light spongy coal remains. Dr Lewis observes, that this experiment does not always succeed, and that there are but few oils which can be fired with certainty, without attending to a particular circumstance first discovered by M. Rouelle, and communicated in the French memoirs for the year 1747. "On letting fall into the oil equal its quantity of acid, the mixture effervesces, fizzes, and a light fungous coal arises: a little more of the acid poured upon this coal sets it instantly on fire: by this method almost all the distilled oils may be fired by spirit of nitre of moderate strength. Expressions oils also may be set on fire by a mixture of the nitrous acid and oil-of-vitriol; the use of which last seems to be absorb the aqueous humidity of the nitrous, and bring it to a greater degree of concentration than it can be brought by itself."

2. Charcoal. By this substance, the nitrous acid cannot be conveniently decomposed, unless it is combined with an alkaline or metallic base. For the purpose of decomposing the acid, common salt-petre is most convenient. The proportions recommended by Dr Lewis for alkali-firing nitre, are four ounces of the salt, to five drachms of powdered charcoal. If these are carefully mixed, and injected by little and little into a tubulated retort made red hot, and fitted with a large receiver and a number of adopters, a violent deflagration will ensue on every addition, attended with a great quantity of air, and some vapours which will circulate for some time, and then condense in the vessels. These vapours, when condensed, seem rather of an alkaline than acid nature. This liquor is called Clystus of nitre. If sulphur is used instead of nitre, the Clystus is of a different kind, consisting of a mixture of the nitrous and vitriolic acids. The residuum, when charcoal is used, is a very strong and pure alkali; with sulphur, it is vitriolated tartar.

3. Vinous spirits. In the process already mentioned for making Spiritus nitri dulcis, a total decomposition of the acid seems to take place; for neither the dulcified spirit itself, nor the acid matter left in the retort, show any signs of deflagration with inflammable matters, which is the peculiar characteristic of nitrous acid.

Mr Pott has given an analysis of the oleaginous residuum of the distillation. Distilled by a stronger fire, it gave over a yellow, acid, slightly empyreumatic spirit; which being saturated with fixed alkali, the liquor evaporated, and the dry neutral salt laid on burning coals did not deflagrate. After this spirit arose a red empyreumatic oil; and in the bottom of the retort was left a shining black mass like foot; which, burnt in a crucible, left a white, fixed, earth, convertible, by a vehement fire, into glaas. Another parcel of the above residuum was evaporated to the consistence of pitch. In this state it gave a yellow tincture to spirit of wine, flamed vividly and quietly on burning coals, and at last swelled up like bitumens. Another portion was saturated with alkaline ley, with which it immediately effervesced, and then evaporated as the former. It gave, as before, a yellow colour to rectified spirit of wine, and a much deeper yellow to dulcified spirit of nitre; and in the fire discovered no footstep of detonation. Mr Macquer supposes this acid to have been not the nitrous, but the acetic, which enters into the composition of the spirit of wine; but it is impossible to account for the total disappearance of the spirit of nitre, unless on the supposition of its decomposition.

III. Of the Marine Acid and its Combinations.

This acid is never, at least very rarely found, but Marine acid is in a state of saturation with the mineral alkali; in which case it forms the common salt used in food. Almost the only exception to this is human urine, and perhaps that of some other animals; for there the marine acid is found saturated, not with the mineral, but the common vegetable fixed alkali. From being found in such plenty in the waters of the ocean, it has the name of marine acid.

It is commonly thought that this acid is no other than the vitriolic, somehow or other disguised by the inflammable principle; to which some have added another, called by them a mercurial earth.

The reasons given for this supposition, however, are but very slight, consisting chiefly in the resemblance between the volatile vitriolic acid and the marine, both in the white colour of their vapours, and likewise the great volatility of both. As to the existence of that principle called a mercurial earth, it hath never been proved; and, till that time, can never be allowed to be an ingredient in the composition of any substance whatever. As we do not remember to have read of any experiments where the marine acid was directly As vitriolated tartar, or Glauber's salt, when fused with charcoal-dust, is converted into an hepatic sulphur, attempts have been made on this principle to separate the pure alkali from the residuum of Glauber's spirit of nitre and spirit of salt. In an attempt of this kind, which, by the bye, proved unsuccessful, as all others of the same kind must do, so or 40 pounds of the mats for Glauber's salt were fused in a strong iron pot, with a sufficient quantity of common coal powdered and fitted. As the quantity of powdered coals was pretty large, the mats was thereby hindered from flowing into thin fusion; and, that the whole might be perfectly alkallated, it was frequently stirred up with an iron ladle, and kept very intensely heated for some hours. The mats was now taken out by means of an iron ladle, and laid on a flat stone; and, as it was but half fluid, every ladleful concreted into a black irregular saline mat, which had the appearance of a cinder; but which, however, consisted of an hepatic sulphur mixed with some coal-dust. As there was a considerable quantity of this matter, and the ladlefuls were thrown at random above one another, it so happened, that between two or three of the pieces, a kind of chimney was formed, so that there being a small draught of air through the interstices, and the masses containing a quantity of coal-dust, the internal parts were in a state of ignition, while the external were quite cold. From these ignited places, a white fume arose; which, being collected on the colder masses, assumed the form of white flowers. These were found to be genuine sal ammoniac, composed of volatile alkali, and marine acid; both of which, we have the greatest reason to think, were produced at that very time, and that a double transmutation took place; namely, of the vitriolic acid into the marine, and of the fixed alkali into the volatile. Our reasons for being of this opinion are, 1. That the matter had been subjected to such an extreme and long continued heat, that, had any sal ammoniac been pre-existent in the mixture, it must have certainly been distillated, as this salt always sublimes with a degree of heat below ignition. 2. Though the matter was taken out of the pot of a very intense red heat, so that the saline part was evidently melted, yet no ammoniacal fume issued from it at that time, nor till the masses had been for some time exposed to the air, and were become cool, excepting only those interstices where the air kept up a burning heat, by a small draught being formed from the situation of the saline masses. 3. In those ignited places, when cool, the fixed salt was entirely decomposed, neither alkaline salt, Glauber's salt, fixed alkali, nor sulphur remaining; but the whole was consumed to a kind of ferruginous ashes. We are therefore of opinion, that the marine acid and volatile alkali are, in some cases, mere creatures of the fire, and most commonly produced at the same time, from the slow combustion of mineral substances. Hence, where heaps of hot cinders are thrown out, small quantities of the true sal ammoniac are always formed, when the ignited ones happen to fall in such a manner as to occasion a small draught of air through them.

The marine acid, or spirit of salt, is weaker than either the vitriolic or nitrous; though Dr Priestley has observed, that, when concentrated to the utmost degree, in which state it was perfectly invisible and elastic as air, it was then able to separate the nitrous acid from an alkali. In some other cases, too, it appears not only stronger than the nitrous, but even than the vitriolic, of which we shall take notice in course.

To procure the Marine Acid by means of the Vitriolic.

Put any quantity of sea-salt, into a tubulated glass retort, to which a large receiver is firmly luted, having a quantity of water in it, more or less, as you want your spirit of salt to be more or less strong. Having placed your retort in a sand-bath, take of concentrated oil of vitriol half as much as you put salt into the retort. Through the aperture in the upper part of the retort, pour a small quantity of the vitriolic acid; a violent effervescence will immediately arise, and white vapours will ascend, and come over into the receiver. These vapours are the marine acid in its most concentrated state; and, as they are very greedy of moisture, they will unite with the water in a very short time, unless too much oil of vitriol is put in at once; in which case, part of them will be distillated through the small hole in the receiver. When you perceive the first fumes condensed, add a little more oil of vitriol, taking care to stop the aperture of the retort as soon as you drop in the vitriolic acid, that the marine acid may not escape. Continue this by intervals, till your acid is all put in; and then make a very gentle fire, that the retort may be no warmer than the hand can bear. This degree of heat must be continued a long time, otherwise very much of the acid will be lost. To perform this operation perfectly, no more acid should be forced over, than what the water in the receiver can take up; and by this means the operator's patience will be rewarded with a vastly larger produce of acid than can be procured by halting distillation. When the vapours become a little more fixed, a greater heat is necessary, but nothing equal to what the nitrous acid requires.

The marine acid cannot be procured by means of Why distillations of the vitriolic acid with metallic and lation of earthy bases, as the nitrous is: for though, by means of calcined vitriol, for instance, the marine acid is effectually expelled from its alkaline basis, yet it immediately combines with the calx of iron left by the vitriolic acid, and not only adheres obstinately, but even sublimes the metal; so that what little spirit can be obtained, is never pure. This inconvenience is not so great when uncalcined copperas is made use of: for the marine acid has a very strong attraction to water; which partly dissolves its union with the metallic calx. If gypsum is used, instead of calcined vitriol, not a drop of spirit will be obtained. Alum and sal catharticus amarus answer better.

To procure the Marine Acid by means of the Nitrous.

Take equal quantities of sea-salt, and Glauber's spirit of nitre; put the salt into a retort, and pour on it the nitrous acid; let them stand for 10 or 12 hours; Practice then distil with a gentle heat; an acid liquor will come over, which is a compound of the nitrous and marine acids, called aqua regis. When the distillation is finished, and the vessels cooled, pour back the distilled liquor on the mass which is left on the retort, and distil again; the second produce will be more of the nature of spirit of sea-salt than the former. Continue to do this, pouring the distilled liquor either on the mass left in the retort, or upon fresh sea-salt, till you observe that no nitrous acid arises. No experiments have been made on this spirit of salt, by which we can judge whether it is different from that procured by the vitriolic acid or not.

To procure the Marine Acid, by distilling Salt per se.

Put into a retort any quantity of common salt which has not been dried, and distil in a slow heat, till nothing more will come over. In the receiver you will have a liquor considerably more acid than vinegar, in weight about the fourth part of the salt employed. On the dry salt left in the retort, pour some water, somewhat less in quantity than the liquor which came over. Let it stand till the salt has thoroughly imbued the moisture, and then distil again. You will again have an acid, but weaker than the former. Repeat this five or seven times; after which you will obtain no more marine acid in this way. It has been thought that sea-salt was capable of total decomposition by means of moisture alone; but that is found to be a mistake. The reason of any acid being procurable in this way, is the impurity of the common salt, which is always mixed with a quantity of sal causticums amarus, and of marine acid combined with magnesia, from which last it is separable by moisture. If a pure salt be formed by combining marine acid with salt of soda, no spirit will be obtained.

Marine Acid combined with Alkaline Salts.

1. Vegetable fixed alkali. This combination is accidentally formed after the distillation of volatile salts, by means of salt of tartar. (See Alkaline Salts.) It was formerly known by the name of sal digestus Sylvii; and a process for making it was inferred in the distillatories, under the name of spiritus salis marini coagulatus; but as it has been found to possess no virtues superior, or even equal, to common salt, it is fallen into disuse.

The crystals of this kind of salt are not cubical, like those of common salt, but parallelopipeds, and if thrown into the fire crack and leap about with violence. They are soluble in greater quantity by hot water than cold; and therefore are crystallized by evaporating the solution to a pellicle, and then letting it cool.—It is very remarkable, that though by a direct combination of vitriolic acid with vegetable fixed alkali, the salt called vitriolated tartar is formed; yet, if this alkali is once saturated with spirit of salt, so as to form a sal digestus, upon the decomposition of this salt by means of oil of vitriol, the residuum of the distillation will not be a vitriolated tartar, but a salt easily soluble in water, and which bears a strong resemblance to Glauber's salt. Whether, by means of spirit of sea-salt, the vegetable alkali could be converted into the mineral, or salt of soda, is a question well worthy of being solved.

2. Mineral alkali. This combination is the common alimentary salt, and is never made but for experiment's sake; as the marine acid cannot be had but from sea-salt. For the extraction of this salt from seawater, see the article SALT.

3. Volatile alkali. The produce of this combination is the common sal ammoniac, which is used in many different arts, and which has the property of making tin unite very readily with iron and copper to its much used by coppersmiths and in the manufactory of tinned iron.

Sal ammoniac is usually sold in large semi-transparent cakes, which are again capable of being sublimed into masses of the like kind. If they are dissolved in water, the salt very easily floats into small crystals like feathers. Exposed to a moist air, it deliquesces. It is one of the salts which produces the most cold by its solution; so as to sink the thermometer 18 or 20 degrees, or more, according to the temperature of the atmosphere. According to Mr. Gellert, a solution of sal ammoniac has the property of dissolving resins. According to Neuman, the volatility of sal ammoniac is so much diminished by repeated sublimations, that at last it remains half fluid in the bottom of the subliming vessel. In its natural state, it sublimes with a degree of heat necessary to melt lead. Pott says, that a small quantity of sal ammoniac may be produced, by distilling sea-salt with charcoal, or with alum, or by distilling marine acid with Armenian bole. The same author affirms, that the inflammability of sulphur is destroyed by subliming it with twice its quantity of sal ammoniac.

The method of making this salt was long unknown; how made, and it was imported from Egypt, where it was said to be prepared by sublimation from foot alone, or from a mixture of sea-salt, urine, and foot. That it should be produced from foot alone is very improbable; and the other method, from the known principles of chemistry, is absolutely impossible. The composition of this salt, however, being once known, there remained no other desideratum than a method of procuring those component parts of sal ammoniac sufficiently cheap, so as to afford sal ammoniac made in Britain at a price equally low with what was imported. The volatile alkali is to be procured in plenty from animal substances, or from foot; and the low price of the vitriolic acid made from sulphur, affords an easy method of decomposing sea-salt, and obtaining its acid at a low rate. A sal ammoniac work has, accordingly, been established for several years past in Edinburgh; the principal material made choice of for procuring the volatile alkali is foot; and though no persons are admitted to see the work, the large quantities of oil of vitriol brought into it, and the quantities of genuine sal mirabile which are there made, evidently show that the process for making sal ammoniac also produces Glauber's salt, by the decomposition of common salt by means of vitriolic acid. The method of conducting the process is unknown; but it is plain that there can be no other difficulty than what arises from the volatility of the vapours of the alkali and of the marine acid. In the common way of distilling those substances, Practice substances, a great part of both is lost; and if it is attempted to make sal ammoniac by combining these two when distilled by the common apparatus, the produce will not pay the cost: a little ingenuity, however, will easily suggest different forms and materials for distilling vessels by which the marine acid and volatile alkali may be united without losing a particle of either.

If a solution of vitriolic or Glauber's secret sal ammoniac is mixed with sea-salt, the vitriolic acid seizes the alkaline basis of the sea-salt, and expels the marine acid; which immediately unites with the volatile alkali left by the vitriolic acid, and forms a true sal ammoniac. If this solution is now evaporated to dryness, and the saline mass sublimed, the sal ammoniac rises, and leaves a combination of vitriolic acid and mineral alkali at the bottom. This fixed mass being dissolved, filtered, and evaporated, affords Glauber's salts. This has sometimes been thought a preferable method of making sal ammoniac, as the trouble of distilling the marine acid was thereby prevented; but it is found vastly inconvenient on another account, namely, that when sal ammoniac is mixed with any fixed salt, it is always more difficult of sublimation, and a part of it even remains entirely fixed, or is destroyed. The mass of Glauber's salt also, by reason of the inflammable and oily matter contained in impure volatile alkalies, is partly changed into a sulphurous mass, so that the solution refuse to crystallize; at least, the operation is attended with intolerable trouble.

Marine Acid combined with Earths.

The combinations of this acid with earths of any kind have never been found applicable to any purpose, and therefore they are seldom made or inquired into. The combination with calcareous earth, is indeed pretty frequently made accidentally, in the distillation of volatile alkali from sal ammoniac by means of chalk, or quicklime. When melted in a crucible and cooled, it appears luminous when struck, and has been called phosphorus scintillans. See Earths.

Marine Acid combined with Metallic Substances.

1. Gold. The marine acid has no action on gold in its metallic state, in whatever manner the acid be applied; but if the metal is previously attenuated, or reduced to a calx, either by precipitation from aqua regis, or by calcination in mixture with calcinable metals, this acid will then perfectly dissolve, and keep it permanently suspended. Gold, precipitated from aqua regis by fixed alkalies, and elaborated by repeated ablutions, may be dissolved even in a very weak spirit of salt by moderate digestion. This solution appears of the same yellow colour as that made in aqua regis; gives the same purple tinge to the skin, feathers, bones, and other solid parts of animals; the same violet tinge to marble; and strikes the same red colour with tin. Even when common aqua regis is made use of for the menstruum, it seems to be chiefly by the marine acid in that compound liquor that the gold is held in solution. In distillation the nitrous acid arises, and the marine acid remains combined with the gold in a blood-red mass, fusible, like most of the combinations of metallic bodies with this acid, in spirit of wine. It, towards the end of the distillation, the fire is raised, part of the gold distils in a high fuming coloured liquor; and part sublimes into the neck of the retort in clutters of long slender crystals of a deep red colour, fusible in a small heat, deliquifying in the air, and easily soluble in water. By repetitions of this process the whole of the gold may be elevated, except a small quantity of white powder whose nature is not known.—This red sublimate of gold is said to be easily fusible with the heat of one's hand, and to be thrown by the Papists for the blood of St Janarius; the sublimate contained in a vial, being warmed by the hands of the priests who hold it, constitutes the miracle of that saint's blood melting on his birthday.

2. Silver. Strong spirit of salt corrodes leaf-silver into a white powder, but has no effect on filings or larger masses of the metal. If applied in the form of vapour, to masses of silver, and strongly heated at the same time, it readily corrodes them. Thus, if filings, grains, or plates, of silver are mixed with about twice their weight of mercury sublimate, and exposed to a moderate fire, in a retort, or other distilling vessel, a part of the marine acid in the sublimate will be separated and unite with the silver, leaving the mercury to arise in the form of mercurius dulcis. Marine acid is commonly supposed to be incapable of dissolving silver into a liquid state; but Hennel relates, that if red silver ore, which consists of silver intimately mixed with red arsenic, be digested in spirit of salt, the silver will be extracted and kept permanently dissolved.

The combination of marine acid with silver is called Luna cornea. The most ready way of preparing it is, by dissolving silver in the nitrous acid, and then adding spirit of salt, or a solution of sea-salt; when a precipitation instantly ensues: the marine acid expels the nitrous, and, uniting with the silver, falls to the bottom in form of a white powder. The same precipitation would take place, if a solution of silver was made in the vitriolic acid. See n° 32.

Luna cornea weighs one-fourth more than the silver is properly employed; yet, when perfectly washed, it is quite impervious to the tafte. It does not dissolve in water, spirit of wine, aqua fortis, or aqua regis; but is in some small degree acted upon by the vitriolic acid. It melts in the fire as soon as it grows red-hot; and, on cooling, forms a ponderous brownish mass, which being cast into thin plates, becomes semi-transparent, and somewhat flexible, like horn; whence its name luna cornea. A stronger fire does not expel the acid from the metal, the whole concrete either subliming entire, or passing through the crucible. It totally dissolves in volatile alkaline spirits without any separation of the metal. Exposed to the fire in a close copper vessel, it penetrates the copper, and tinges it throughout of a silver color. Kunckel observes, that when carefully prepared, melted in a glass vessel, and suffered to cool slowly, to prevent its cracking, it proves clear and transparent; and may be turned upon a lathe, and formed into elegant figures. He supposes this to be the preparation which gave rise to the notion of malleable glass.

3. Copper. In the marine acid, copper dissolves but slowly. The solution, if made without heat, appears at first brown; but, on standing for some time, deposits a white sediment, and becomes green. On adding fresh copper, Practice copper, it becomes brown again, and now recovers its greenness more slowly than before. The white sediment, on being barely melted, proves pure and perfect copper of the same colour as at first. Copper calcined by fire, communicates a reddish colour to this acid.

4. Iron. The marine acid acts upon iron less vehemently than the nitrous, and does not dissolve so much; nevertheless it attacks the metal briskly, so as to raise considerable heat and effervescence, and dissolve it into a yellow liquor. During the solution, an inflammable vapour arises as in the solution of this metal by vitriolic acid. This solution of iron does not crystallize. If it is evaporated, it leaves a greenish saline mass, which is soluble in spirit of wine, and runs in the air into an astringent yellow liquor. If this solution of iron is distilled, some of the acid separates, and towards the end of the distillation the spirit becomes yellow. This is followed by a yellowish, or deep reddish sublimate, which glitters like the scales of fishes; leaving behind a substance which consists of thin, glossy plates, like talc.

The solution of iron in spirit of salt, with the addition of some spirit of wine, is used in medicine as a corrosive, under the name of tinctura martis. The sublimate of iron is also used for the same purpose, and called ens veneris, or ferris martiales. It is commonly directed to be prepared by subliming iron filings and sal ammoniac together. In the process, the sal ammoniac is partly decomposed, and a caustic alkaline liquor distils. Then the undecomposed sal ammoniac, and the martial sublimate above mentioned, arise together. The sublimate has a deeper or lighter yellow colour, according as it contains more or less sal ammoniac. The name ens veneris is improper. It was given by Mr Boyle, who discovered this medicine. He imagined it to be a preparation of copper, having made use of a colochoar of vitriol containing both iron and copper.

5. Tin. Though the concentrated marine acid has a greater attraction for tin than any other acid, it does not readily dissolve this metal while the acid is in its liquid state; but may be made to dissolve it perfectly, by the addition of a small quantity of spirit of nitre. Neuman observes, that an ounce of spirit of salt, with only a scruple of spirit of nitre, dissolved tin perfectly; but on inverting the proportions, and taking a scruple of marine acid to an ounce of the nitrous, four scruples, or four and an half, of tin, were dissolved into a thick pulp; some more of the marine acid being gradually added, the whole was dissolved into a clear liquor. In making these solutions, a small quantity of black matter usually subsides.

The solution of tin is sometimes colourless; sometimes of a bluish, or yellow colour, according to different circumstances of the process. It is of the greatest consequence in dyeing, by not only heightening the colours, but making them more durable. (See Dyeing.) It floats into small crystals; and, if inflamed, deliquesces in the air.

Marine acid in its concentrated state volatilizes tin, and forms with it a thick liquor, which, from its inventor, is called smoking liquor of Libavius. To make this smoking liquor, an amalgam (See Sect. III.) must be made of four parts of tin and five of mercury. This amalgam is to be mixed with an equal weight of corrosive mercury, by triturating the whole together in a glass mortar. The mixture is then to be put into a glass retort, and the distillation performed with a fire gradually increased. A very smoking liquor passes into the receiver; and towards the end of the distillation, a thick, and even concrete matter. When the operation is finished, the liquor is to be poured quickly into a crystal glass-bottle, with a glass stopper. When this bottle is opened, a white, copious, thick, and poignant fume issues, which remains long in the air without disappearing.

The acid in this liquor is far from being saturated, and is capable of still dissolving much tin in the ordinary way. From this imperfect saturation, together with its concentration, proceeds partly its property of smoking so considerably: nevertheless, some other cause probably concurs to give it this property; for though it smokes infinitely more than the most concentrated spirit of salt, its vapours are, notwithstanding, much less elastic. It has all the other properties of concentrated marine acid when imperfectly saturated with tin. If it is diluted with much water, most of the metal separates in light white floes. In dyeing it produces the same effects as solution of tin made in the common way. If the distillation is continued after the smoking liquor of Libavius has come over, the mercury of the corrosive sublimate will then arise in its proper form.

6. Lead. Marine acid, whether in its concentrated or diluted state, has little effect upon lead, unless assisted by heat. If spirit of salt is poured on filings of lead, and the heat is increased so as to make the liquor boil and distil, a part of the acid will be retained by the metal, which will be corroded into a saline mass; and this, by a repetition of the process, may be dissolved into a limpid liquor. If lead is dissolved in aqua fortis, and spirit of tea-falt, or tea-falt itself, added, a precipitation of the metal ensues; but if some aqua regia is added, the precipitate is redissolved.

The combination of lead with marine acid, has, when melted, some degree of transparency and flexibility like horn; whence, and from its resemblance to luna cornecum, it is called plumbum cornicum. This substance is used in preparing phosphorus, according to Mr Margraaf's method.

7. Quicksilver. Marine acid in its limpid state, whether concentrated or diluted, has no effect upon quicksilver, even when assisted by a boiling heat; but if mercury is dissolved in the vitriolic or nitrous acids, and tea-falt, or its spirit, is added to the solution, it immediately precipitates the quicksilver in the same manner as it does silver or lead. If concentrated marine acid, in the form of vapour, and strongly heated, meets with mercury in the same state, a very intimate union takes place; and the produce is a most violent corrosive and poisonous salt, called corrosive sublimate mercury. This salt is soluble, though sparingly, in water; but is far from being perfectly saturated with mercury; for it will readily unite with almost its own weight of fresh quicksilver, and sublime with it into a solid white mass (which, when levigated, assumes a yellowish colour) called mercurius dulcis, aquila alba, or calomel.

There There have been many different ways of preparing corrosive mercury, recommended by different chemists. Neuman mentions no fewer than ten.

1. From mercury, common salt, nitre, and vitriol. 2. From mercury, common salt, and vitriol. 3. Mercury, common salt, and spirit of nitre. 4. Solution of mercury in aqua fortis and salt. 5. Solution of mercury in aqua fortis, and spirit of salt, or the white precipitate. 6. Mercury, common salt, nitre, and oil of vitriol. 7. Edulcorated turbith mineral, and common salt. 8. Red precipitate, common salt, and oil of vitriol. 9. Edulcorated turbith mineral, and spirit of salt. 10. Mercury, sal ammoniac, and oil of vitriol.

From a view of these different methods, it is evident, that the intention of them all is to combine the marine acid with quicksilver; and as this combination can be effected without making use of the nitrous acid, the greatest chemists have imagined that this acid, which is by far the most expensive of the three, might be thrown out of the process altogether, and sublimate be more conveniently made by directly combining marine acid and mercury in a process similar to the distillation of spirit of salt. This method was formerly recommended by Kunckel; then published in the memoirs of the Academy of Sciences for 1730; and has been adopted and recommended by Dr Lewis.

The process consists in dissolving mercury in the vitriolic acid, as directed for making turbith mineral (see no 153). The white mass remaining on the evaporation of this solution is to be triturated with an equal weight of dried salt, and the mixture is then to be sublimed in a sand-pan; gradually increasing the fire till nothing more arises.

Neuman observes, that there is a considerable difference in the quality of sublimes made by the different methods he mentions; particularly in those made with, or without nitre. This we have also found to be the case; and that sublimate made without the nitrous acid is never so corrosive, or soluble in water, as that which is made with it; nor will it afterwards take up so large a quantity of crude mercury as it otherwise would, when it is to be formed into calomel. The above process, therefore, tho' very convenient and easy, is to be rejected; and some other, in which the nitrous acid is used, substituted in its stead. This is another instance where a rigid adherence to the established rules of chemistry will lead people into a mistake. See no 117.

From Tacheius, Neuman gives us the following process, which he says was the method of making sublimate at London, Venice, and Amsterdam. Two hundred and eighty pounds of quicksilver; 400 pounds of calcined vitriol, 200 pounds of nitre, the same quantity of common salt, and 50 pounds of the caput mortuum remaining after a former sublimation, or (in want of it) of the caput mortuum of aqua fortis, making, in all, 1130 pounds, are well ground, and mixed together; then let to sublime in proper glasises placed in warm ashes; the fire is increased by degrees, and continued for five days and nights. In the making such large quantities, he says, some precautions are necessary, and which those constantly employed herein are best acquainted with. The principal are, the due mixture of the ingredients, which in some places is performed in the same manner as that of the ingredients for gun-powder, (see Gun-powder): that a head and receiver be adapted to the subliming glasise, to save some spirit of nitre which will come over. (Here a bent tube of glass will answer the purpose, as already mentioned, no 80.) The fire must not be raised too hastily. When the sublimate begins to form, the ashes must be removed a little from the sides of the glasise, or the glasise cautiously raised up a little from the ashes. (This last, we think, is highly imprudent.) Lastly, the laboratory must have a good chimney, capable of carrying off the noxious fumes.

The above-mentioned quantities commonly yield 360 pounds of sublimate; the 280 pounds of quicksilver gaining 80 from the 200 pounds of sea-salt. The makers of sublimate in France, he says, employ, in one operation, only 20 pounds of mercury. They dissolve in aqua fortis, evaporate the solution to dryness, mix the dry matter with 20 pounds of decrepitated sea-salt and 60 of calcined vitriol, and then proceed to sublimate.

The above processes, particularly the last, are unexceptionable as to the production of a sublimate perfectly corrosive; but the operation, it is evident, must differ; be attended with considerable difficulty, by reason of the large quantity of matter put into the glasise at once. We must remember, that always on mixing a volatile salt with a quantity of fixed matter, the sublimation of it becomes more difficult than it would have been had no such matter been mixed with it. It is of considerable consequence, therefore, in all sublimations, to make the quantity of matter put into the glasise as little as possible. It would seem more proper, therefore, instead of the calcined vitriol used in the processes last mentioned, to dissolve the mercury in the vitriolic acid, as directed for turbith mineral, and sublimate the dry mass mixed with nitre and sea-salt.

It has been said, that corrosive sublimate mercury was frequently adulterated with arsenic; and means have even been pointed out for detecting this supposed adulteration. These means are, to dissolve a little of the suspected salt in water, and add an alkaline lixivium to precipitate the mercury. If the precipitate was of a black colour, it was said to be a certain sign of arsenic. This, however, shows nothing at all, but that either the alkali contains some inflammable matter, which, joining with the precipitate, makes it appear black; or that the sublimate is not perfectly corrosive; for if a volatile alkali is poured on levigated mercurius dulcis, the place it touches is instantly turned black.

Mercurius dulcis, or calomel, is prepared by mixing equal parts, or, at least, three of quicksilver, with four of sublimate; after being thoroughly ground together in a glasise or stone mortar, they are to be poured through a long funnel into a bolt-head, and then sublimed. The medicine has been thought to be improved by repeated sublimations, but this is found to be a mistake.

g. Zinc. This semi-metal dissolves readily in the zinc volatilising acid, into a transparent colourless liquor. It is volatilized, as well as most other metallic substances, by this combination, as appears from the following process delivered by Neuman. Equal parts of filings of zinc and powdered sal ammoniac being mixed together, and urged with a gradual fire in a retort; at first arose, in a very gentle heat, an excessively penetrating volatile spirit, so strong as to strike a man down, who should inadvertently receive its vapour freely into the nose. This came over in subtile vapours, and was followed by a spirit of salt in dense white fumes. In an open fire, white flowers succeeded; and, at length, a reddish and a black butter. In the bottom of the retort was found a portion of the zinc, in its metallic form, with a little ponderous and fixed butyraeous matter, which liquefied in the air. The lump was far more brittle than zinc ordinarily is; of a reddish colour on the outside, and blackish within. The bottom of the retort was variegated with yellow and red colours, and looked extremely beautiful. The remaining zinc was mixed afresh with equal its weight of sal ammoniac, and the process repeated. A volatile alkaline spirit and marine acid were obtained as at first; and in the retort was found only a little black matter. When the zinc was taken at first in twice the quantity of the sal ammoniac, the part that preserved its metallic form proved less brittle than in the foregoing experiment; and the retort appeared variegated in the same manner; on endeavouring to rectify the butter, the retort parted in two, by the time that one half had distilled." The nature of this combination is unknown.

10. Regulus of antimony. This semi-metal cannot be united with the marine acid, unless the latter is in its most concentrated state. The produce is an excessively caustic thick liquid, called butter of antimony. The process for obtaining this butter, is similar to that for distilling the smoking spirit of Libavius. (See no 247). Either crude antimony, or its regulus, may be used; for the spirit of salt will attack the reguline part of this mineral, without touching the sulphureous. Three parts of corrosive sublimate are to be mixed with one of crude antimony; the mixture to be digested in a retort set in a sand bath; the marine acid in the sublimate will unite with the reguline part of the antimony. Upon increasing the fire, the regulus arises, dissolved in the concentrated acid, not into a liquid form, but that of a thick unctuous substance like butter, from whence it takes its name. This substance liquefies by heat, and requires the cautious application of a live coal to melt it down from the neck of the retort. By rectification, or exposure to the air, it becomes fluid, like oil, but still retains the name of butter. If water is added to butter of antimony, either when in a butyraeous form, or when become fluid by rectification, the antimony is precipitated in a white powder called powder of argoth, and improperly mercurius vitae. This powder is a violent and very unsafe emetic. The butter itself was formerly used as a caustic; but it seems totally neglected in the present practice.

When the mercurius vitae precipitates, the union between the marine acid and regulus is totally dissolved; so that the powder, by frequent washings, becomes perfectly free from every particle of acid, which unites with the water made use of, and is then called, very improperly, philosophic spirit of vitriol.

11. Regulus of cobalt. Pure spirit of salt dissolves this semi-metal into a reddish yellow liquor, which immediately becomes green from a very gentle warmth. On fuming the solution with urinous spirits, the precipitate appears at first white, but afterwards becomes blue, and at length yellow. If the nitrous acid is added to solutions of regulus of cobalt, they assume a deep emerald green when moderately heated, and on cooling become red as at first. Duly evaporated, they yield rose-coloured crystals, which change their colour by heat in the same manner. This solution makes a curious sympathetic ink, the invention of which is commonly ascribed to M. Hellot, though he himself acknowledges that he received the first hint of it from a German chemist in 1736. Anything wrote with this solution is invisible when dry and cold; but assumes a fine green colour when warm, and will again disappear on being cooled; but if the heat has been too violent, the writing still appears. Mr Hellot observes, that if nitre or borax be added to the nitrous solution, the characters wrote with it become rose-coloured when heated; and if sea-salt is afterwards passed over them, they become blue; that with alkali sufficient to saturate the acid, they change purple, and red with heat.

Arsenic. This substance is soluble in all acids; but the nature of the compounds formed by such an union is little known. If half a pound of arsenic is distilled with one pound of corrosive sublimate, a thin smoking liquor and a butyraeous substance will be obtained, as in making the smoking liquor of Libavius, (see no 247). By repeated rectifications, this butter may be almost all converted into spirit. If equal parts of the arsenic and sublimate are used, a ponderous black oil comes over along with the spirit, which cannot be mixed with it. By rectification in a clean retort, they will become clear, but still will not incorporate. If they are now returned upon the red mass remaining in the first retort, and again distilled, a much more ponderous oil than the former will be obtained.

Marine Acid combined with Inflammable Substances. The acid of sea-salt is very little disposed to combine with any union with the phlogiston, while in a liquid ether state; and much less so, even in its most concentrated state, than either the vitriolic or nitrous. Mr Beaumé, however, has found that a small quantity of ether, similar to that prepared with the vitriolic and nitrous acids, may be obtained by causing the fumes of the marine acid unite with those of spirit of wine. Others, and particularly some German chemists, attempted to make this liquor, by employing a marine acid previously combined with metallic substances, such as butter of antimony. The smoking liquor of Libavius, (no 247), succeeds best. If equal parts of this liquor, and highly rectified spirit of wine are distilled together, a considerable quantity of true ether is produced; but which, like the vitriolic and nitrous ether, must be rectified, in order to its greater purity. The tin contained in the smoking liquor is separated, and precipitated in white powder. In this process, the acid is probably more disposed to unite with the spirit of wine, by having already begun to combine with the inflammable principle of the metal. Dr Priestley has observed, that the pure marine acid, when reduced to an invisible aerial state, has a strong affinity with phlogiston, so that it decomposes many substances that contain it, and forms with them an air permanently inflammable. By giving it more time, it will extract phlogiston from dry wood, crusts of bread not burnt, dry flesh; and, what is still more extraordinary, from flints.

Essential oil of mint absorbed the marine acid air pretty fast, and presently became of a deep brown colour. When taken out of this air, it was of the consistence of treacle, and sunk in water, smelling differently from what it did before; but still the smell of the mint was predominant. Oil of turpentine was also much thickened; and became of a deep brown colour, by being saturated with acid air. Ether absorbed the air very fast; and became first of a turbid white, and then of a yellow and brown colour. In one night a considerable quantity of strongly inflammable air was produced.

Having once saturated a quantity of ether with acid air, he admitted bubbles of common air to it, through the quicksilver by which it was confined, (see Air, n° 49.) and observed that white fumes were made in it, at the entrance of every bubble, for a considerable time. Having, at another time, saturated a small quantity of ether with this kind of air, and the vial which contained it happening to be overturned, the whole room was instantly filled with a white cloud, which had very much the smell of ether, but peculiarly offensive. Opening the door and window of the room, this light cloud filled a long passage, and another room. The ether, in the mean time, was seemingly all vanished; but, some time after, the surface of the quicksilver in which the experiment had been made, was covered with a very acid liquor, arising probably from the moisture in the atmosphere, attracted by the acid vapour with which the ether had been impregnated. This seems to show, that, however much disposed the marine acid may be to unite with phlogistic matters when in its aerial state, the attraction it has for them is but very slight, and still inferior to what it has for water.

Camphor was presently reduced into a fluid state by inhaling this acid air; but there seemed to be something of a whitish sediment in it. After continuing two days in this situation, water was admitted to it, upon which the camphor immediately resumed its former solid state; and to appearance was the same substance that it had been before.

Strong concentrated oil of vitriol, being put to marine acid air, was not at all affected by it in a day and a night. In order to try whether it would not have more power in a condensed state, it was compressed with an additional atmosphere; but, on taking off this, the air expanded again, and was not in the least diminished. A quantity of strong spirit of nitre was also put to it without any sensible effect. From these last experiments it appears, that the marine acid is not able to dilodge the other acids from their union with water.

IV. Of the Fluor Acid.

The discovery of this curious acid we owe to Mr Scheele, a Swedish chemist, from whom it is often distinguished by the name of the Swedish acid. Mr Scheele was of opinion, that this acid is one of the component parts of a sparry substance called fluor sparosus. This substance he reckons to be composed of a calcareous earth, and the particular species of acid obtained from it by distillation; and accordingly relates, that he produced the same kind of spar by adding this acid to lime-water. The most remarkable circumstance, however, attending this acid is, that when the vapour of it is mingled with water placed in the receiver for that purpose, a white spot is formed on the surface of the water, which, by degrees, spreads entirely over the surface of it. On agitating the receiver, this crust, being broken into several pieces, was thereby sunk to the bottom. On the contact of the succeeding vapours, a new crust was immediately formed, and the water soon became considerably acid. The white crust, which first appeared on the surface of the water in the receiver, and which afterwards sunk to the bottom, was found by him to possess all the properties of a real flex, or flinty substance. It could not be dissolved in any of the acids, nor would it form any paste with water. It dissolved in an alkaline lixivium; suffered no change from fire, when exposed to its single action; but, on the addition of an alkali, melted into glass. This glass, mixed with thrice its own quantity of vegetable fixed alkali, melted into a blue mass; which, being pounded, and put into a damp cellar, very soon ran per deliquitum, and turned into a gelatinous substance. An acid precipitated a powder from it; and lastly, it was dissolved in borax without the least effervescence.

The inference drawn by the author from these circumstances is, that this flex, or flinty crust, thus produced from the sparry fluor, is solely compounded of the acid of spar united with the particles of the water in the receiver. From some other processes he concludes, that the whole of this singular acid may be converted into flint by the addition of water; and that the water is a necessary ingredient in this compound body, he infers from other processes; in which it appears, that when the receiver contained alcohol, oil of olives, or oil of vitriol, no flinty crust was formed; and that it appeared only when there was water in the receiver.

His method of operating upon this substance was, glass cups to dilute it with oil of vitriol in glass vessels; and another very remarkable fact concerning it was, that all those vessels were so corroded, that holes were made through them. This occasioned some doubt with respect to the flinty crust formed on the water in the receiver, as it might reasonably enough be imagined that it proceeded from the particles of sand or flint originally existing in the glass, which the acid parted with on its meeting with the water. Mr Scheele, however, fell upon a way to obviate this objection, by exposing a piece of wet charcoal to the vapours of the acid, as they arose from the mixture of oil of vitriol and spar, and found the same flinty crust formed upon it as when the vapour was suffered to mingle directly with water in the receiver.

Mr Boulanger, who examined this acid with great care, concludes, that it is the acid of sea-salt, joined with Practice with an earthy substance, but does not pretend to decide this matter with certainty. Mr Scheele himself was of opinion, that the fluor acid was distinct from the marine and all other acids; because he found it dislodged from the spar by the nitrous and marine, as well as by the vitriolic acid: but those who repeated the experiments after him could procure no fluor acid, except by using the vitriolic.

Dr Priestley, who had exhibited other acids in the form of air, was very desirous of making similar experiments on this. Accordingly, having put some powdered spar into a vial, and poured oil of vitriol upon it, he filled it with a tube, and the other apparatus for receiving the air which should be expelled from it; (see Air, No. 49.) He observed, that when the fluor acid vapour issued out of the tube, and mingled with the external air, a permanent white cloud was formed; which he attributes to the attachment of the acid to the water contained in the air. The moment that water came into contact with this air, the surface of it became white and opake, by a stony film; which, forming a separation between the air above and the water below, considerably retarded the ascent of the water; till, the air infusing itself through the pores and cracks of the crust, the water necessarily rose as the air diminished; and, breaking the crust, presented a new surface to the air, which immediately was covered with another crust. Thus one stony incrustation was formed after another, till every particle of the air was united to the water, and the different films being collected and dried, formed a white powdery substance, generally a little acid to the taste; but when washed in much pure water, was perfectly infusible. The property of corroding glass he found to belong to this air only when pretty hot.

The Doctor is of opinion, that this acid is only the vitriolic, loaded with plenty of sparry crust, and volatilized by a little phlogiston. What he reckons an experiment sufficient to determine this matter, is, that having pressed out the stony matter with which the acid liquor in the receiver was impregnated, he found it to yield air which formed no crust on the surface of other water, but was imbibed by it in the same manner as the vitriolic acid air he had formerly made experiments on. The proof, however, would have been more convincing to chemists, had he formed a little vitriolated tartar, or Glauber's salt, by uniting it with a fixed alkali.

V. Of the Sal Sedativus, or Acid of Borax.

This is a saline substance of a very singular nature, and hitherto found nowhere but in borax itself. From this it is separable either by sublimation or crystallization. The method by sublimation, is that recommended by Homberg, who first discovered the sedative salt. His process consists in mixing green vitriol with borax, dissolving them in water, filtering the solution, and evaporating till a pellicle appears: the liquor is then to be put into a small glass alembic, and the sublimation promoted till only a dry matter remains in the cucurbit. During this operation, the liquor passes into the receiver; but the internal surface of the capital is covered with a saline matter forming very small, thin, laminated crystals, very shining, and very light. This is the sedative salt. The capital is then to be unloated, and the adhering salt swept off with a feather; the part of the liquor, which passed salt into the receiver, is to be poured on the dry matter in the cucurbit; and a new sublimation is to be promoted as before, by distilling till the matter in the cucurbit is dry. These operations are to be frequently repeated, in the same manner, till no more sedative salt can be obtained.

To obtain the sedative salt by crystallization, borax is to be dissolved in hot water; and to this solution any one of the three mineral acids is to be gradually added, by a little at a time, till the liquor be saturated, and even have an excess of acid, according to Mr Beaumé's process. The liquor is then to be left in a cold place; and a great number of small, shining, laminated crystals will be formed; these must be washed with a little very cold water, and drained upon brown paper. The sedative salt obtained by this process is somewhat denser than that obtained by sublimation; the latter being so light, that 72 grains are sufficient to fill a large vial.

Sedative salt, though thus capable of being once fixed in sublimed, is not, however, volatile: for it arises only the fire, by means of the water of its crystallization; and when it has once lost its water by drying, it cannot be raised into vapours by the most violent fire, but remains fixed, and melts into a vitreous matter, like borax itself. This glass is soluble in water, and then becomes sedative salt again. A great quantity of water is required to dissolve the sedative salt, and much more of cold than of boiling water; whence it is crystallizable by cold, as it also is by evaporation; a singular property, which scarce belongs to any other known salt.

This substance has not an acid, but a somewhat bitterish taste, accompanied with a slight impression of heat and coolness. It nevertheless unites with alkaline salts as acids do, and forms with them neutral salts. It is soluble in spirit of wine, to which it communicates the property of burning with a green flame. It makes no change on the blue colour of vegetables, as other acids do. It expels the other acids from their basis, when distilled with a strong heat; though these are all capable of expelling it in the cold, the acid of vinegar not excepted.

The composition of sedative salt is very much unknown, as no means sufficient for its decomposition have hitherto been found out. Mr Bourdelin, who made many experiments on this salt, found that it was unalterable by treatment with inflammable matters, with sulphur, with mineral acids disengaged, or united with metallic substances, and with spirit of wine. He could only perceive some marks of an inflammable matter, and a little marine acid. The former discovered itself by its communicating a sulphurous smell to the vitriolic acid employed; and the latter by a white precipitate formed, in a solution of mercury in the nitrous acid, by the liquor which came over on distilling the salt with powdered charcoal.

Mr Cadet, in the Memoirs of the Royal Academy of Sciences for 1766, has given an account of some experiments made by him on borax and its acid: from which he infers, (1.) That the acid contained in borax itself is the marine, and not sedative, salt. (2.) That it Practice it is the marine, he proves by having made a corrosive sublimate with this acid and mercurius precipitatus per ft. That sedative salt does not enter the composition of borax itself; he proves, by the impossibility of recomposing borax from uniting the sedative salt with fusible alkali. The salt so produced, he owns, is very like borax, but unfit for the purposes of soldering metals as borax is. He therefore thinks, that, in the decomposition of borax, the principles of the salt are somewhat changed, by the addition of that acid which extricates the sedative salt; and that this salt is composed of the marine acid originally existing in the borax, of the vitriolic acid employed in the operation, and of a vitreifiable earth. (If this is true, then sedative salt either cannot be procured by any other acid than the vitriolic, or it must have different properties according to the acid which procures it.) The vitreifiable earth, he says, is that which separates from borax during its solution in water, and which abounds more in the unrefined than refined borax, and which he thinks consists of a calx of copper, having obtained a regulus of copper from it. As he has never been able, however, to compose borax by the union of these ingredients, his experiments are by no means decisive.

Sedative Salt combined with Alkalis.

With the vegetable alkali this salt forms a compound very much resembling borax itself in quality; but in what respects it differs from, or how far it is applicable to the purposes of borax, hath not yet been determined.

With the mineral alkali, this salt has generally been thought to recombine borax; and, though Mr. Cader has denied this, yet as his experiments are hitherto imperfect and unsupported, we shall here give the history of that salt, as far as it is yet known.

This salt is prepared in the East Indies. It is said, that from certain hills in these countries there runs a green saline liquor, which is received in pits lined with clay, and suffered to evaporate with the sun's heat; that a bluish mud which the liquor brings along with it is frequently stirred up, and a bituminous matter, which floats upon the surface, taken off; that when the whole is reduced to a thick confluence, some melted fat is mixed, the matter covered with vegetable substances and a thin coat of clay; and that when the salt has crystallized, it is separated from the earth by a sieve. In the same countries is found native the mineral alkali in considerable quantity; sometimes tolerably pure, at other times blended with heterogeneous matters of various kinds. This alkali appears to exist in borax, as a Glauber's salt may be formed from a combination of borax with vitriolic acid.

Borax, when imported from the East Indies, consists of small, yellow, and glutinous crystals. It is refined, some say, by dissolving it in lime-water; others, in alkaline lixivium, or in a lixivium of caustic alkali; and, by others, in alum-water. Refined borax consists of large, eight-sided crystals, each of which is composed of small, soft, and bitterish scales. Crystals of this size can by no means be obtained by dissolving unrefined borax in common water. The crystals obtained in this way are extremely small, and differ considerably from the refined borax of the shops; inasmuch that Cramer calls the large crystals, not a purified, but an adulterated borax. When dissolved in lime-water, the borax flooats into larger crystals; and largest of all, when the vessel is covered, and a gentle warmth continued during the crystallization. During the dissolution, borax appears glutinous, and adheres in part to the bottom of the vessel. From this glutinous quality, peculiar to borax among the salts, it is used by dyers for giving a glost to silks.

All acids dissolve borax slowly, and without effervescence. It precipitates from them most, but not all, metallic substances; along with which a considerable part of the borax is generally deposited. It does not absorb the marine acid of luna cornea, or of mercury sublimate. It melts upon the surface of the first without uniting, and suffers the latter to rise unchanged; the borax in both cases becomes coloured; in the first, milky with red streaks; in the latter, amethyst or purple. Mixed with sal ammoniac, it extricates the volatile alkali, and retains the acid; but mixed with a combination of the marine acid with calcareous earths, it unites with the earth, and extricates the acid. It extricates the acid of nitre without seeming to unite with the alkaline basis of that salt; nor does it mingle in fusion with the common fixed alkaline salts, the borax flowing distinct upon their surface. A mixture of borax with twice its weight of tartar, dissolves in one fifth of the quantity of water that would be necessary to dissolve them separately: the liquor yields, on insipration, a viscid, tenacious mass like glue; which refuses to crystallize, and which deliquesces in the air. Borax affords likewise a glutinous compound with the other acids, except the vitriolic; whence this salt is generally preferred for making the sedative salt. It proves most glutinous with the vegetable, and least with the marine. With oils both expressed and distilled, it forms a milky, semi-saponaceous compound. It partially dissolves in spirit of wine. In conjunction with any acid, it tinges the flame of burning matters green; the precipitate thrown down by it from metallic solutions has this effect. It does not deflagrate with nitre. Fused with inflammable matters, it yields nothing sulphureous as those salts do which contain vitriolic acid. By repeatedly moistening it when considerably heated, it may be entirely sublimed.

Borax renders all earths and stones fusible by fire, and hence is used for the efflorescence of ores. It also facilitates the fusion of metals; and is particularly useful when small particles of metal, mixed with dirt and ashes, are to be melted together; as it promotes the fusion of the metal, and the vitrification of the other matters, by which the particles of metal may disengage themselves, and collect into one mass. It is further useful in the fusion of metals, as it defends their surfaces from the combined action of air and fire, by which imperfect metals are calcined. A principal use of borax is in the soldering of metals; which it probably does by accelerating the fusion of the surfaces of the metals to be joined, and by clearing them of any calx or other matter by which they might be prevented from being perfectly joined to one another. VI. Of the Acetous Acid and its Combinations.

This acid is plentifully obtained from all vinous liquors, by a fermentation of a particular kind, (see Fermentation, and Vinegar.) It appears first in the form of an acid liquor, more or less deeply coloured, as the vinegar is more or less pure. By distillation in a common copper-bill, with a pewter head and worm, this acid may be separated from many of its oily and impure parts. Distilled vinegar is a purer, but not a stronger acid, than the vinegar itself; for the acid is originally less volatile than water, though, by certain operations, it becomes more so. After vinegar has been distilled to about \( \frac{1}{4} \) of its original bulk, it is still very acid, but thick and black. This matter continues to yield, by distillation, a strong acid spirit, but tainted with an empyreumatic oil. If the distillation is still continued, a thick black oil continues to come over; and at last some volatile alkali, as in the distillation of animal substances. The caput mortuum left in the distilling vessel, being calcined in an open fire, and afterwards lixiviated, yields some fixed alkaline salt.

Acetous Acid combined with Alkaline Salts.

1. Vegetable Alkali. The produce of this combination is the terra foliata tartari, or sal diuretus of the shops; but to prepare this salt of a fine white flaky appearance, which is necessary for sale, is a matter of some difficulty. The best method of performing this operation is, after having saturated the alkali with the vinegar, which requires about 15 parts of common distilled vinegar to one of alkali, to evaporate the liquor to dryness; then melt the saline mass which remains with a gentle heat; after which it is to be dissolved in water, then filtered, and again evaporated to dryness. If it is now dissolved in spirit of wine, and the liquid abstracted by distillation, the remaining mass being melted a second time, will, on cooling, have the flaky appearance desired.

A good deal of caution is necessary in the first melting; for the acetous acid is easily dissipated, even when combined with fixed alkali, by fire. It is proper, therefore, that, when the salt is melted, a little should be occasionally taken out, and put into water; and, when it readily parts with its blackness to the water, must then be removed from the fire.

2. Fusible Alkali. This alkali combined with the acetous acid, forms a salt whose properties are not well known. Dr. Lewis affirms, that it is nearly similar to the terra foliata tartari. The author of the Chemical Dictionary, again, maintains it to be quite different; particularly that it crystallizes well, and is not deliquescent in the air; whereas the former cannot be crystallized; and even when obtained in a dry form, unless great care is taken to exclude the air, will perfectly deliquesce.

3. Volatile Alkali. This combination produces a salt to exceedingly deliquescent that it cannot be procured in a dry form without the greatest difficulty. In a liquid state, it is well known in medicine, as a sudorific, by the name of spiritus windeleri. It may, however, be procured in a dry form, by mixing equal parts of vitriolic sal ammoniac and terra foliata tartari, and subliming the mixture with a very gentle heat. When the salt is once procured, the utmost care is requisite to preserve it from the air.

Acetous Acid combined with Earths.

Combinations of this kind are but little known. Anomalous with the calcareous and argillaceous earths compounds salts of an astringent nature are formed. According to the author of the Chemical Dictionary, the salt resulting from a combination of vinegar with calcareous earth easily crystallizes, and does not deliquesce. With magnesia the acetous acid does not crystallize; but, when infusitated, forms a tough mass, of which two drachms, or two and a half, are a brisk purgative.

Acetous Acid combined with Metallic Bodies.

1. Copper. Upon this metal the acid of vinegar does not act briskly, until it is partly at least calcined. If the copper is previously dissolved in a mineral acid, and then precipitated, the calx will be readily diffused by the acetous acid. The solution is of a green colour, and beautiful green crystals may be obtained from it. The solution, however, is much more easily effected, by employing verdigris, which is copper already united with a kind of acetous or tartarous acid, and very readily dissolves in vinegar. The crystals obtained by this process are used in painting, under the name of distilled verdigris.

2. Iron. Vinegar acts very readily upon iron, and iron liquor dissolves it into a very brown, and almost black liquor, which does not easily crystallize, but, if infusitated, runs per deliquium. This liquor is employed in the printing of linens, calicoes, &c., being found to strike a finer black with madder, and to injure the cloth less, than solutions of iron in the other acids.

3. Lead. The acetous acid dissolves lead in its metallic state very sparingly; but if the metal is calcined, it acts upon it very strongly. Even after lead is melted into glass, the acetous acid will receive a strong impregnation from it; and hence it is dangerous to put vinegar into such earthen vessels as are glazed with lead. In the metallic state, only a drachm of lead can be dissolved in eight ounces of distilled vinegar.

If lead is exposed to the vapours of warm vinegar, it is corroded into a kind of calx, which is used in great quantities in painting, and is known by the name of cerus, or white lead. The preparation of this pigment has become a distinct trade, and is practised in some places of this kingdom where lead is procurable at the lowest price. The process for making cerus is thus given by the author of the Chemical Dictionary.

"To make cerus, leaden plates rolled spirally, so that the space of an inch shall be left between each circumvoluted, must be placed vertically in earthen pots of a proper size, containing some good vinegar. These leaden rolls ought to be so supported in the pots..." that they do not touch the vinegar, but that the acid vapour may circulate freely between the circumvolutions. The pots are to be covered, and placed in a bed of dung, or in a sand-bath, by which a gentle heat may be applied. The acid of vinegar being thus reduced into vapour, easily attaches itself to the surface of these plates, penetrates them, and is impregnated with the metal, which it reduces to a beautiful white powder called cerus. When a sufficient quantity of it is collected on the plates, the rolls are taken out of the pots, and unfolded; the cerus is then taken off, and they are again rolled up, that the operation may be repeated.

"In this operation, the acid being overcharged with lead, this metal is not properly in a false state; hence cerus is not in crystals, nor is soluble in water: but a false property would render it unfit for painting, in which it is chiefly employed."

Though this process may in general be just, yet there are certainly some particulars necessary to make cerus of a proper colour, which this author has omitted; for though we have carefully treated thin plates of lead in the manner he directs, yet the calx always turned out of a dirty grey colour. It is probable, therefore, that after the lead has been corroded by the steam of vinegar, it may be washed with water slightly impregnated with the vitriolic and nitrous acids.

This preparation is the only white hitherto found fit for painting in oil: but the discovery of another would be very desirable, not only from the faults of cerus as a paint, but also from its injuring the health of persons employed in its manufacture, by afflicting them with a fever colic; which lead, and all its preparations, frequently occasion.

If distilled vinegar is poured on white lead, it will dissolve it in much greater quantity than either the lead in its metallic form, or any of its calces. This solution, filtered and evaporated, shoots into small crystals, of an austerest twelfth taste, called sugar of lead. These are used in dyeing, and externally in medicines. They have been even given internally for spitting of blood. This they will very certainly cure; but, at the same time, they as certainly kill the patient by bringing on other diseases. If these crystals are repeatedly dissolved in fresh acids, and the solutions evaporated, an oily kind of substance will at last be obtained, which can scarcely be dried.

From all the metallic combinations of the acetous acid, it may be recovered in an exceedingly concentrated form, by simple distillation, sugar of lead only excepted. If this substance is distilled in a retort with a strong heat, it hath been said that an inflammable spirit, and not an acid, comes over; but this is denied by Dr Black.

4. Tin. The combination of acetous acid with tin is so little known, that many have doubted whether distilled vinegar is capable of dissolving tin or not.

Dr Lewis observes, "That plates of pure tin put into common vinegar begin in a few hours to be corroded, without the application of heat. By degrees a portion of the metal was taken up by the acid, but did not seem to be perfectly dissolved, the liquor appearing quite opaque and turbid, and depositing great part of the corroded tin to the bottom, in a whitish powder. A part of the tin, if not truly dissolved, is exquisitely divided in the liquor: for, after standing many days, and after passing through a filter, so much remained suspended as to give a whitishness and opacity to the fluid. Acid juices of fruits, introduced to the vinegar, exhibited the same phenomena. These experiments are not fully conclusive for the real solubility of tin in these acids, with regard to the purposes for which chemists have wanted such a solution; but they prove what is more important; that tin, or tinied vessels, however pure the tin be, will give a metallic impregnation to light vegetable acids suffered to stand in them for a few hours."

With regard to other metallic substances, neither the degree of attraction which the acetous acid has for them, nor the nature of the compounds formed by the union of it with such substances, are known; only, that as much of the reguline part of antimony is dissolved in this acid as to give it a violent emetic quality. See Regulus of Antimony.

Concentration of the Acetous Acid.

Common vinegar, as any other weak acid, may be advantageously concentrated by froth; as also may the vinegar spirit, or the distilled vinegar of the shops: but as the cold, in this country, is seldom or never so intense as to freeze vinegar, this method of concentration cannot be made use of here. If distilled vinegar be set in a water-bath, the most aqueous part will arise, and leave the more concentrated acid behind. This method, however, is tedious, and no great degree of concentration can be produced, even when the operation is carried to its utmost length. A much more concentrated acid may be obtained by distilling in a retort the crystals of copper, mentioned (no 278) under the name of distilled verdigris. A very strong acid may thus be obtained, which has a very pungent smell, almost as suffocating as volatile sulphurous acid. The count de Lauragnais discovered that this spirit, if heated in a wide-mouthed pan, would take fire on the contact of flaming substances, and burn entirely away, like spirit of wine, without any residuum. The same nobleman also observed, that this spirit, salt of vinegar when well concentrated, easily crystallizes without addition.

This may seem to be the most proper method of obtaining the acetous acid in its greatest degree of strength and purity; but as the process requires a very strong heat to be used towards the end of the operation, it is probable that part of the acetous acid may be by that means entirely decomposed. It would seem preferable, therefore, to decompose pure tartaric acid by means of the vitriolic acid, in the same manner as nitre or sea-salt are decomposed for obtaining their acids. In this case, indeed, the acetous acid might be a little mixed with the vitriolic; but that could easily be separated by a second distillation.

Dr Priestley, who gives us several experiments on the vegetable acid when reduced to the form of air, says experiments his being easily able to expel it from some extremely strong concentrated vinegar, by means of heat alone. This seems somewhat contrary to the count de Lauragnais's observation of the deposition of Practice the spirit of verdigris, as it is commonly called, to crystallize; but a still greater difference is, that the vegetable acid air extinguished a candle, when, according to the count's observation, it ought to have been inflammable. The most curious property observed by Dr Priestley is, that the vegetable acid air being imbibed by oil olive, the oil was rendered less viscid, and clearer, almost like an essential oil. This is an useful hint; and, if purified, might lead to important discoveries.

Acetous Acid combined with Inflammable Matter.

The only method yet known, of combining acetous acid with the principle of inflammability, is by mixing together equal parts of the strongly concentrated acid called spirit of verdigris, and spirit of wine. The result is, a new kind of ether, similar to the vitriolic, nitrous, and marine. This ether, however, retains some of the acidity and peculiar smell of the vinegar. By rectification with fixed alkali, it may be freed from this acidity, and then remains more like true ether, but still retaining something of the smell, not of the acid, but the inflammable part of the vinegar.

In this process a greater quantity of ether is obtained than by employing the vitriolic acid; which shows that the vegetable acid is essentially fitter to produce ether than the vitriolic. This difference must undoubtedly be attributed to the great quantity of ardent spirit which enters into the composition of the acetous acid, and perhaps already approaches the state of ether.

VII. Of the Acid of Tartar.

Tartar is a substance thrown off from wine, after it is put into casks to departure. The more tartar that is separated, the more smooth and palatable the wine is. This substance forms a thick hard crust on the sides of the casks; and, as part of the fine dregs of the wine adhere to it, the tartar of the white wines is of a greyish white colour, called white tartar; and that of red wine has a red colour, and is called red tartar.

When separated from the casks on which it is formed, tartar is mixed with much heterogeneous matter, from which, for the purposes of medicine and chemistry, it requires to be purified. This purification is performed at Montpelier; and consists first in boiling the tartar in water, filtrating the solution, and allowing the salt to crystallize, which it very soon does; as tartar requires nearly twenty times its weight of water to dissolve it.

The crystals of tartar obtained by this operation are far from being perfectly pure; and therefore they are again boiled in water, with an addition of clay, which absorbs the colouring matter; and thus, on a second crystallization, a very pure and white salt is obtained. The crystals now obtained are called cream, or crystals of tartar; and are commonly sold under these names.

To obtain the pure Acid of Tartar.

For a long time the cream or crystals of tartar were considered as the purest acid which could be obtained from this substance; but, in the year 1779, an analysis of tartar was published in the Swedish transactions, by Mr Scheele, a Swedish chemist. His method of decomposing the salt was, to dissolve it in a sufficient quantity of boiling water, then to add chalk in fine powder till the effervescence ceased. A copious precipitation ensued; and the remaining liquor being evaporated, afforded a soluble tartar. This proved, that cream of tartar is not, as was commonly supposed, an acid of a peculiar kind, joined with a great deal of earthy impurities; but really a compound salt, containing an alkali joined with an acid; and that the alkali produced from burnt tartar, is not generated in the fire, but pre-existent in the salt.

The whole sediment obtained in this experiment, is the calcareous earth combined with the acid of tartar, which may justly be called felenites tartaricus. (See n° 34). If some diluted vitriolic acid is poured upon this felenites tartaricus, the vitriolic acid expels the acid of tartar, forming a true felenite with the earth, while the liquor contains the pure acid of tartar. By infusion this acid may be made stronger; and even formed into small white crystals, which do not deliquesce in the air. A particular species of tartar extracted from forrel hath been sold for taking spots out of clothes, under the name of essential salt of lemon.

This experiment was repeated by Dr Black; who farther observed, that if quicklime was used instead of chalk, the whole acid would be absorbed by the lime; and the remaining liquor, instead of being a solution of soluble tartar, would be a caustic lixivium.

Acid of tartar combined with Alkalies.

1. Vegetable Alkali. If the pure acid of tartar be combined with this alkali to the point of saturation, a neutral salt is produced, which deliquesces in the air, and is not easily crystallized, unless the liquor be kept warm, and likewise be somewhat alkaline. This salt, called soluble tartar, is used in medicine as a purgative; but as its deliquescence does not admit of its being kept in a crystalline form, it is always sold in powder. Hence, those who prepare soluble tartar, take no further trouble than merely to rub one part of fixed alkaline salt with three of cream of tartar, which renders the compound sufficiently neutral, and answers all the purposes of medicine.

According to Mr Scheele, cream of tartar may be recomposed from the pure acid and alkali, in the following manner: "Upon fixed vegetable alkali pour a solution of the acid of tartar. Continue this till the effervescence is over; the fluid will then be transparent; but if more of the acid is added, it will become turbid, and white, and small crystals like white sand will be formed in it. These crystals are a perfect cream of tartar."

Upon these principles, another method of decomposing cream of tartar might be tried; namely, adding to it as much oil of vitriol as would saturate the alkali, then dissolving and crystallizing the salt; but, by this method, there would be danger of the acid being adulterated with vitriolated tartar.

2. Fossil Alkali. The salt produced from an union of cream of tartar with fossil alkali, has been long known under the names of Seignette's salt, sal Rupelensis, or Rochelle salt; but as the cream of tartar is now Practice now discovered to be not a pure acid, but adulterated with a portion of soluble tartar; possibly some differences might be observed if the pure acid was used.

This fact was first invented, and brought into vogue, by one Seignette, an apothecary at Rochelle, who kept the composition a secret as long as he could. Meff. Boulud and Geoffroy afterwards discovered and published its composition.

To prepare this salt, crystals of mineral alkali are to be dissolved in hot water, and powdered cream of tartar thrown in as long as any effervescence arises. For the better crystallization of the salt, the alkali ought to prevail. The liquor must then be filtered and evaporated, and very fine large crystals may be obtained by cold, each of which is the half of a polygonous prism cut in the direction of its axis. This section, which forms a face much larger than the rest, is, like them, a regular rectangle, distinguishable from the others, not only by its breadth, but also by two distinct diagonal lines which intersect each other in the middle.

3. Volatile Alkali. With regard to this combination, all we know as yet, is, that if the alkali is over saturated with acid, a cream of tartar, almost as difficult of solution as that of fixed alkali, will be obtained.

Acid of Tartar combined with Earths.

All that is as yet known concerning these combinations, is, that with the calcareous earth a compound not easily soluble in water is formed. The other properties of this substance, and the nature of combinations of tartarous acid with other earths, are entirely unknown.

Acid of Tartar combined with Metallic Substances.

1. Copper. In its metallic state, cream of tartar acts but weakly on this metal, but dissolves verdigris much more perfectly than distilled vinegar can. The solution with cream of tartar, being evaporated, does not crystallize, but runs into a gummy kind of matter; which, however, does not attract the moisture of the air. It readily dissolves in water, and makes a beautiful bluish green on paper, which has the property of always shining, as if covered with varnish. The effects of the pure acid on this metal have not yet been tried.

2. Iron. The effects of a combination of iron with the pure acid have not hitherto been tried. Cream of tartar dissolves this metal into a green liquor, which being evaporated runs per deliquium. It has been attempted to substitute a solution of this kind to the liquor used in printing calicoes formed of iron and four beer; but this gave a very dull brownish colour with madder. Possibly, if the pure acid was used, the colour might be improved. In medicine, a combination of cream of tartar with iron is used, and probably may be a useful chalybeate.

3. Regulus of Antimony. See Sect. III.

VIII. Of the Acid of Sugar.

That sugar contains an acid, which on distillation by a strong fire arises in a liquid form, in common with that of most other vegetable substances, has been generally known; but how to obtain this acid in a concrete form, and to appearance as pure and crystalizable as the acid of tartar, we were entirely ignorant, till the appearance of a treatise entitled, Differatio Chemica, de Acido Sacchari, auctore Johanne Azzelio Arvidsson, 4to, Upsalia.

Of the method of procuring, and the properties of, this new acid, we have the following account in the Edinburgh Medical Commentaries, vol. iv.

"1. To an ounce of the finest white sugar in powder, in a retort with a neck, add three ounces of strong spirit of nitre.

"2. The solution being finished, and the phlogiston of the spirit of nitre mostly exhaled, let a receiver be properly fitted to the retort and luted, and the liquor then made to boil gently.

"3. When the solution has obtained a brownish colour, add three ounces more of spirit of nitre, and let the ebullition be continued till the fumes of the acid are almost gone.

"4. The liquor being at length emptied into a larger vessel, and exposed to a proper degree of cold, quadrangular prismatic crystals are observed to form; which being collected, and dried on soft paper, are found to weigh about 100 grains.

"5. The remaining liquor being again boiled in the same retort, with two ounces of fresh spirit of nitre, till the red vapours begin to disappear; and being then in the same manner exposed to crystallize, about 43 grains of saline spiculae are obtained.

"6. To the liquid that still remains, about two ounces more of spirit of nitre being added, and afterwards the whole being, both by boiling and evaporation, reduced to a dry mass, a brown, saline, gelatinous kind of substance is produced, which, when thoroughly dry, is found to weigh about half a drachm.

"In the same manner, a similar acid, we are told, may be obtained from different saccharine substances, as gum-arabic, honey, &c.; but from none in such quantities, or so pure, as from fine sugar."

This salt possesses some very singular properties, of which what appears to us the most remarkable, and which we cannot help reading with some degree of doubt, is, that it produces an effervescence on being added to such alkaline, earthy, or metallic substances, as contain the vitriolic acid. From this we should be apt to think, that this acid was capable of dislodging even the vitriolic acid from its basis.

Acid of sugar, being distilled in a retort, gives over about 1/3 of its weight of water. By an intense heat it melts, and is partly sublimed; leaving in the retort a dark grey mass, of about the fifth part of the weight of the crystals made use of. The sublimed salt easily recovers its crystalline form, and seems to have undergone no further change by sublimation than being rendered more pure. During the distillation a great quantity of elastic vapour rushes out, (about 100 cubic inches from half an ounce of the crystals), which, from the distilled liquor's precipitating lime-water, we may judge to be fixed air. In a second sublimation, white fumes are seen over, which, when cold, appear to be an acid, glaify-coloured liquor, but cannot be again crystallized.

"Such parts of the salts as adhere to the sides and necks of the vessels, do not appear to be in the least changed in the process." (What these parts are, we do not comprehend). On a third sublimation, these parts This singular salt has a considerable acid power; twenty grains of it giving a very considerable degree of acidity to a large tankard of water. It dissolves in an equal weight of distilled water, but concretes on the liquor's growing cool. It is also soluble in spirit of wine; 100 parts of boiling spirit of wine dissolving 56 of the saccharine crystals, but no more than 40 when cold. The solution in spirit of wine soon becomes turbid; and deposits a mucous sediment, in quantity about 1/8 of the acid made use of. When cold, irregular fealy crystals are formed, which when dry are perfectly white.

With vegetable alkali, the acid of sugar can scarcely be formed into crystals, unless either the alkali or acid predominate. With mineral alkali, a salt very difficult of solution is formed. The quantity of volatile alkali saturated by this acid is incredible. "Six parts of a pure volatile alkali (we suppose volatile fats are meant) may be saturated with one of the acid of sugar!" The produce is a quadrangular prismatic salt. With lime this acid unites so strongly, as to be separable by no other means than a strong heat. What kind of a salt results from this combination we are not told; but the author is of opinion, that this shows the use of lime in the purification of sugar, in order to absorb the superfluous acid. Being saturated with some of the terra ponderosa, the acid of sugar immediately deposits a quantity of pellucid angular crystals, scarcely soluble in water. With magnesia the salt appears in form of a white powder, soluble neither in water nor spirit of wine, unless the acid prevails. It has a stronger affinity with magnesia, than any of the alkaline salts. With earth of alum, no crystals are obtained; but a yellow pellucid mass, of a sweetish and somewhat astringent taste; which, in a moist air, liquefies, and increases two-thirds in weight.

This acid acts upon all metals, gold, silver, platinum, and quicksilver, not excepted, if they have been previously dissolved in an acid, and then precipitated. Iron in its metallic state is dissolved in very large quantity by the saccharine acid; 45 parts of iron being soluble in 55 of acid. By evaporation, the liquor flows into yellow prismatic crystals, which are easily soluble in water. With cobalt, a quantity of yellow-coloured crystals are obtained, which being dissolved in water, and tea-salt added to the solution, form a sympathetic ink. The elective attractions of this singular acid are, first, Lime; then the terra ponderosa, magnesia, vegetable alkali, mineral alkali, and lastly clays. With spirit of wine an ether was obtained, which cannot easily be set on fire unless previously heated, and burns with a blue instead of a white flame.

Towards the conclusion of his dissertation the author observes, that some may imagine that the acid of nitre, made use of in these experiments, may have a considerable share in the production of what he has termed acid of sugar. But, though he acknowledges that this acid cannot in any way be obtained but by the assistance of spirit of nitre, he is thoroughly convinced that it does not, in any degree, enter into its composition.

What occurs to us on this subject is, that if the acid really pre-exists in the sugar, it must give some tokens of its existence by mixing the sugar with other substances besides spirit of nitre. The author himself thinks that lime acts upon the acid part of the sugar: from whence we are apt to conclude, that by mixing lime, in a certain proportion, with sugar, a compound should be obtained somewhat similar to what was formed by a direct combination of lime with the pure acid. In this case, we might conclude that the nitrous acid produces this salt, by combining with the inflammable part of the sugar, becoming thereby volatile, and flying entirely off, so as to leave the acid of the sugar pure. In the distillation of deliquescent spirit of nitre, however, we have an influence of the nitrous acid itself being very much altered. This must therefore suggest a doubt, that the acid salt obtained in the present case is only the nitrous acid deprived of its phlogiston, and united with some earthy particles.

IX. Of the Acid of Phosphorus.

This acid as yet is but little known. It is obtained phosphorously in the greatest quantities from human urine, where it is combined with a volatile alkali, forming a singular kind of ammoniacal salts. It is there also found in combination with the vegetable fixed alkali; (see Sect. VI.) It may also be obtained from most vegetable substances, by distillation with a very violent heat; but in small quantity. When obtained in this manner, it combines with the phlogiston of the matter distilled, and affirms the form of phosphorus. As the only method of procuring this acid, without the trouble of a very tedious, and even dangerous, distillation, is by evaporation and crystallization, we shall here give the process for extracting the microcosmic salt from urine.

"A large quantity of urine is to be evaporated to Microcosm, the consistence of a thin syrup; which, being let in a nice salt, cold place, will yield, in three or four weeks, four how procure brown-coloured crystals, which are the microcosmic red salt, mixed with the marine, and other salts of urine. These crystals are to be dissolved in hot water; the solution filtered whilst it continues hot, and let to crystallize again; and the solution, filtration, and crystallization, repeated till the salt becomes pure and white. In all the crystallizations the microcosmic salt floats first, and is easily distinguished and separated from the others. If the urine which remains after the first crystallization be further evaporated, and again let in the cold, it will yield more crystals; but browner and more impure than the former; and therefore requiring to be purified by themselves. From twenty gallons of urine, may be obtained four ounces of pure salt; a considerable part being still left in the reticulum.

"In these operations the heat ought to be gentle, and the vessels either of glass or compact stone-ware. Urine being evaporated in a copper vessel, afforded only a green solution of that metal."

Concerning the nature of the microcosmic salt obtained by the above process, Mr. Margraaff gives the following account in the Berlin Memoirs for 1796.

"Sixteen ounces of the salt, distilled in a glass retort, in a heat gradually raised, gave over eight ounces of a volatile urinous spirit, resembling that made..." The vitreous matter dissolved in twice or thrice its quantity of water, into a clear, transparent, acid liquor, somewhat thick, not ill resembling in consistence concentrated oil of vitriol. This liquor totally corroded zinc into a white powder, which, being diluted with water, appeared in great part to dissolve, fixed alkalies occasioning a plentiful precipitation. It acted powerfully upon iron, with some effervescence; and changed the metal into a kind of muddy substance inclining to bluish, in part soluble in water like the preceding. It dissolved likewise a portion of regulus of antimony, and extracted a red tincture from cobalt. On lead and tin it had very little action. Copper it corroded but slightly. On bismuth, silver, and gold, it had no effect at all, either by strong digestion, or a boiling heat. Nor did the addition of a considerable portion of nitrous acid enable it to act upon gold.

"The vitreous salt in its dry form, melted with metallic bodies with a strong fire, acts upon them more powerfully. In each of the following experiments, two drachms of the salt were taken to two scruples of the metal reduced to finall parts. (1.) Gold communicated a purple colour to the vitreous salt; on weighing the metal, however, its diminution was not considerable. (2.) Silver lost four grains, or \( \frac{1}{3} \); and rendered the salt yellowish, and moderately opake. (3.) Copper lost only two grains, or \( \frac{1}{6} \), though the salt was tinged of a deep green colour. It seemed as if a portion of the salt had been retained by the metal, which, after the fusion, was found to be whiter and more brittle than before. (4.) During the fusion with iron, flashes like lightning were continually thrown out; a phosphorus being generated from the combination of the acid with the inflammable principle of the iron. Great part of the mixture rises up in froth; which, when cold, appears a vitreous scoria, covered on the surface with a kind of metallic film, which, on being rubbed, changes its green colour to a yellowish. The rest of the iron remains at the bottom of the crucible, half melted, half vitrified, and spongy. (5.) Tin lost 18 grains, or nearly one-half its weight, and rendered the salt whitish; the remaining metal being at the same time remarkably changed. It was all over leafy and brilliant, very brittle, internally like zinc. Laid on burning coals, it first began to melt, then burnt like zinc, or phosphorus. (6.) Lead lost 16 grains, and gave the same whitish colour to the scoria that tin does. The remaining lead was in like manner inflammable, but burnt less vehemently than the tin; from which it differed also in retaining its malleability. (7.) Mercury precipitated from aqua fortis, and well edulcorated, being treated with the salt in a glass retort, with a fire raised to the utmost, only 12 grains of mercury sublimed; 23 remaining united with the acid, in a whitish, semi-opaque mass. A solution of this, mixed in distilled water, deposited a quantity of a yellowish powder; which, by distillation in a glass retort, was in great part revived into running mercury. A part also remained dissolved in the clear liquor; for a drop let fall on polished copper instantly whitened it. (8.) Regulus of antimony melted with the vitreous salt, lost eight or nine grains, (about \( \frac{1}{2} \)); the regulus assumed a fine, brilliant, striated appearance; the scoria were somewhat opake. (9.) Bismuth lost eight grains; the scoria were like the preceding, but the bismuth itself suffered little change. (10.) Zinc, mixed with the salt, and distilled in a glass retort, yielded a true phosphorus, which arose in a very moderate heat. The residuum was of a grey colour, a little melted at the bottom, in weight not exceeding two drachms, so that two scruples had sublimed. This residuum, urged further in a small Hessian crucible, to perfect fusion, emitted an infinity of phosphorine flashes, with a kind of detonation. The matter, grown cold, looked like the scoria of melted glasfs. (11.) White arsenic, mixed with this salt, separated in the fire; greatest part of it subliming, and only so much remaining behind as increased the weight of the salt eight or nine grains. This compound appeared at first transparent; but, on being exposed to the air, became moist, and of an opake whiteness, much resembling crystalline arsenic. (12.) Cinabar totally sublimed; suffering no change itself, and occasioning none in the salt. Sulphur did the same. (13.) One part of the salt, mixed with ten of manganese, and melted in a close vessel, gave a semi-transparent mass, some parts of which were bluish. The crucible was lined with a fine purple glazing, and the edges of the mass itself appeared of the same colour.

"The vitreous salt distilled also, in fusion, metallic calces, and earths. Chalk, with one third its weight of the salt, formed a semi-transparent vitreous mass; calcined marble, with the same proportion, flowed so thin as to run all through the crucible; gypsum, likewise, ran mostly through the crucible; what remained was semi-transparent. Lapis specularis ran entirely through the vessel. Spanish chalk gave a semi-transparent mass, which sparkled on breaking; and fine white clay, a similar one. Saxon topaz and flint were changed into beautiful opal-coloured masses; the earth of alum into a semi-transparent mass, and quick lime into an opake white one. The mass with flints imbibed moisture from the air; the others not.

"Oil of vitriol, poured upon one fourth its weight of this salt in a retort, raised an effervescence, acquired a brownish colour, and afterwards became turbid and white. On raising the fire, the oil of vitriol distilled, and the matter in the bottom of the retort melted. In the neck was found a little sublimate, which grew moist in the air; as did likewise the remaining salt, which was opake and whitish. Concentrated spirit of nitre, distilled with this salt in the above proportion, came over unchanged; no sublimate appeared; the residuum looked like glasfs of borax. The distilled spirit did not act in the least upon gold, even by coction. Strong spirit of sea-salt being distilled in the same manner, no sensible change was made either in the spirit or the salt.

"Equal parts of the vitrified microcosmic salt and salt of tartar being urged with the strongest fire that a glass retort could bear, nothing sensible came over," Practice nor did the mixture appear in thin fusion. Dissolved in water, filtered, and duly evaporated, it afforded, very difficultly, oblong crystals, somewhat alkaline; the quantity of alkali having been more than enough to saturate the acid. A whitish matter remained on the filter, amounting to seven or eight grains, from two drachms of the mixture; this, after being washed and dried, melted before a blow-pipe, as did likewise the crystals.

"This salt seems to extricate, in part, the acids of vitriolated tartar, nitre, and sea-salt. (1.) On distilling a mixture of it with an equal quantity of vitriolated tartar, there came over some ponderous acid drops, which, saturated with fixed alkali, formed a neutral salt greatly resembling the vitriolated tartar. The residuum readily dissolved in water, and difficultly crystallized. (2.) Nitre, treated with the same proportion of the salt, began to emit red vapours. The residuum was of a peach-blossom colour, appeared to have melted less perfectly than the preceding, and dissolved more difficulty in water. The solution deposited a little earthy matter; and, on being slowly evaporated, shot into crystals, which did not deliquesce in the fire. (3.) Sea-salt, distilled in the same manner, manifestly parted with its acid; the residuum was whitish, readily dissolved in water, and afforded some cubical crystals. (4.) Sal ammoniac suffered no change. (5.) Borax, with an equal quantity of vitreous salt, run all through the crucibles.

Solutions of this salt precipitated the earthy part of lime-water, solutions of alum, of flint dissolved in fixed alkali, and the combination of marine acid with chalk or quicklime. The precipitate from this last liquor is tenacious like glue, and does not dissolve even in boiling water; exposed to a strong fire, it froths prodigiously, and at last melts into a thick scoria.

Solutions of this salt precipitate also fumery metallic solutions; as butter of antimony, solutions of silver, copper, lead, iron, mercury, and bitumen, in the nitrous acid; and of tin in aqua regis. The precipitate of iron from spirit of salt is a tenacious mass; that of silver from aqua fortis, sometimes a white powder, sometimes tenacious. Copper from aqua fortis is sometimes thrown down in form of a white powder, and sometimes in that of a green oil, according to the proportions and diluteness of the liquor. Silver is not precipitated at all by this acid from its solution in vinegar, nor gold from aqua regis.

An ounce of the vitreous salt, well mixed with half an ounce of foot, and committed to distillation, yielded a drachm of fine phosphorus. The black residuum, being eluated with boiling water, and the liquor passed through a filter, there remained upon the filter eight scruples of a black matter; and, on evaporating and crystallizing the liquor, about seven drachms were obtained of oblong crystals, which did not deliquesce in a moist air, but became powdery in a warm one. These crystals, treated afresh with inflammable matter, yielded no phosphorus. Before a blow-pipe they melted into a transparent globular mass, which, on cooling, became turbid and opake. Dissolved in water, they precipitated solutions of silver, mercury, copper, and of chalk; though they did not act upon the latter so powerfully, nor produce with it a gluey mass, as before they had been deprived of their phosphoric acid."

On this account of the phosphoric, or rather microcosmic, acid, which we have taken from Lewis's notes on Neuman's chemistry, we have only to observe, that the microcosmic salt either contains two distinct acids, one of which only is capable of being united with inflammable matter so as to produce phosphorus; or that the true phosphoric acid itself is capable of a kind of decomposition, by which it may be made incapable of uniting any more with phlogiston. It would seem likewise, that some substances are much more proper for producing phosphorus than others; and that those which contain the greatest quantity of inflammable matter are not the most proper. Thus, half an ounce of foot mixed with a whole ounce of acid, yielded only a drachm of phosphorus; while two drachms of acid, with no more than two scruples of zinc, gave two scruples of phosphorus; nor did the distillation seem to be finished.

X. Of the Acid of Ants.

The acid may be obtained from these insects either how procured by distillation, or simple infusion in water. From red twenty-four ounces of ants, Neuman obtained eleven ounces and an half of acid as strong as good vinegar, by distillation in balneo mariae. Of this acid, Mr Margraff gives the following account in the Berlin Memoirs for 1749.

"The acid of ants effervesces with alkaline salts, its proper both fixed and volatile. With volatile alkalies it forms a neutral liquor, which, like that composed of the same alkalies and vinegar, yields no concrete salt on distillation. With fixed alkalies it concretes, upon proper exhalation into oblong crystals, which deliquesce in the air. The crystals, or the saturated neutral liquor uncrystallized, on being distilled with a fire increased till the retort begun to melt, yielded a liquor scarce feebly acid, and afterwards a small quantity of an urinous and partly ammoniacal liquor. The remaining black matter, dissolved in distilled water, filtered and evaporated, shot into large crystals which did not deliquesce in the air, though they were in taff strongly alkaline, effervesced with acids, and had all the other properties by which fixed alkalies are distinguished.

"This acid dissolves, with great effervescence, coral, chalk, and quicklime; and concretes with them all into crystals which do deliquesce in the air.

"It does not precipitate silver, lead, or mercury, from the nitrous acid; nor quicklime from the marine. Hence it appears to have no analogy to the marine or vitriolic acids; the first of which constantly precipitates the metallic solutions, and the other the earthy.

"It does not act upon filings of silver; but (like vegetable acids), it totally dissolves, by the affluence of heat, the calx of silver precipitated from aqua fortis by salt of tartar.

"It does not dissolve calces of mercury, (as vegetable acids do); but revives them into running quicksilver.

"It acts very weakly upon filings of copper; but perfectly dissolves copper that has been calcined. The solution yields beautiful, compact, green crystals.

"It It dissolves iron-filings with violence; the solution, duly evaporated, floats into crystals more readily than that made in distilled vinegar. It scarcely acts at all upon filings of tin.

"It does not, according to Mr Margraff, corrode filings of lead; but dissolves, by the affluence of heat, the red calx of lead. The solution crystallizes into a saccharum saturni. In Mr Ray's philosophical letters, it is said, that "lead put into the acid spirit, or fair water, together with the animals themselves, makes a good saccharum saturni;" and that this saccharum, on being distilled, "will afford the same acid spirit again, which the saccharum saturni made with vinegar will not do, but returns an inflammable oil with water, but nothing that is acid: and saccharum saturni made with spirit of verdigrase doth the same in this respect with spirit of pilmires.

"It dissolves zinc with vehemence, and floats, upon due evaporation, into elegant crystals, not at all like those produced with distilled vinegar. On bismuth, or regulus of antimony, it has little effect, either when calcined, or in their metallic state."

XI. Of the Acid of Amber.

The nature of this acid is as yet but little known, and Mr Pott is the only chemist who seems to have examined it with accuracy. We shall therefore give an abstract of the principal observations and experiments he has made on this salt.

"Salt of amber requires a large quantity of water for its solution. In the first crystallization (being much impregnated with the oil, which rises from the amber along with it), it floats into spongy flakes, in colour resembling brown sugar-candy; the crystals which succeed prove darker and darker coloured. On repeating the separation, the crystals appear at top of a clear yellow, or whitish colour, in form of long needles or feathers; at bottom, darker, and more irregular, as are likewise the crystals which float afterwards. The crystals neither liquefy nor become powdery in the air: rubbed, they emit a pungent smell like that of radishes, especially if warmed a little; their taste is acid, not in the least corrosive, but with a kind of oily pungency.

"This salt, kept in the heat of boiling water, loses nothing of its weight, and suffers no alteration. In a great heat it melts like oil; after which a little oily acid arises, then oily fires appear in the lower part of the retort, and the salt sublimes into the neck, partly in the form of a dark yellow butter, and partly in that of feathers, a black coaly matter remaining at bottom; so that, by this process, a part of the salt is destroyed.

"Oil of turpentine has no action on this salt. Highly rectified spirit of wine gains from it a yellow colour in the cold; and, on the application of heat, dissolves a considerable quantity, but deposits great part of it on cooling. The salt thus deposited is somewhat whiter than before, but still continues sensibly yellow. The dulcified spirit of sal ammoniac dissolves it readily, without effervescence, into a yellow liquor; if the salt was foul, the solution proves of a red colour; on burning of the vinous spirit, a neutral liquor remains.

"A solution of salt of amber in water, saturated with a pure alkaline lixivium, yielded, on inspissation, a saline matter, which would not crystallize, and which, when effloresced by heat, deliquescent in the air, leaving a considerable proportion of an earthy, unctuous matter. Being again gently inspissated, it left a brownish felt, very soluble, weighing one half more than the salt of amber employed. This salt effervesced with the vitriolic and nitrous acids: the vapour, which exhaled, was not acid, but oily and sulphureous. On repeating the experiment, and fully saturating the alkali with the salt of amber, the neutral salt made no effervescence with these acids. This salt did not perfectly melt before a blow-pipe; continued in the fire for some time, it effervesced with aqua fortis. In distillation it yielded a bitter, oily, alkaline spirit, much resembling the spirit of tartar; and towards the end, an empyreumatic oil. The residuum elixiated, yielded the alkaline salt again of a brown colour.

"Salt of amber effervesces strongly with volatile alkalies; and, on saturation, forms with them an oily ammoniacal liquor, which, in distillation, totally arises in a fluid form, except that a small portion of a penetrating, oily, saline matter concretes towards the end.

"On distilling salt of amber with an equal quantity of common sal ammoniac, an acid marine spirit came over, of a strong finell, and a brown colour: afterwards, a little white sal ammoniac sublimed; at length arose suddenly a large quantity of a fuliginous or bituminous matter, leaving behind a small portion of a like shining black substance. The coaly matter was considerably more in quantity than the salt of amber employed. On treating it with nitre, red vapours arose, and the mixture detonated with violence. A mixture of it with borax, frothed and swelled up much more than borax by itself; and, on raising the fire, yielded only some oily drops; the acid being destroyed by this salt, as by fixed alkalies and quicklime.

"Spirit of sea-salt, poured upon one-fourth its weight of salt of amber, made scarce any solution in the marine acid: on the application of heat, nearly the whole coagulated into the consistence of a jelly. In distillation, the spirit of salt arose first; then almost the whole of the salt of amber, partly like firm butter, partly like long striated plumpous alum, very pure, and of a fine white colour, its oily matter being changed into a coal at the bottom. The salt, thus purified, makes no precipitation in the solution of silver, and consequently retains nothing of the marine acid; nor does it precipitate solution of quicklime made in spirit of salt, and consequently contains nothing vitriolic. If any of the mineral acids was contained in this salt, it could not here escape discovery; the oil, which in the rough salt is supposed to conceal the acid, being in this process separated.

"Aqua fortis being poured upon one-fourth its weight of salt of amber, extracted a yellowish colour from it in the cold, but dissolved little: on the application of heat, the whole dissolves into a clear liquor, without any coagulation: if the salt is very oily, the solution proves red. In distillation, greatest part arises in a liquid form, with only a very small quantity of concrete salt. The spirit does not act upon gold, but but dissolves silver, and quicksilver, as at first; a proof that it has received no marine acid from the salt of amber.

"Oil of vitriol being added to twice its weight of salt of amber diluted with a little water, a moderate fire elevated an acidulous liquor, which appeared to proceed from the salt of amber; for its making no change in solution of fixed sal ammoniac, shewed it not to be vitriolic. On continuing the distillation by a stronger fire, greatest part of the salt arises undestroyed, and the oil of vitriol along with it; a black, light, porous earth remaining.

"Equal parts of quicklime and salt of amber gave over in distillation only an acidulous phlegm; the residuum, elixated with water, yielded a solution of the lime in the acid of amber, resembling a solution of the same earth in vegetable acids, precipitable by alkaline salts, and by the vitriolic acid. Lime added to a watery solution of salt of amber, dissolves with some effervescence; after which, the whole congeals into the confluence of a jelly; this, diluted with water, proves similar to the foregoing solution.

"Solution of salt of amber makes no precipitation in solutions of silver or quicksilver. It dissolves zinc, as all acids do; fixed alkalies precipitate the zinc; the volatile do not; and when a sufficient quantity of the volatile has been added, the fixed make no precipitation. It acts exceeding slowly and difficulty upon copper; but corrodes calcined copper in a shorter time. It soon corrodes iron, by coction, into a crocus, and dissolves a part into a liquid form; the solution has little colour; but alkaline salts readily discover that it holds iron, by rendering it turbid and whitish, and throwing down a considerable quantity of a greenish calx."

XII. Of Fixed Alkaline Salts.

Of these there are two kinds; the vegetable, and mineral, (see no. 23.) The first is never found by itself, and but rarely in combination with any acid; but is always prepared from the ashes of burnt vegetables. The second is found native in some parts of the earth. It is likewise found, in very large quantities, combined with the marine acid, in the waters of the ocean, and in the bowels of the earth; thus forming the common alimentary salt. It is also produced from the ashes of certain sea-plants, and of the plant called kali; from whence both the mineral and vegetable alkalies have taken their name.

The vegetable alkali difficulty afflumes a crystalline form; nevertheless, it may be partially united with some acids in such a manner as to crystallize, and lose its property of deliquiating in the air, without, at the same time, ceasing to be an alkali. Of this we have an example in the acid of ants above mentioned. Something of the same kind we have observed in treating vegetable fixed alkali with spirit of wine. A gallon of pretty strong spirit of wine being drawn over from a pound of salt of tartar, a black unctuous liquor was left, which shot into crystals very much resembling vitriolated tartar, and which did not deliquiate in the air, but were nevertheless strongly alkaline.

The mineral alkali in its natural state always afflumes a crystalline form, somewhat resembling that of sal mirabile. It does not deliquiate in the air, nor does it seem to have so strong an attraction for water, even when in its most caustic state, as the vegetable alkali; hence mineral alkali is preferable to it in making soap, which is always of a firmer confluence with mineral than with vegetable alkali. If vegetable alkali is combined with spirit of salt, some change seems to be thereby induced upon it; as the salt produced by expelling the marine acid by means of the vitriolic, and then crystallizing the mats, crystallizes differently from vitriolated tartar. Whether the vegetable alkali might by this means be entirely converted into the mineral, deserves a further inquiry.

Both vegetable and mineral alkalies, when applied to the tongue, have a very sharp, pungent, and urinous taste; but the vegetable considerably more so than the mineral. They both unite with acids, and form different neutral salts with them; but the vegetable alkali seems to have rather a greater attraction for acids than the other; although this difference is not so great, as that a neutral salt, formed by the union of mineral alkali with any acid, can be decomposed by an addition of the vegetable alkali.

Both vegetable and mineral alkali appear to be composed of an exceedingly caustic salt united with a certain quantity of fixed air: (See Art. no. 10.) This may be increased so far, as to make the vegetable alkali assume a crystalline form and lose great part of its alkaline properties; but as the adhesion of great part of this air is very slight, it easily separates by a gentle heat. Some part, however, is obstinately retained; and the alkali cannot be deprived of it by the most violent calcination per se. The only method of depriving it entirely of its fixed air, is by mixing an alkaline solution with quicklime.

Fixed Alkalies combined with Sulphur.

The produce of this is the red fetid compound called hepatic sulphuris, or liver of sulphur. It may be made by melting sulphur with a gentle heat, and stirring into it, while melted, four times its weight of dry alkaline salt. The whole readily melts and forms a red mass of a very fetid smell, and which deliquesces in the air. If sulphur is boiled in a solution of fixed alkaline salt, a like combination will take place.

In this process, when the hepar is made either in the dry or the moist way, the fixed air of the alkali is discharged, according to Dr Priestley's observation. Neither does a fixed alkali, when combined with fixed air, seem capable of uniting with sulphur; nor will the union be accomplished without heat, unless the alkali is already in a caustic state. Hence a cold solution of hepatic sulphur may be decomposed, partly at least, by fixed air. On adding an acid, however, the decomposition takes place much more rapidly; and the sulphur is precipitated to the bottom, in form of a white powder.

During the precipitation of the sulphur from an alkali, by means of acids, a thick white smoke arises, of a most fetid smell, and suffocating nature; this smoke seems to approach more nearly to the nature of pure phlogiston, than any substance that hath been hitherto observed. It burns quietly, without explosion, on a candle's being held in it. Calces of silver, lead, iron, or bismuth, are rendered black by it. Hence, if any thing... thing is wrote with a solution of lead, and a solution of hepar sulphuris is palled over it when dry, the writing, formerly invisible, will immediately appear of a blackish brown colour. Silver, in its metallic state, is prodigiously blackened either by the contact of this vapour, or by being immersed in a solution of the hepar sulphuris itself. Litharge is instantly restored to its metallic state, on being immersed even in a cold solution of hepar sulphuris.

By being united with an alkali, the acid of sulphur seems very much disposed to quit the phlogiston. If a solution of hepar sulphur is exposed to the air for some time, it is spontaneously decomposed; the phlogiston of the sulphur flying off, and the acid remaining united with the alkali into a vitriolated tartar. This decomposition takes place so remarkably, when liver of sulphur is dissolved in water, that, by a single evaporation to dryness, it will be almost totally changed into vitriolated tartar. If this substance, in a dry state, be exposed to a moderate degree of heat, and the mass kept constantly stirring, a like decomposition will follow; the phlogiston of the sulphur will fly off, and the acid unite with the alkali.

Liver of sulphur is a great solvent of metallic matters; all of which, except zinc, it attacks, particularly in fusion. It seems to dissolve gold more effectually than other metals; (see Sect. III.) This compound also dissolves vegetable coals, even by the humid way; and these solutions, if suffered to stand in the open air, always precipitate a black powder, no other than the coal they had dissolved, in proportion to the quantity of hepar sulphuris decomposed. When vegetable coal is thus dissolved by liver of sulphur in fusion, it is of a much deeper red than in its natural state. The solution in water is of a green colour.

Fixed Alkalies combined with Expressed Oils.

The result of this combination is soap; for the preparation of which in large quantities in the way of trade, see Soap. The soap which is used in medicine, is prepared without heat, in the following manner, according to the author of the Chemical Dictionary.

"One part of quicklime, and two parts of good Spanish soda" (the salt prepared from the ashes of the herb kali), "are boiled together during a short time in an iron caldron. This lixivium is to be filtered, and evaporated by heat, till a vial, capable of containing an ounce of water, shall contain an ounce and 216 grains of this lixivium. One part of this lixivium is to be mixed with two parts of oil of olives, or of sweet almonds, in a glass or stone ware vessel. The mixture soon becomes thick and white; and must be stirred from time to time with an iron spatula. The combination is gradually completed, and in seven or eight days a very white and firm soap is obtained."

In attempting combinations of this kind, it is absolutely necessary that the alkali be deprived of its fixed air as much as possible; otherwise the soap will be quite unctuous and soft; for fixed alkalies have a greater attraction for fixed air than for oil, and hence soap is decompounded by blowing fixed air into a solution of it in water.

Fixed Alkalies combined with Essential Oils.

The volatility of these oils in a great measure hinders them from being acted upon by alkalies; nevertheless combinations of this kind have been attempted; and the compounds so produced have been called Starkey's soap, from one Starkey a chemist who endeavoured to volatilize salt of tartar by combining it with oil of turpentine. His method was, to put dry salt of tartar into a matras, and pour upon it essential oil of turpentine to the height of two or three fingers breadth. In five or six months, a part of the alkali and oil were combined into a white saponaceous compound. This must be separated from the mixture, and more of it will afterwards be formed by the same method.

Chemists, imagining this soap to be possessed of considerable medical virtues, have endeavoured by various methods to shorten this tedious process. Of these one of the most expeditious is that recommended by Mr Beaumé; which consists in triturating, for a long time, alkaline salt upon a porphyry, and adding oil of turpentine during the trituration. According to him, the thick resinous part of the oil only can combine with the salt; and, during the time this combination is effected, the more subtile and attenuated parts will fly off. Hence he finds, that the operation is considerably abridged by the addition of a little turpentine, or common soap. The most expeditious of all, however, is that mentioned by Dr Lewis; which consists in heating the alkali red hot, and then throwing it into oil of turpentine, stirring them well together; on which they immediately unite into a saponaceous mass.

This kind of soap is subject to great alterations from keeping; particularly the loss of its colour, and a kind of decomposition occasioned by the extraction of an acid from the oil of turpentine, which unites with the alkali, and crystallizes not only all over the surface, but in the very substance of the soap. The nature of this salt is unknown, but certainly deserves consideration.

Fixed Alkalies combined with Phlogiston.

This combination is effected by calcining them with the charcoal either of vegetable or animal matters. The consequence is, that they are greatly altered in their properties; sometimes too much, as to be unable to precipitate calcareous earths from their solutions in acids. Metallic solutions precipitated by them in this state, assume different colours. (See Sect. III. Iron.)

Differences observed between Fixed Alkalies obtained from different Vegetables.

These differences we must conceive to arise from some proportion of the oily and phlogistic matter of the vegetable remaining in the ashes from whence the salts are extracted; for when reduced to their utmost purity, by repeated calcinations in a strong fire, and deliquiations in the air, all of them, the marine alkali excepted, appear to be the very same.

On this subject Mr Gmelin has given a great number of experiments in the fifth volume of the Commentaria Petropolitana; and found very considerable differences, not only between the alkaline salts, but likewise... likewise the pure vegetable earths obtained from different vegetables by burning. (See Sect. II. Vegetable Earth.) The salts of the fever plants examined were prepared with great care, and all of them exactly in the same manner; each vegetable being burnt in a separate crucible, with the same degree of fire, till no remains of coaly matter could any longer be perceived; and the ashes eluted in glass vessels with cold distilled water. The salts, thus obtained, were found to produce different colours, on mixture with certain liquors; and to effervesce in very different degrees with acids: certain metallic solutions were by some precipitated, by others only rendered thicker, by others both precipitated and rendered thick; whilst some occasioned neither the one nor the other of these changes, but left the fluid clear and transparent. Thus, with the vitriolic acid, the salts of southernwood and sage struck a pale brown colour; those of pine-tops and rue, a yellow; that of fern, a reddish yellow; and that of fanicle, a dark leek-green: that of dill yielded a leek-green precipitate, with elegant green flakes floating in the liquor. This last salt also gave a greenish precipitate with the marine acid, and a red one with the nitrous. Solution of corrosive sublimate was changed yellow by salt of southernwood; of a brownish colour, by that of colt's-foot; of a deep red, by that of wormwood; and of a pitch colour, by that of dill. That of fern threw down an opal-colour; of sage, a sulphur-yellow; of elder flowers, a citron-yellow; of fanicle, a saffron-colour; and of milfoil a deep-red precipitate. From solution of silver, salt of cardus benedictus threw down a white; of camomile, a grey; of hyssop, a brownish; of dill, a blackish brown; of scabious, a yellowish; and that of pine-tree tops, a sulphur-yellow precipitate. Solution of vitriol of copper was changed by salt of southernwood to a bright sea-green, by that of dill to an unflightly-green, of agrimony to a greenish-blue, and by that of milfoil to a bright sky-blue: the salt of penny-royal made the liquor thick as well as blue, and that of feverfew made it thick and green: the salt of hyssop threw down a green precipitate, that of fever-grass a blue one, and that of fumitory a greenish-blue; whilst the salt of fern made scarcely any change either in the colour or consistency of the liquor.

XIII. Of Volatile Alkali.

This is a kind of salt obtained from all animal, some vegetable substances, and from root, by distillation with a strong heat; and from all vegetable substances by putrefaction. Though a volatile alkali is procurable from all putrid animal substances by distillation, yet the putrefactive process does not seem to prepare volatile alkali in all of these. Putrid urine, indeed, contains a great quantity of alkali ready formed, whence its use in scouring, &c., but the case is not so with putrid blood or flesh. These afford no alkali till after the phlegm has arisen; and this they would do, though they had not been putrefied.

Volatile alkali, when pure, appears of a snowy whiteness; has a very pungent smell, without any disagreeable empyreuma; is very easily evaporable, without leaving any residuum; effervesces with acids much more strongly than fixed alkali; and forms with them neutral compounds called ammoniacal salts, which we have already described, and which are different according to the nature of the acid made use of; for all volatile alkalis, when perfectly purified, appear to be the very same, without the smallest difference.

Like fixed alkalis, these salts contain a great quantity of fixed air, on which their solidity depends; and which may be increased as perfectly to neutralize, and deprive them of their peculiar taste and smell. When neutralized by fixed air, they have a very agreeable pungent taste, somewhat resembling that of weak fermenting liquors. When totally deprived of fixed air, by means of lime, they cannot be reduced to a solid form; but are dissipated in an invisible and exceedingly pungent vapour, called by Dr Priestley alkaline air. When volatile alkaline salt is dissolved in water, the solution is called a volatile alkaline spirit.

Distillation and Purification of Volatile Alkalis.

The materials most commonly used for preparing volatile alkalis are the solid parts of animals, as bones, horns, &c. These are to be put into an iron pot of the shape recommended for solution; (see n° 68). To this must be fitted a flat head having a hole in the middle, about two inches diameter. From this a tube of plate-iron must issue, which is to be bent in such a manner, that the extremity of it may enter an oil-jar, through an hole made in its upper part, and dip about half an inch under some water placed in the lower part. The mouth of the jar is to be fitted with a cover, luted on very exactly; and having a small hole, which may be occasionally flopped with a wooden peg. The junctures are to be all luted as close as possible, with a mixture of clay, sand, and some oil; and those which are not exposed to a burning heat, may be further secured by quicklime and the white of an egg, or by means of glue. A fire being now kindled, the air contained in the distilling vessel is first expelled, which is known by the bubbling of the water; and to this vent must be given by pulling out the wooden peg. A considerable quantity of phlegm will then come over, along with some volatile alkali, a great quantity of fixable air, and some oil. The alkali will unite with the water, and likewise some part of the fixed air, the oil swimming above. A great many incoercible vapours, however, will come over, to which vent must be given from time to time, by pulling out the peg. The distillation is to be continued till all is come over; which may be known by the cessation, or very slow bubbling of the water. The iron-pipe must then be separated from the cover of the distilling vessel, lest the liquid in the jar should return into it, on the air being condensed by its cooling. In the jar will be a volatile spirit, more or less strong according as there was less or more water put in, with an exceedingly fetid black oil floating upon it.

The rectification of the volatile alkali is most commodiously performed at once by combining it with an acid; and, as spirit of salt has the least affinity with inflammable matter, it is to be chosen for this purpose, in preference to the vitriolic, or nitrous. As the spirit is excessively oily, though already much weakened by the admixture of the water in the jar, if a very large quantity was not originally put in, an equal equal quantity of water may still be added, on drawing off the spirit. That as little may be lost as possible, the spirit should be received in a stone bottle; and the marine acid, likewise in a distilled state, added by little and little, till the effervescence ceases. The liquor, which is now an impure solution of sal ammoniac, is to be left for some time, that the oil may separate itself; it is then to be filtered, evaporated, and crystallized in a leaden vessel. If the crystals are not sufficiently pure at the first, they will easily become so on a second dissolution.

From sal ammoniac thus obtained pure, the vitriolic alkali may be extracted by distillation with chalk, alkaline salts, or quicklime. Alkaline salts act more briskly than chalk, and give a much stronger volatile alkali. The strength of this, however, we know, may be altered at pleasure, by adding to, or depriving it of, its natural quantity of fixed air. Hence, perhaps, the best method would be, to prepare volatile alkalies altogether in a fluid state, by means of quicklime; and then add fixed air to them, by means of an apparatus similar to that directed by Dr Priestley for impregnating water with fixed air.

Volatile Alkalis combined with Metals.

There are only three metals, viz. copper, iron, and lead, upon which, while in their metallic form, volatile alkalies are capable of acting. Copper-spirits are dissolved by volatile alkali, especially in their caustic state, into a liquor of a most admirable blue colour. It is remarkable, that this colour depends entirely upon the air having access to this solution: for if the bottle containing it is close stopp'd, the liquor becomes colourless; but, however, resumes its blue colour, on being exposed to the air. On evaporation, a blue saline mass is obtained, which, mixed with fats, or other inflammable matters, tinges their flame green, leaving a red calx of copper, soluble again in volatile spirits as at first. This saline substance has been received into the last edition of the Edinburgh Dispensatory, under the name of cuprum ammoniacale, as an antiepileptic.

The blue mixture of solution of copper in aqua fortis with volatile spirits, yields sapphire-coloured crystals, which dissolve in spirit of wine, and impart their colour to it. If, instead of crystallization, the liquor be totally evaporated, the remaining dry matter explodes, in a moderate heat, like aurum fulminans. This is given as a fact by Dr Lewis; but hath not succeeded upon trial by Dr Black.

On the other two metals, the action of volatile alkali is by no means so evident: it dissolves iron very slowly into a liquor, the nature of which is not known; and lead is corroded by it into a mucilaginous substance.

Volatile Alkalis combined with Inflammable Substances.

With expressed oils, the caustic volatile alkali unites into a soft unctuous mass, of a very white colour, imperfectly soluble in water; and which is soon decomposed spontaneously. Compositions of this kind are frequently used for removing pains, and sometimes with success. With essential oils, volatile alkalies may be united, either in their dry or liquid form, by means of distillation. The produce is called sal volatile olfactum; it is much more frequently used in a liquid practice than in a dry form. The general method of preparation is by distilling volatile alkali along with essential oils and spirit of wine, or the aromatic substances from whence the essential oils are drawn. These compositions are variable at pleasure; but certain forms are laid down in the dispensatories, with which it is expected that all the chemists should comply in the preparation of these medicines.

Eau de Luce.

This is the name given to an exceedingly volatile spirit, which some years ago was pretty much in vogue; and indeed seems very well calculated to answer all the purposes for which volatile alkalies can be useful. It was of a thick white colour, and smelled somewhat of oil of amber. A receipt appeared in Lewis's dispensatory for the preparation of this fluid, under the name of spiritus volatilis succinatus. The method there directed, however, did not succeed; because though the alkaline spirit is capable of keeping a small quantity of oil of amber suspended, the colour is greatly more dilute than that of genuine eau de luce. In the Chemical Dictionary we have the following receipt:

"Take four ounces of rectified spirit of wine, and in it dissolve 10 or 12 grains of white soap; filter this solution; then dissolve in it a drachm of rectified oil of amber, and filter again. Mix as much of this solution with the strongest volatile spirit of sal ammoniac, as will be sufficient, when thoroughly shook, to give it a beautiful milky appearance. If upon its surface be formed a cream, some more of the oily spirit must be added."

This receipt likewise seems insufficient. For the oil of amber does not dissolve in spirit of wine: neither is it probable that the small quantity of soap made use of could be of any service; for the soap would dissolve perfectly in the alkaline spirit, without suffering any decomposition. The only method which we have found to answer is the following. Take an ounce, or any quantity at pleasure, of ballamum Canadense; place it in a small china basin, in a pan of boiling water, and keep it there till a drop of it taken out appears of a resinous consistence when cold. Extract a tincture from this resin with good spirit of wine; and having impregnated your volatile spirit with oil of amber, lavender, or any other essential oil, drop in as much of the spiritous tincture as will give it the desired colour. If the volatile spirit is very strong, the eau de luce will be thick and white, like the cream of new milk; nor is it subject to turn brown with keeping.

Volatile Tincture of Sulphur.

This is a combination of the caustic volatile alkali, or spirit of sal ammoniac, with quicklime. It is usually directed to be made by grinding lime with the sulphur, and afterwards with the sal ammoniac, and distilling the whole in a retort; but the produce is by this method very small, and even the success uncertain. A preferable method seems to be, to impregnate the strongest caustic volatile spirit with the vapour which arises in the decomposition of hepar sulphuris by means of an acid, in the same manner as directed for impregnating water with fixed air. See Art. no. 49. This preparation has a most nauseous fetid smell, which spreads to a considerable distance; and the effluvia will blacken silver or copper, if barely placed in the neighbourhood of the untopped bottle. This property renders it capable of forming a curious kind of sympathetic ink; for if paper is wrote upon with a solution of saccharum saturni, the writing, which disappears when dry, will appear legible, and of a brownish black, by barely holding it near the mouth of the bottle containing volatile tincture of sulphur. The vapours of this tincture are so exceedingly penetrating, that it is said they will even penetrate through a wall, so as to make a writing with saccharum saturni appear legible on the other side; but this is much to be doubted.

XIV. Of the Phenomena resulting from different mixtures of the Acid, Neutral, and Alkaline Salts, already treated of.

1. If concentrated oil of vitriol is mixed with strong spirit of nitre, or spirit of salt, the weaker acid will become exceedingly volatile, and emit very elastic fumes; so that if a mixture of this kind is put into a close stoppered bottle, it will almost certainly burst it. The same effect follows upon mixing spirit of salt and spirit of nitre together. In this case, both acids become surprisingly volatile; and much of the liquor will be dissipated in fumes, if the mixture is suffered to stand for any considerable time. Such mixtures ought therefore to be made only at the time they are to be used.

2. If vitriolated tartar is dissolved in an equal quantity of strong spirit of nitre, by heating them together in a matrix, the stronger vitriolic acid will be displaced by the weaker nitrous one, and the liquor, on cooling, will float into crystals of nitre. The same thing happens also upon dissolving vitriolated tartar, or Glauber's salt, in spirit of salt. This observation we owe to Monf. Beaumé. It seems to strike directly against the commonly received opinion of an essential superiority of strength in the vitriolic acid over the nitrous and marine acids; and would intimate that one acid displaced another, not according to its quality but according to its quantity.

3. If vitriolated tartar, or Glauber's salt, is dissolved in water, and this solution mixed with another consisting of calcareous earth, silver, mercury, lead, or tin, dissolved in the nitrous or marine acids, the vitriolic acid will leave the fixed alkali with which it was combined, and, uniting with the calcareous earth or metal, fall with it to the bottom of the vessel. This decomposition takes place only when the vitriolic acid meets with such bodies as it cannot easily dissolve into a liquid, such as those we have just now mentioned: for though vitriolated tartar is mixed with a solution of iron, copper, &c., in the nitrous or marine acids, no decomposition takes place. The case is not altered, whatever acid is made use of; for the marine acid will effectually separate silver, mercury, or lead, from the vitriolic or nitrous acids. This, as well as the last observation, shews us, that the attractive power between acids and alkalies or metals, is exceedingly weakened by water.

4. According to Dr Lewis, if a solution of vitriolated tartar is dropped into lime-water, the acid will unite with the lime, and precipitate with it in an indissoluble selenite, the alkali remaining in the water in a pure and caustic state.

5. If green vitriol is mixed with any solution containing substances which cannot be dissolved into a liquid by the vitriolic acid, the vitriol will be immediately decomposed, and the liquor will become a solution of iron only. Thus, if green vitriol is mixed with a solution of saccharum saturni, the vitriolic acid immediately quits the iron for the lead, and falls to the bottom with the latter, leaving the vegetable acid of the saccharum saturni to combine with the iron.

6. If solution of tin in aqua regia is mixed with solution of saccharum saturni, the marine acid quits the tin for the lead contained in the saccharum; at the same time, the acetous acid, which was combined with the lead, is unable to dissolve the tin which was before kept fulminated by the marine acid. Hence, both the saccharum saturni, and solution of tin, are very effectually decomposed, and the mixture becomes entirely useless. Dyers and calico-printers ought to attend to this, who are very apt to mix these two solutions together; and no doubt many of the faults of colours dyed or printed in particular places, arise from injudicious mixtures of a similar kind. See Dyeing.

7. If mild volatile alkali, that is, such as remains in a concrete form, by being united with a large quantity of fixed air, is poured into a solution of chalk in the nitrous or marine acids, the earth will be precipitated, and a true sal ammoniac formed. If the whole is evaporated to dryness, and a considerable heat applied, the acid will again part with the alkali, and combine with the chalk. Thus, in the purification of volatile alkalies by means of spirit of salt, the same quantity of acid may be made to serve a number of times. This will not hold in volatile spirits prepared with quicklime.

8. If equal parts of sal ammoniac and corrosive sublimate mercury are mixed together and sublimed, they unite in such a manner as never to be separable from one another without decomposition. The compound is called sal alumbroth; which is said to be a very powerful solvent of metallic substances, gold itself not excepted. Its powers in this, or any other respect, are at present but little known. By repeated sublimations, it is said, this salt becomes entirely fluid, and refuses to arise in the strongest heat.

9. If vitriolic acid is poured upon any salt difficult of solution in water, it becomes then very easily fusible. By this means, vitriolated tartar, or cream of tartar, may be dissolved in a very small quantity of water.

Sect. II. Earths.

The general divisions and characters of these substances we have given, n° 33.; and most of their combinations with saline substances have been already mentioned. In this section, therefore, we have to take notice only of their various combinations with one another, with inflammable, or metallic substances, &c. As they do not, however, act upon one another till subjected to a vitrifying heat, the changes then induced upon them come more properly to be treated of under the article Glass. Upon metallic, and inflammable substances, (sulphur alone excepted), they have very very little effect; and therefore, what relates to these combinations shall be taken notice of in the following sections. We shall here confine ourselves to some remarkable alterations in the nature of particular earths by combination with certain substances, and to the phosphoric quality of others.

§ 1. Transmutation of Flints into an Earth soluble in acids.

This is effected by mixing powdered flints with alkaline salt, and melting the mixture by a strong fire. The melted mass deliquesces in the air, like alkaline salts; and if the flint is then precipitated, it becomes soluble in acids, which it entirely resists before.

In this process the alkali, by its union with the flint, is deprived of its fixed air, and becomes caustic. To this causticity its solvent power is owing; and therefore the flint may be precipitated from the alkali, not only by acids, but by any substance capable of furnishing fixed air; such as magnesia alba, or volatile alkali. The precipitate in both cases proves the same; but the nature of it hath not hitherto been determined. Some have conjectured that the vitriolic acid existed in the flint; in which case, the alkali made use of in this process, ought to be partly converted into vitriolated tar.

§ 2. Of Phosphoric Earths.

These are so called from their property of shining in the dark. The most celebrated and anciently known of this kind is that called the Bolognian stone, from Bologna, a city in Italy, near which it is found. The discovery, according to Lemery, was accidentally made by a flint-maker called Vincenzo Caffiaro, who used to make chemical experiments. This man, having been induced to think, from the great weight and lustre of these stones, that they contained silver, gathered some, and calcined them; when carrying them into a dark place, probably by accident, he observed them shining like hot coals.

Mr Margraff describes the Bolognian stone to be an heavy, soft, friable, and crystallized substance, incapable of effervescence with acids before calcination in contact with burning fuel. These properties seem to indicate this stone to be of a felsitic or gypseous nature.

When these stones are to be rendered phosphoric, such of them ought to be chosen as are the clearest, best crystallized, most friable and heavy; which exfoliate when broken, and which contain no heterogeneous parts. They are to be made red hot in a crucible; and reduced to a very fine powder in a glass mortar, or upon a porphyry. Being thus reduced to powder, they are to be formed into a paste with mucilage of gum tragacanth, and divided into thin cakes. These are to be dried with a heat which at last is to be made pretty considerable. An ordinary reverberating furnace is to be filled to three quarters of its height with charcoal, and the fire is to be kindled. Upon this charcoal, the flat surfaces of the cakes are to rest, and more charcoal to be placed above them, so as to fill the furnace. The furnace is then to be covered with its dome, the tube of which is to remain open; all the coal is to be consumed, and the furnace is to be left cool; the cakes are then to be cleaned from the ashes by blowing with bellows upon them. When they have been exposed during some minutes, to light, and afterwards carried to a dark place, they will seem to shine like hot coals; particularly if the person observing them has been sometime in the dark, or have shut his eyes, that the pupils may be sufficiently expanded. After this calcination through the coals, if the stones be exposed to a stronger calcination, during a full half hour, under a muffe, their phosphoric quality will be rendered stronger.

From attending to the qualities of this stone, and analysis of the requisites for making this phosphorus, we are naturally led to think, that the Bolognian phosphorus is nothing other than a composition of sulphur and quicklime. The stone itself, in its natural state, evidently contains vitriolic acid, from its not effervescing with acids of any kind. This acid cannot be expelled from earthly substances by almost any degree of fire, unless inflammable matter is admitted to it. In this case, part of the acid becomes fulphurous, and flies off; while part is converted into sulphur, and combines with the earth. In the above mentioned process, the inflammable matter is furnished by the coals in contact with which the cakes are calcined, and by the mucilage of gum tragacanth with which the cakes are made up. A true fulphur must therefore be formed by the union of this inflammable matter with the vitriolic acid contained in the stone; and part of this sulphur must remain united to the earth left in a calcareous state, by the dissipation, or conversion into sulphur, of its acid.

In the year 1736, a memoir was published by Mr Allard du Fay, wherein he affirms, that all calcareous stones, whether they contain vitriolic acid or not, are capable of becoming luminous by calcination; with this difference only, that the pure calcareous stones require a stronger, or more frequently repeated, calcination to convert them into phosphorus; whereas those which contain an acid, as felsites, gypsum, fairs, &c., become phosphoric by a lighter calcination. On the contrary, Mr Margraff affirms, that no other stones can be rendered phosphoric but those which are saturated with an acid; that purely calcareous stones, such as marble, chalk, limestone, flintstones, &c., cannot be rendered luminous, till saturated with an acid, previously to their calcination.

We have already taken notice, no 195, 225, that the compounds formed by uniting calcareous earths with the nitrous and marine acids become a kind of phosphorus; the former of which emits light in the dark, after having been exposed to the sun through the day; and the latter becomes luminous by being struck. Signor Beccaria found, that this phosphoric quality was capable of being given to almost all substances in nature, metals perhaps excepted. He found that this quality was widely diffused among animals, and that even his own hand and arm possessed it in a very considerable degree. In the year 1775, a treatise on this kind of phosphorus was published by B. Wilson, F. R. S. and Mr Wilson, member of the Royal Academy at Upsal. In this treatise he shews, that oyster-shells, by calcination, acquire the phosphoric quality in a very great degree, either. The first experiment made by our author was the pouring lime aqua fortis, previously impregnated with copper, on a quantity of calcined oyster-shells, so as to form them into a kind of paste; he put this paste into a crucible, which was kept in a pretty hot fire for about 40 minutes. Having taken out the mass, and waited till it was cool, he presented it to the external light. On bringing it back suddenly into the dark, he was surprised with the appearance of a variety of colours like those of the rainbow, but much more vivid. In consequence of this appearance of the primitive colours, he repeated the experiment in various ways, combining the calcined oyster-shells with different metals and metallic solutions, with the different acids, alkaline and neutral salts, as well as with sulphur, charcoal, and other inflammable substances; and by all these he produced phosphorus, which emitted variously coloured light.

What is more remarkable, he found that oyster-shells possessed the phosphoric quality in a surprising degree; and for this purpose nothing more was requisite than putting them into a good tea-coal fire, and keeping them there for some time. On scaling off the internal yellowish surface of each shell, they become excellent phosphorus, and exhibit the most vivid and beautiful colours. As we know that neither the vitriolic nor any other acid is contained in oyster-shells, we cannot as yet say anything satisfactory concerning the nature of this phosphorus.

§ 3. Of the Vegetable Earth.

This is produced from vegetables by burning; and, when perfectly pure, by lixiviating the ashes with water, to extract the fat; and then repeatedly calcining them, to burn out all the inflammable matter; and is perhaps the same, from whatever substance it was obtained: in this state, according to Dr Lewis, it is of the same nature with magnesia. In the state, however, in which this earth is procurable by simply burning the plant, and lixiviating the ashes, it is considerably different, according to the different plants from which it is obtained. The ashes of mugwort, small centaury, chervil, and dill, are of a brownish grey; goat's-beard and lungwort afford white ashes; those of fennel are whitish; those of Roman wormwood of a greenish grey; those of rue, agrimony, saxifrage, brown; those of tansy, of a dusky green; those of dodder, of a fine green; eyebright, southern-wood, common wormwood, and fleabane, afford them grey; scurvy-grass, of a whitish grey; hyssop, yarrow, and sowbane, of a dusky grey; melilot, and oak-leaves, as also plantain, colts-foot, pine-tops, and fenitory, of a dusky brown; penny-royal, of a pale brown, with some spots of white; elder-flowers, sage, and mother of thyme, afford yellow ashes; those of strawberry-leaves are of a pale brimstone colour; those of cat-mint, of a dusky red; of prunella, brick-coloured; of honey-fuckle, blue; of fern, blackish; and those of St John's-wort, feverfew, origanum, and pimpernel, are all of a deep black. The only use to which this kind of earth has yet been put, is that of glass-making and manure. (See Glass, and Agriculture.)

This metal is reckoned of all others the most perfect and indestructible. When in its greatest purity, it has very little elasticity, is not sonorous, its colour is yellow, it is exceedingly soft and flexible, and is more ductile than any other metal whatever. (See Gold Leaf, and Wire-Drawing.) Of all bodies it is the most ponderous; its gravity being to gravity that of water, according to Dr Lewis, as 19,280, or 19,290, to 1. For its fusion it requires a low degree of white heat, somewhat greater than that in which silver melts. Whilst fluid, it appears of a bluish green colour; when cold, its surface looks smooth, bright, and considerably concave: it seems to expand more in the act of fusion, and to shrink more in its return to solidity, than any of the other metals; whence the greater concavity of its surface. Before fusion it expands the least of all metals, except iron. By sudden cooling it becomes, as well as other metals, brittle; which effect has been erroneously attributed to the contact of fuel during fusion.

Gold amalgamates very readily with mercury, and mingles in fusion with all the metals. It is remarkably disposed to unite with iron; of which it dissolves many times its own weight, in a heat not much greater than that in which gold itself melts; the mixture is of a silver colour, very brittle, and hard. All the metals, except copper, debauch the colour of gold; and, if their quantity is nearly equal to that of the gold, almost entirely conceal it. All but copper and silver destroy its malleability; but none so remarkably as tin and lead; a most minute portion, even the vapour, of these metals, renders gold extremely brittle; though a small proportion of gold forms with them compounds sufficiently ductile; more so than either the lead or tin by themselves. When gold is struck during a certain time by a hammer, or when violently compressed, as by the wire-drawers, it becomes more hard, elastic, and less ductile; so that it is apt to be cracked and torn. Its ductility is, however, restored by the same means used with other metals, namely, heating it red hot, and letting it cool slowly. This is called annealing metals; and gold seems to be more affected by this operation than any other metal. The tenacity of the parts of gold is also very surprising; for a wire tensely of \( \frac{1}{16} \) of an inch in diameter will support a weight of 500 pounds.

Gold is unalterable by air, or water. It never contracts rust like other metals. The action of the fiercest furnace-fires occasions no alteration in it. Kunkel kept gold in a glass-house furnace for a month, and Boyle kept some exposed to a great heat for a still longer time, without the loss of a single grain. It is said, however, to be disipable in the focus of a large burning mirror.

Mr Boyle relates a very curious and extraordinary experiment, which he thought was sufficient to prove the total destructibility of gold. About an eighth part of a grain of powder, communicated by a stranger, was projected upon two drachms of fine gold in fusion, and the matter kept melted for a quarter of an hour. During the fusion, the matter looked like ordinary gold; except only once, that his afflatus observed it to look exactly of the colour of opal. When cold, it was of a dirty colour, and, as it were, overcast with a thin coat, almost like half-vitrified litharge: the bottom of the crucible was overlaid with a vitrified substance, partly yellow, and partly reddish brown; with a few small globules, more like impure silver than gold. The metal was brittle, internally like brass, or bell-metal; on the touchstone more like silver than gold: its specific gravity was to that of water only as 152 to 1. There was no abfolute loss of weight.

By cupellation, 60 grains of this mass yielded 53 grains of pure gold; with seven grains of a ponderous, fixed, dark-coloured sublimate.

We have already mentioned, that in certain circumstances gold is soluble in the nitrous and marine acids separately. It is, however, always soluble by the two united, but dissolves slowly even then. The most commodious method of obtaining this solution is, by putting the gold, either in leaves, or granulated, or cut into small thin pieces, into a proper quantity of aqua fortis; then adding, by degrees, some powdered sal ammoniac, till the whole of the gold is dissolved. By this means a much smaller quantity of the menstruum proves sufficient, than if the sal ammoniac was previously dissolved in the aqua fortis; the conflict, which each addition of the salt raises with the acid, greatly promoting the dissolution. Aqua fortis of moderate strength will, in this way, take up about one-third of its weight of gold; whereas an aqua regia, ready prepared from the same aqua fortis, will not take up above one-fifth its weight. Common salt answers better for the preparation of the aqua regia, than sal ammoniac.

This solution, like all other metallic ones, is corrosive. It gives a violet colour to the fingers, or to any animal matters. If the solution is evaporated and cooled, yellow transparent crystals will be formed; but, if the evaporation is carried too far, the acids with which the gold is combined may be driven from it, by heat alone; and the gold will be left in the state of a yellow powder, called calx of gold.

Gold may be precipitated from its solution by those substances which commonly precipitate metals, such as alkaline salts, and calcareous earths. It may also be precipitated in a fine purple powder, by tin, or its solution.

When fixed alkalis are made use of, the precipitate weighs about one-fourth more than the gold employed. With volatile alkalis also, if they are added in no greater proportion than is sufficient to saturate the acid, the quantity of precipitate proves nearly the same: but if volatile spirit is added in an over-proportion, it re-dissolves part of the gold which it had before precipitated, and the liquor becomes again considerably yellow. The whole of the precipitate, however, could not be re-dissolved, either by the mild or caustic alkali; nor did either of these spirits sensibly dissolve, or extract any tinge from precipitates of gold which had been thoroughly edulcorated with boiling water.

All the metallic bodies which dissolve in aqua regia, precipitate gold from it. Mercury and copper throw down the gold in its bright metallic form; the others, in that of a calx or powder, which has no metallic aspect. Vitriol of iron, though it precipitates separated gold, yet has no effect upon any other metal; hence from it affords an easy method of separating gold from all metals by other metals. The precipitation with tin succeeds certainly, only when the metal in sublimate is used, and the folation of gold largely diluted with water.

It is observable, that though the gold is precipitated from the diluted folation by tin, yet, if the whole is suffered to stand till the water has in a great measure exhaled, the gold is taken up afresh, and only a white calx of tin remains.

If gold is precipitated from its folation in aqua regia, by a volatile alkali, the precipitate will explode with a prodigious force and noise if too strongly heated. This preparation is called aurum fulminans. The reason of this explosion probably is, the sudden expansion of the fixed air contained in the calx, occasioned by the deflagration of a small quantity of nitrous ammoniacal salt produced during the precipitation.

The explosion of aurum fulminans is one of the most violent known in chemistry. The report is preceded by a flash, visible in the dark; and, during the explosion, the gold is revived into little granules, which may be caught by a proper apparatus. It is not necessary that fulminating gold should be touched by an ignited body, or made red hot, in order to make it explode. The heat requisite for this purpose, is intermediate between that of boiling water and the heat which makes metals of an obscure red colour. If a little of it is laid upon a smooth piece of metal, and then heated so as to explode, the vehement quickness and strength of the explosion will make a small hollow in the metal. From this it has been thought that fulminating gold directed its force only, or chiefly, downwards. This, however, seems not to be the case; but rather, that it acts equally in all directions. Friction, likewise, and even a friction that is not very considerable, is sufficient to make this substance explode; and these circumstances render fulminating gold very dangerous. The author of the Chemical Dictionary relates the following accident, to which he says he was witness: "A young man who worked in a laboratory, had put a drachm of fulminating gold into a bottle, and had neglected to wipe the inner surface of the neck of the bottle, to which some of the powder adhered. When he endeavoured to close the bottle, the turning of the glass stopper round, in order to make it fit more closely, occasioned such a friction, that heat enough was produced to make part of the powder explode. By this explosion the young man was thrown some steps behind, his face and hands were wounded by the fragments of the bottle, and his eyes were put out. Notwithstanding this violent explosion, the whole drachm of fulminating gold certainly was not exploded; for much of it was afterwards found scattered about the laboratory." Of this mischiefous quality the gold may easily be deprived, by boiling it in oil of vitriol, or mixing it with sulphur, and burning away the sulphur.

If gold is melted with an hepar sulphuris, composed of equal parts of sulphur and fixed alkaline salt, the metal readily unites with it into an uniform mass, capable of dissolution in water without any separation of its parts. The solution, besides a nauseous taste from the sulphur, has a peculiar penetrating bitterness, not discoverable in any other metallic solution made by the same means.

Though the compositions of sulphur and alkali seem to unite more intimately with gold than any other metal, their affinity with it is but slight; copper, or iron, added to the matter in fusion, dilute, and precipitate the gold. The metal thus recovered, and purified by the common processes, proves remarkably paler-coloured than at first. In an experiment related by Dr Brandt, in the Swedish Memoirs, the purified gold turned out nearly as pale as silver, without any diminution of weight.

Gold has been thought to be possessed of many extraordinary virtues as a medicine; which, however, are long ago determined to be only imaginary. It is not indeed very easy to prepare this metal in such a manner that it can be safely taken into the human body. The solution in aqua regia is poisonous; but if any essential oil is poured on this solution, the gold will be separated from the acid, and united to the essential oil, with which, however, it contracts no lasting union, but in a few hours separates in bright yellow phlegm to the sides of the glass. Vitriolic ether dissolves the gold more readily and perfectly than the common essential oils; and keeps it permanently suspended, the acid liquor underneath appearing colourless. The yellow ethereal solution poured off, and kept for some time in a glass flask with a cork, so that the spirit may slowly exhale, yields long, transparent, prismatic crystals, in shape like those of nitre, and yellow like topaz. What the nature of these crystals is, either as to medicinal effects, or other purposes, is as yet unknown.

Rectified spirit of wine mingles uniformly with the solution of gold made in acids; if the mixture is suffered to stand, for some days, in a glass slightly covered, the gold is by degrees revived, and arises in bright pellicles to the surface. Greasier inflammable matters, wine, vinegar, solutions of tartar, throw down the gold, in its metallic form, to the bottom. Gold is the only metal which is thus separable from its solution in acids by these substances; and hence gold may be purified by these means from all admixtures, and small proportions of it in liquors readily discovered.

When the colour of gold is by any means rendered pale, it may be recovered again by melting it with copper, and afterwards separating the copper; or by a mixture of verdigrisate and sal ammoniac with vitriol or nitre. The colour is also improved by fusion with nitre, injecting sal ammoniac upon it in the fusion, quenching it in urine, or boiling it in a solution of alum. When borax is used as a flux, it is customary to add a little nitre or sal ammoniac, to prevent its being made pale by the borax. Juncker reports, that by melting gold with four times its weight of copper, separating the copper by aqua fortis unpurified, then melting the gold with the same quantity of fresh copper, and repeating this process eight or nine times, the gold becomes at length of a deep red colour, which sustains the action of lead, antimony, and aqua fortis.

2. Silver.

This, next to gold, is the most perfect, fixed, and ductile of all the metals. Its specific gravity is to that of water nearly as 11 to 1. A single grain has been drawn into a wire three yards long, and flattened into a plate an inch broad. In common fires it suffers no diminution of its weight; and, kept in the vehement heat of a glass-house for a month, it loses no more than one sixty-fourth. In the focus of a large burning-glass, it fumes for a long while, then contracts a greyish ash on the surface, and at length is totally dissipated.

Silver is somewhat harder and more sonorous than gold, and is fusible with a less degree of heat. The tenacity of its parts also is nearly one half less than that of gold; a silver wire of \( \frac{1}{4} \) of an inch diameter being unable to bear more than 270 pounds.

Mercury unites very readily with silver-leaf, or with the calx of silver precipitated by copper; but does not touch the calces precipitated by alkaline salts. The vapours of sulphureous solutions stain silver yellow or black. Sulphur, melted with silver, debases its colour to a leaden hue, renders it more easily fusible than before, and makes it flow too thin as to be apt in a little time to penetrate the crucible; in a heat just below fusion, a part of the silver floats up, all over the surface, into capillary efflorescences. Aqua fortis does not act upon silver in this compound; but fixed alkaline salts will absorb the sulphur, and form a heap sulphuris, which, however, is capable of again dissolving the metal. If the sulphurated silver is mixed with mercury sublimate, and exposed to the fire, the mercury of the sublimate will unite with the sulphur, and carry it up in the form of cinnabar, whilst the marine acid of the sublimate unites with the silver, into a luna cornea, (see no 239), which remains at the bottom of the glass. Fire alone is sufficient, if continued for some time, to expel the sulphur from silver.

From the base metals, silver is purified by cupellation with lead. (See Refining.) It always retains, however, after that operation, some small portion of copper, sufficient to give a blue colour to volatile spirits, which has been erroneously thought to proceed from the silver itself. It is purified from this admixture by melting it twice or thrice with nitre and borax. The foams, on the first fusion, are commonly blue; on the second, green; and on the third, white, which is a mark of the purification being completed.

The most effectual means, however, of purifying silver, is by reviving it from luna cornea; because spirit of salt will not precipitate copper as it does silver. The silver may be recovered from luna cornea, by fusion with alkaline and inflammable fluxes; but, in these operations, some loss is always occasioned by the distillation of part of the volatile calx, before the alkali or metal can absorb its acid.

Mr Margraaff has discovered a method of recovering Mr Mar's silver with little or no loss; mercury assisted by grass's volatile salts, imbuing it by trituration without heat. One part of luna cornea, and two of volatile salt, are to be ground together in a glass-mortar, with so much water as will reduce them to the consistence of a thin paste, for a quarter of an hour or more; five parts of pure pure quicksilver are then to be added, with a little more water, and the trituration to be continued for some hours. A fine amalgam will thus be obtained; which is to be washed with fresh parcels of water, as long as any white powder separates. Nearly the whole of the silver is contained in the amalgam, and may be obtained perfectly pure by distilling off the mercury. The white powder holds a small proportion separable by gentle sublimation; the matter which sublimes is nearly similar to mercurius dulcis.

The colour of silver is debased by all the metals, and its malleability greatly injured by all but gold and copper. The English standard-silver contains one part of copper to twelve and one-third of pure silver. This metal discovers in some circumstances a great attraction for lead; though it does not retain any of that metal in cupellation. If a mixture of silver and copper be melted with lead in certain proportions, and the compound afterwards exposed to a moderate fire, the lead and silver will melt out together, bringing very little of the copper with them; by this means silver is often separated from copper in large works. The effect does not wholly depend upon the different fusibility of the metals; for if tin, which is still more fusible than lead, be treated in the same manner with a mixture of silver and copper, the three ingredients are found to attract one another so strongly as to come all into fusion together. Again, if silver be melted with iron, and lead added to the mixture, the silver will forsake the iron to unite with the lead, and the iron will float by itself on the surface.

Silver is purified and whitened externally by boiling in a solution of tartar and common salt. This is no other than an extraction of the cuprous particles from the surface of the silver, by the acid of the tartar actuated by the common salt.

3. Copper.

This is one of those metals, which, from their destructibility by fire, and contracting rust in the air, are called imperfect. Of these, however, it is the most perfect and indestructible. It is of a reddish colour when pure; easily tarnishes in moist air, and contracts a green rust. It is the most famous of all the metals, and the hardest and most elastic of all but iron. In some of its flates, copper is as difficultly extended under the hammer as iron, but always proves softer to the file; and is never found hard enough to strike a spark with flint or other stones; whence its use for chisels, hammers, hoops, &c., in the gunpowder works. When broke by often bending backwards and forwards, it appears internally of a dull red colour without any brightness, and of a fine granulated texture, resembling some kinds of earthen ware. It is considerably ductile, though less so than either gold or silver; and may be drawn into wire as fine as hair, or beaten into leaves almost as thin as those of silver. The tenacity of its parts is very considerable; for a copper wire of 1/4 of an inch diameter will support a weight of 29½ pounds without breaking. The specific gravity of this metal, according to Dr Lewis, is to that of water as 8.850 to 1.

Copper continues malleable when heated red; in which respect it agrees with iron; but is not, like iron, capable of being welded, or having two pieces joined into one. It requires for its fusion a stronger heat than either gold or silver, though less than that requisite to melt iron. When in fusion, it is remarkably impatient of moisture; the contact of a little water occasioning the melted copper to be thrown about with violence, to the great danger of the bystanders. It is, nevertheless, said to be granulated in the brass-works at Bristol, without explosion or danger, by letting it fall in little drops, into a large cistern of cold water covered with a brass-plate. In the middle of the plate is an aperture, in which is secured with Sturbridge clay a small vessel, whose capacity is not above a spoonful, perforated with a number of minute holes, through which the melted copper passes. A stream of cold water passes through the cistern. If suffered to grow hot, the copper falls liquid to the bottom, and runs into plates.

Copper, in fusion, appears of a bluish green colour, calcined, nearly like that of melted gold. Kept in fusion for a long time, it becomes gradually more and more brittle; but does not corrode considerably, nor lose much of its weight. It is much less destructible than any of the imperfect metals, being very difficultly sublimed even by lead or bismuth. It kept in a heat below fusion, it contracts on the surface thin powdery scales; which, being rubbed off, are succeeded by others, till the whole quantity of the metal is thus changed into a scoria or calx, of a dark reddish colour. This calx does not melt in the strongest furnace fires; but, in the focus of a large burning mirror, runs easily into a deep red, and almost opaque, glass. A flaming fire, and strong draught of air over the surface of the metal, greatly promote its calcination. The flame being tinged of a green, bluish, or rainbow colour, is a mark that the copper burns.

This metal is very readily soluble by almost all fusable substances; even common water, suffered to stand long in copper-vessels, extracts so much as to gain a coppery taste. It is observable, that water is much more impregnated with this taste, on being suffered to stand in the cold, than if boiled for a longer time in the vessel. The same thing happens in regard to the mild vegetable acids. The confectioners prepare the most acid syrups, even those of lemons and oranges, by boiling in clean copper-vessels, without the preparations receiving any ill taste from the metal; whereas, either the juices themselves, or the syrups made from them, if kept cold in copper-vessels, soon become impregnated with a disagreeable taste, and with the pernicious qualities of the copper.

By combination with vegetable acids, copper becomes in some respects remarkably altered. Verdigris, which is a combination of copper with a kind of acetic or tartarous acid, is partially soluble in distilled vinegar; the residuum, on being melted with borax and linseed oil, yields a brittle metallic substance, of a whitish colour, not unlike bell-metal. The copper also, when revived from the distilled verdigris, was found by Dr Lewis to be different from the metal before distillation; but neither of these changes have yet been sufficiently examined.

Copper, in its metallic state, is very difficultly amalgamated with mercury; but unites with it more easily than iron. Practice fily if divided by certain admixtures. If mercury and verdigrase be triturated together with common salt, vinegar, and water, the copper in the verdigrase will be imbued by the mercury, and form with it, as Boyle observes, a curious amalgam, at first so soft as to receive any impression, and which, on standing, becomes hard like brittle metals. Brass-leaf likewise gives its copper to mercury, the other ingredient of the brass separating in the form of powder.

Earlier methods of amalgamating copper are published by Dr Lewis in his notes on Wilton's chemistry, p. 432. His receipts are—“Dissolve some fine copper in aqua fortis: when the menstruum will take up no more of the metal, pour it into an iron mortar, and add six times the weight of the copper, of mercury, and a little common salt: grind the whole well together with an iron pestle; and, in a little time, the copper will be imbued by the mercury, and an amalgama formed, which may be rendered bright by washing it well with repeated additions of water.”

Another method. Take the muddy substance which is procured in the polishing of copper plates with a pumice stone, and grind it well with a suitable portion of mercury, a little common salt, and some vinegar, in an iron mortar, (a marble one will do, if you make use of an iron pestle,) till you perceive the mercury has taken up the copper.” The copper recovered from these amalgams retains its original colour, without any tendency to yellow. Even when brass is made use of for making the amalgam, the recovered metal is perfect red copper; the ingredient from which the brass received its yellowness being, as above observed, separated in the amalgamation.

Copper is the basis of several metals for mechanic uses; as brass, prince’s metal, bell-metal, bath-metal, white copper, &c. Brass is prepared from copper and calamine, with the addition of powdered charcoal, cemented together, and at last brought into fusion. The calamine is to be previously prepared by cleansing it from adhering earth, stone, or other matters; by roasting, or calcining it; and by grinding it into a fine powder. The length of time, and degree of heat, requisite for the calcination of the calamine, are different according to the qualities of that mineral. The calamine, thus calcined, cleansed, and ground, is to be mixed with about a third or fourth part of charcoal dust, or powdered pit-coal, as is done in some parts of England. The malleability of the basis is diminished by the use of pit-coal, which is therefore only employed for the preparation of the coarser kinds. To this composition of calamine and coal, some manufacturers add common salt, by which the process of making brass is said to be hastened. In Goflar, where the calamine adhering to the inside of the furnaces is used instead of the native calamine, a small quantity of alum is added, by which they pretend the colour of the brass is heightened. With this composition, and with thin plates or grains of copper, the crucibles are to be nearly filled. The proportion of the calamine to the copper varies according to the richness of the former, but is generally as three to two. The copper must be dispersed through the composition of calamine and coal; and the whole must be covered with more coal, till the crucibles are full. The crucibles, thus filled, are to be placed in a furnace sunk in the ground, the form of which is that of the frustum of a hollow cone. At the bottom of the furnace, or greater basis of the frustum, is a circular grate, or iron plate. This plate is covered with a coat of clay and horse-dung, to defend it from the action of the fire; and pierced with holes, through which the air maintaining the fire passes. The crucibles stand upon the circular plate, forming a circular row, with one in the middle. The fuel is placed betwixt the crucibles, and is thrown into the furnace at the upper part of it, or the lesser basis of the frustum. To this upper part or mouth of the furnace is fitted a cover made of bricks or clay, kept together with bars of iron, and pierced with holes. This cover serves as a register. When the heat is to be increased, the cover must be partly or entirely taken off, and a free draught is permitted to the external air, which passes along a vault underground to the ash-hole, through the holes in the circular grate or plate, betwixt the crucibles, and through the upper mouth, along with the smoke and flame, into an area where the workmen stand, which is covered with a large dome or chimney, through which the smoke and air ascend. When the heat is to be diminished, the mouth of the furnace is closed with the lid; through the holes of which the air, smoke, and flame pass. The crucibles are to be kept red-hot during eight or ten hours, and in some places much longer, even several days, according to the nature of the calamine. During this time, the zinc rises in vapour from the calamine, unites with the copper, and renders that metal considerably more fusible than it is by itself. To render the metal very fluid, that it may flow into one uniform mass at the bottom, the fire is to be increased a little before the crucibles are taken out, for pouring off the fluid metal into molds. From 60 pounds of good calamine, and 40 of copper, 60 pounds of brass may be obtained, notwithstanding a considerable quantity of the zinc is dissipated in the operation. The quantity of brass obtained has been considerably augmented since the introduction of the method now commonly practised, of granulating the copper; by which means a larger surface of this metal is exposed to the vapour of zinc, and consequently less of that vapour escapes. To make the finer and more malleable kinds of brass, besides the choice of pure calamine and pure copper, some manufacturers cement the brass a second time with calamine and charcoal; and sometimes add to it old brass, by which the new is said to be meliorated.

Brass is brittle when hot; but so ductile when cold, that it may be drawn into very fine wire, and bent into very thin leaves. Its beautiful colour, malleability, and its fusibility, by which it may be easily cast into moulds, together with its being less liable to rust than copper, render it fit for the fabrication of many utensils.

Although zinc be fixed to a certain degree in brass, by the adhesion which it contracts with the copper; yet when brass is melted, and exposed to a violent fire, during a certain time, the zinc dissipates in vapours, and even flames away, if the heat be strong enough; and if the fire is long enough continued, all the zinc will be evaporated and destroyed, so that what remains is copper. Prince's metal is made by melting zinc in substance with copper; and all the yellow compound metals prepared in imitation of gold, are no other than mixtures of copper with different proportions of that ferri-metal, taken either in its pure state, or in its natural ore calamine, with an addition sometimes of iron-flings, &c. Zinc itself unites most easily with the copper; but calamine makes the most ductile compound, and gives the yellowest colour. Dr Lewis observes, that a little of the calamine renders the copper pale; that when it has imbibed about half its own weight, the colour inclines to yellow; that the yellowness increases more and more, till the proportion comes to almost one half; that on further augmenting the calamine, the compound becomes paler and paler, and at last white. The crucibles, in which the fusion is performed in large works, are commonly tinged by the matter of a deep blue colour.

Bell-metal is a mixture of copper and tin; though both these metals singly are malleable, the compound proves extremely brittle. Copper is dissolved by melted tin easily and intimately, far more so than by lead. A small portion of tin renders this metal dull-coloured, hard, and brittle. Bell-metal is composed of about ten parts of copper to one of tin, with the addition commonly of a little brass or zinc. A small proportion of copper, on the other hand, improves the colour and consistence of tin, without much injuring its ductility. Pewter is sometimes made from one part of copper, and twenty or more of tin.

It has long been observed, that though tin is specifically much lighter than copper, yet the gravity of the compound, bell-metal, is greater than that of the copper itself. The same augmentation of gravity also takes place where the lighter metal is in the greatest proportion; a mixture even of one part of tin with two of copper, turning out specifically heavier than pure copper. Most metallic mixtures answer to the mean gravity of the ingredients, or such as would result from a bare apportionment of parts. Of those tried by Dr Lewis, some exceeded the mean, but the greater number fell short of it; tin and copper were the only ones that formed a compound heavier than the heaviest of the metals separately.

White copper is prepared by mixing together equal parts of arsenic and nitre, injecting the mixture into a red-hot crucible, which is to be kept in a moderate fire, till they fusible, and flow like wax. One part of this mixture is injected upon four parts of melted copper, and the metal, as soon as they appear thoroughly united together, immediately poured out. The copper, thus whitened, is commonly melted with a considerable proportion of silver, by which its colour is both improved and rendered more permanent. The white copper of China and Japan appears to be no other than a mixture of copper and arsenic. Geoffroy relates, that, on repeated fusions, it exhaled arsenical fumes, and became red copper, losing, with its whiteness, one seventh of its weight.

4. Iron.

Iron is a metal of a greyish colour; soon tarnishing in the air into a dusky blackish hue; and in a short time contracting a yellowish, or reddish rust. It is the hardest of all metals; the most elastic; and, excepting platinum, the most difficult to be fused. Next to gold, iron has the greatest tenacity of parts; an iron wire, the tensile diameter of which is the tenth part of an inch, being capable of sustaining 450 pounds. Next to tin, it is the lightest of all the metals, losing between a seventh and eighth part of its weight when immersed in water. When very pure, it may be drawn into wire as fine as horse-hair; but is much less capable of being beaten into thin leaves than the other metals, excepting only lead.

Iron grows red-hot much sooner than any other metal; and this, not only from the application of actual fire, but likewise from strong hammering, friction, or other mechanic violence. It nevertheless melts the most difficulty of all metals except platinum; requiring, in its most fusible state, an intense, bright, white heat. When perfectly malleable, it is not fusible at all by the heat of furnaces, without the additions or the immediate contact of burning fuel; and, when melted, loses its malleability: all the common operations which communicate one of these qualities deprive it at the same time of the other; as if infusibility and malleability were in this metal incompatible. When exposed to the focus of a large burning mirror, however, it quickly fused, boiled, and emitted an ardent flame, the lower part of which was a true flame. At length it was changed into a blackish, vitrified scoria.

From the great waste occasioned by exposing iron to a red hot especially to a white heat, this metal appears to be a combustible substance. This combustion is maintained, like that of all other combustible substances, by contact of air. Dr Hook, having heated a bar of iron to that degree called white heat, he placed it upon an anvil, and blowed air upon it by means of bellows, by which it burnt brighter and hotter. Exposed to a white heat, it contracts a semi-vitreous coat, which bursts at times, and flies off in sparkles. No other metallic body exhibits any such appearance. On continuing the fire, it changes by degrees into a dark red calx, which does not melt in the most vehement heat procurable by furnaces, and, if brought into fusion by additions, yields an opake black glass. When strongly heated, it appears covered on the surface with a soft vitreous matter like varnish. In this state pieces of it cohere; and, on being hammered together, weld, or unite, without discovering a juncture. As iron is the only metal which exhibits this appearance in the fire, so it is the only one capable of being welded. Those operations which prevent the superficial scoriification, deprive it likewise of this valuable property: which may be restored again, by suffering the iron to resume its vitreous aspect; and, in some measure, by the interpolation of foreign vitreifiable matters; whilst none of the other metals will unite in the smallest degree, even with its own scoria.

Iron expands the least of all metals by heat. In the act of fusion, instead of continuing to expand, like the other metals, it shrinks; and thus becomes so much more dense as to throw up such parts as is unmelted, to the surface; whilst pieces of gold, silver, copper, lead, or tin, put into the respective metals in fusion, sink freely to the bottom. In its return to a confluent state, instead of shrinking like the other metals, it expands; Practice expands sensibly rising in the vessel, and assuming a convex surface, while the others become concave. This property, first observed by Reaumur, excellently fits it for receiving impressions from moulds. By the increase of bulk which the metal receives in conglomeration, it is forced into the minutest cavities, so as to take the impression far more exactly than the other metals which shrink.

Iron is dissolved, by all the metals made fluid, except lead; though none of them act so powerfully upon it as gold; but, as Cramer observes, if the iron contains any portion of sulphur, it can scarcely be made to unite at all with gold.

Among the semi-metallic bodies, it is adverse to an union with mercury; no method of amalgamating these two having yet been discovered; though quicksilver, in certain circumstances, seems in some small degree to act upon it. A plate of tough iron, kept immersed in mercury for some days, becomes brittle; and mercury will often adhere to and coat the ends of iron pebbles used in triturating certain amalgams with saline liquors.

Next to mercury, zinc is the most difficultly combined with iron; not from any natural indisposition to unite, but from the zinc being difficultly made to fulfill the heat requisite. The mixture is hard, somewhat malleable, of a white colour approaching to that of silver. Regulus of antimony, as soon as it melts, begins to act on iron, and dissolves a considerable quantity. If the regulus be stirred with an iron rod, it will melt off a part of it. Arsenic likewise easily mingles with iron, and has a strong attraction for it; forsaking all the other metals, to unite with this. It renders the iron white, very hard, and brittle.

This metal is the basis of the fine blue pigment, called, from the place where it was first discovered, Berlin or Prussian blue. This colour was accidentally discovered about the beginning of the present century, by a chemist of Berlin, who, having successively thrown upon the ground several liquors from his laboratory, was much surprized to see it suddenly stained with a beautiful blue colour. Recollecting what liquors he had thrown out, and observing the same effects from a similar mixture, he prepared the blue for the use of painters; who found that it might be substituted to ultramarine, and accordingly have used it ever since.

Several chemists immediately endeavoured to discover the composition of this pigment; and, in the year 1724, Dr Woodward published the following process, in the Philosophical Transactions, for making it. "Alkalize together four ounces of nitre, and as much tartar: (See No. 220). Mix this alkali well with four ounces of dried bullocks blood; and put the whole in a crucible covered with a lid, in which there is a small hole. Calcine with a moderate heat, till the blood be reduced to a perfect coal; that is, till it emits no more smoke or flame capable of blackening any white bodies that are exposed to it. Increase the fire towards the end, so that the whole matter contained in the crucible shall be moderately, but sensibly, red.

Throw into two pints of water the matter contained in the crucible, while yet red, and give it half an hour's boiling; decant this first water; and pour more upon the black charry coal, till it becomes almost infipid. Mix together all these waters; and reduce them, by boiling, to about two pints.

Dissolve also two ounces of martial vitriol, and eight ounces of alum, in two pints of boiling water. Mix this solution when hot with the preceding lixivium also hot. A great effervescence will then be made: the liquors will be rendered turbid; and will become of a green colour, more or less blue; and a precipitate will be formed of the same colour. Filter, in order to separate this precipitate; upon which pour spirit of salt, and mix them well together; by which means the precipitate will become of a fine blue colour. It is necessary to add rather too much of the salt than too little, and till it no longer increases the beauty of the precipitate. The next day wash this blue, till the water comes off from it infipid; and then gently dry it."

Mr Geoffroy was the first who gave any plausible theory of this process, or any rational means of improving it. He observes, that the Prussian blue is no other than the iron of the vitriol revived by the inflammable matter of the alkaline lixivium, and perhaps a little brightened by the earth of alum; that the green colour proceeds from a part of the yellow ferruginous ochre, or ochre, unrevived, mixing with the blue; and that the spirit of salt dissolves this ochre more readily than the blue part; though it will dissolve that also by long standing, or if used in too large quantity. From these principles, he was led to increase the quantity of inflammable matter; that there might be enough to revive the whole of the ferruginous ochre, and produce a blue colour at once, without the use of the acid spirit. In this he perfectly succeeded; and found, at the same time, that the colour might be rendered of any degree of depth, or lightness, at pleasure. If the alkali is calcined with twice its weight of dried blood, and the lixivium obtained from it poured into a solution of one part of vitriol to six of alum, the liquor acquires a very pale blue colour, and deposits as pale a precipitate. On adding more and more of a fresh solution of vitriol, the colour becomes deeper and deeper, almost to blackness. He imagines with great probability, that the blue pigment, thus prepared, will prove more durable in the air, mingle more perfectly with other colours, and be less apt to injure the lustre of such as are mixed with or applied in its neighbourhood, than that made in the common manner; the tarnish to which common Prussian blue is subject, seeming to proceed from the acid, which cannot be separated by any ablution.

He takes notice of an amusing phenomenon, which happens upon mixture. When the liquors are well shaken together; and the circular motion, as soon as possible, flopped; some drops of solution of vitriol, prepared (deprated by long settling), let fall on different parts of the surface, divide, spread, and form curious representations of flowers, trees, shrubs, flying insects, &c., in great regularity and perfection. These continue 10 or 12 minutes; and on flinging the liquor again, and dropping in some more of the solution of vitriol, are succeeded by a new picture.

This theory is confirmed by Mr Macquer, in a Memoir printed in the year 1752. He observes, that the quantity of phlogiston communicated to the iron by... Practice in this process is so great, as not only to cause the metal itself in a great measure the action of acids, and become totally unaffected by the magnet; but by a slight calcination it becomes entirely similar to other iron, and is at once deprived of its blue colour. He further observes, that fire is not the only means by which Prussian blue may be deprived of all the properties which distinguish it from ordinary iron. A very pure alkali produces the same effect. He has also discovered, that the alkali which has thus deprived the Prussian blue of all the properties which distinguish it from ordinary iron, becomes, by that operation, entirely similar to the phlogisticated alkali used for the preparation of Prussian blue.

By a more particular examination, he found, that the alkali might become perfectly saturated with the colouring matter; so that, when boiled on Prussian blue, the alkali extracted none of its colour. When the salt was thus perfectly saturated, it seemed no longer to possess any alkaline qualities. If poured into a solution of iron in any acid, a single, homogeneous, and perfect precipitate was formed; not green, as in Dr. Woodward's process, but a perfect Prussian blue; which needed no acid to brighten its colour. A pure acid added to the alkali was not in the least neutralized, nor in the least precipitated the colouring matter. From hence Mr. Macquer concludes, that, in the making of Prussian blue, vitriol is decomposed; because the iron has a strong attraction for the colouring matter, as well as the acid for the alkali; and the sum of the attraction of the acid to the alkali, joined to that of the iron for the colouring matter, is greater than the single attraction of the acid to the metal.

Another very important phenomenon is, that earths have not the same attraction for this colouring matter that metallic substances have. Hence, if an alkali saturated with this colouring matter be poured into a solution of alum, no decomposition is effected, nor any precipitate formed. The alum continues alum, and the alkali remains unchanged. From this experiment Mr. Macquer concludes that alum does not directly contribute to the formation of the Prussian blue. The purpose he thinks it answers is as follows. Fixed alkaline salts can never be perfectly saturated with phlogistic matter by calcination; alkalis, therefore, though calcined with inflammable substances, so as to make a proper lixivium for Prussian blue, remains still alkaline. Hence, when mixed with a solution of green vitriol, they form, by their purely alkaline part, a yellow precipitate, far much more copious, as the alkali is less saturated with phlogiston. But nothing is more capable of spoiling the fine colour of the Prussian blue, than an admixture of this yellow precipitate; it is therefore necessary to add a quantity of alum, which will take up the greatest part of the purely alkaline salt; and, of consequence, the quantity of yellow ferruginous precipitate is much diminished. But the earth of alum, being of a fine shining white, does not in the least alter the purity of the blue colour, but is rather necessary to dilute it. From all this it follows, that it is a matter of indifference whether the green precipitate is to be again dissolved by an acid, or the alkaline part of the lixivium saturated with alum, or with an acid, before the precipitate is formed. The latter indeed seems to be the most eligible method.

Most alkalis obtained from the ashes of vegetables, being combined, by their combustion, with a portion of inflammable matter, are capable of furnishing a close from quantity of Prussian blue, proportionable to the quantity of colouring matter they contain, even without the necessity of mixing them with a solution of iron; because they always contain a little of this metal dissolved, some of which may be found in almost all vegetables; therefore, it is sufficient to saturate them with an acid. Henckel observed the production of this blue in the saturation of the fusible alkali, and recommended to chemists to inquire into its nature.

Iron deflagrates with nitre, and renders the salt alkaline and caustic. A part of the iron is thus rendered soluble, along with the alkalized salt. A mixture of equal parts of iron filings and nitre, injected into a strongly heated crucible, and, after the detonation, thrown into water, tinges the liquor of a violet or purplish blue colour. This solution, however, is not permanent. Though the liquor at first passes through a filter, without any separation of the iron; yet, on standing for a few hours, the metal falls to the bottom, in form of a brick-coloured powder. Volatile alkalis instantly precipitate the iron from this fixed alkaline solution.

Iron readily unites with sulphur; and when combined with it, proves much easier of fusion than by itself. A mixture of iron filings and sulphur, moistened with water, and pressed down close, in a few hours swells and grows hot; and, if the quantity is large, bursts into flame.

By cementation with inflammable matters, iron imbibes a larger quantity of phlogiston; and becomes much harder, less malleable, and more fusible. It is then called steel. See Metallurgy, and Steel.

5. Lead.

Lead is a pale or livid-white metal, soon losing its brightness in the air, and contracting a blackish or greyish ash-colour. It is the softest and most flexible of all metallic bodies; but not ductile to any great degree, either in the form of wire, or leaf; coming far short, in this respect, of all other metals. It has also the least tenacity of all metallic bodies; a leaden wire little tensed of \( \frac{1}{4} \) of an inch diameter being capable of supporting only \( 29\frac{1}{4} \) pounds. Lead has, however, a considerable specific gravity; losing, when immersed in water, between \( \frac{3}{4} \) and \( \frac{1}{2} \) of its weight. It is of all metals the most fusible, excepting only tin and bismuth. The plumbers cast thin sheets of lead upon a table or mould, covered with a woollen, and above this with a linen-cloth, without burning or scorching the cloths. The melted lead is received in a wooden case without a bottom; which being drawn down the sloping table by a man on each side, leaves a sheet of its own width, and more or less thin according to the greater or less celerity of its descent. For thick plates, the table is covered over with moistened sand, and the liquid metal conducted evenly over it, by a wooden strike, which bears on a ledge at each side.

Some have preferred, for mechanic uses, the milled lead, or flattened sheets, to the cast; as being more equal, smooth, and solid. But whatever advantage of this kind... kind the milled fort may appear to have at first, they are not found to be very durable. When the lead is stretched between the rollers, its cavities must necessarily be enlarged. The particles of metal that may be squeezed into them can have no union or adhesion with the contiguous particles; and, of consequence, must be liable, from bending, blows, jars, &c., to start out again, and leave the mass spongy and porous.

Lead yields the dullest and weakest sound of all metallic bodies. Reaumur observes, that it is rendered sonorous by casting a small quantity into a spherical or elliptical segment, as in the bottom of an iron-kettle; from hence he conjectures, that the sound of the sonorous metals might be improved for the bells of clocks, &c., by giving them a similar form.

Though this metal very soon loses its lustre, and tarnishes in the air, it resists much longer than iron or copper the combined action of air and water, before it is decomposed or destroyed; and hence it is exceedingly useful for many purposes to which these metals can by no means be applied. When just become fluid, lead looks bright like quicksilver; but immediately contracts a variously coloured pellicle on the surface. If this is taken off, and the fire continued, a fresh pellicle will always be formed, till the metal is by degrees changed into a dusty powder, or calx. The injection of a little fat, charcoal-powder, or other inflammable matter, prevents this change, and readily revives the calx into lead again. It is said, that lead, recovered from its calx, proves somewhat harder and whiter than at first, as well as less subject to tarnish in the air.

The blackish calx or ashes of lead become of a very different appearance if the calcination is continued with a fire so moderate as not to melt them, and particularly if exposed to flame. By this treatment, they become first yellow; and are then called massicot, or yellow lead. This colour becomes gradually more and more intense, till at last the calx is of a deep red; and then is called minium, or red lead.

The preparation of this substance, which is much used in painting, has become a trade by itself. It is made from lead calcined to a greyish powder by keeping it melted over the fire. This powder, ground in mills, is further calcined in a reverberatory furnace, under a low arch, and frequently flumed with an iron-rake, to prevent its running and melting into clots, and to expose a fresh surface to the air and flame. The calcination lasts two or three days. The increase of the weight by fume is said to amount to $\frac{1}{5}$, by others, to no less than $\frac{1}{2}$ of the lead employed; and the lead recovered from the minium to be $\frac{3}{4}$ less than the original weight of the metal. If the minium be further calcined, the increase is no more, but the quantity of lead recoverable from the calx proves less in proportion to the vehemence and continuance of the calcination.

If, instead of keeping this calx in a continued moderate heat, it is suddenly fused, the matter then puts on a foliated appearance, and changes to a dull kind of brick-colour when powdered, and is then called litharge. Most of this substance is produced by refining silver with lead, (see Refining); and is of two kinds, white, and red. These two are distinguished by the names of litharge of gold, and litharge of silver. The perfect is that called litharge of gold; the pale fort contains a considerable proportion of lead in its metallic state; and even the highest coloured litharge is seldom free from a little metallic lead, discoverable and separable by melting the mass in a crucible; when the lead sublimes to the bottom.

Lead mingles in fusion with all the metals except iron, with which it refuses any degree of union as long as the lead preserves its metallic form. On continuing the fire, the lead, scoriifying or calcining, absorbs the phlogistic principle of the iron, and consequently promotes the calcination of that metal; both being at length reduced to calces. The fusible calx of lead easily unites with the calx of iron, and both melt together into an opake brown or blackish glass. Copper does not unite with melted lead, till the fire is raised so high as to make the lead smoke and boil, and of a bright red heat. Pieces of copper, now thrown in, soon dissolve and disappear in the lead; the mixture, when cold, is brittle, and of a granulated texture. The union of these two metals is remarkably slight. If a mixture of copper and lead is exposed to a fire no greater than that in which lead melts, the lead almost entirely runs off by itself; a separation, of which no other example is known. What little lead is retained in the pores of the copper, may be scorified, and melted out, by a fire considerably less than is sufficient to fuse copper. If any of the copper is carried off by the lead, it swims unmelted on the surface.

Gold and silver are both dissolved by lead in a slight red heat. They are both rendered extremely brittle by the minute quantity of this metal, though lead is rendered more ductile by a small quantity of either of them. In cupellation, a portion of lead is retained by gold, but silver parts with it all. On the other hand, in its separation from copper, if the copper contains any of the precious metals, the silver will totally melt out with the lead, but the gold will not. The attraction of lead to copper, however slight, is greater than that of copper to iron: a mixture of copper and iron being boiled in melted lead, the copper is imbibed by the lead, and the iron thrown up to the top. Silver is in like manner imbibed from iron by lead; whilst tin, on the contrary, is imbibed from lead by iron. If two mixtures, one of lead and tin, and another of iron and silver, be melted together, the result will be two new combinations, one of the tin with the iron at the top, the other with the lead and silver at the bottom: how carefully soever the matter be stirred and mixed in fusion, the two compounds, when grown cold, are found distinct, so as to be parted with a blow.

This metal is soluble in alkaline lixivias and expressed oils. Plates of lead boiled in alkaline lixivias, have a small part dissolved, and a considerable quantity corroded; the solution stains hair black. Lead, fused with fixed alkaline fats, is in part corroded into a dark-coloured feoria, which partially dissolves in water. Expressions oils dissolve the calces of lead, by boiling, in such large quantities as to become thick and congealed: hence plasters, cements for water-works, paint for preserving nets, &c. Acids have a greater affinity affinity with lead than oils have. If the common plaster, composed of oil and litharge, be boiled in distilled vinegar, the litharge will be dissolved, and the oil thrown up to the top. The oil thus recovered, proves soluble like essential oils in spirit of wine; a phenomenon first taken notice of by Mr Geoffroy.

6. Tin.

The colour of this metal resembles silver, but somewhat darker. It is softer, less elastic, and sonorous, than any other metal except lead. When bent backwards and forwards, it occasions a crackling sound, as if torn asunder. It is the lightest of all the malleable metals, being little more than seven times specifically heavier than water. The tenacity of its parts also is not very considerable; a thin wire of 1/4 of an inch diameter, being able to support only 49½ pounds.

Tin is commonly reckoned the least ductile of all metals except lead; and certainly is so, in regard to ductility into wire, but not in regard to extensibility into leaves. These two properties seem not to be so much connected with one another as is generally imagined. Iron and steel may be drawn into very fine wire, but cannot be beat into leaves. Tin, on the other hand, may be beat into very thin leaves, but cannot be drawn into wire; gold and silver possess both properties in a very eminent degree; whilst lead, notwithstanding its flexibility and softness, cannot be drawn into fine wire, or beat into thin leaves. It melts the most easily of all the metals; about the 450th degree of Fahrenheit's thermometer. Heated till almost ready to melt, it becomes so brittle that large blocks may be easily beat to pieces by a blow. The purer sort, from its facility of breaking into long shining pieces, is called grain-tin. Melted, and mildly agitated at the instant of its beginning to congeal, it is reduced into small grains, or powder.

With the heat necessary for fusion, it may also be calcined; or at least so far deprived of its phlogiston as to appear in the form of a grey calx, which may be entirely reduced to tin by the addition of inflammable matter. The calcination of tin, like that of lead, begins by the melted metal losing its brightness, and contracting a pellicle on its surface. If the fire is raised to a cherry-red, the pellicle swells and bursts, discharging a small bright flame of an arsenical smell. By longer continuance in the fire, the metal is converted first into a greyish, and then into a perfectly white calx, called putty, which is used for polishing glafts, and other hard bodies.

The calx of tin is the most refractory of all others, that of platinum excepted. Even in the focus of a large burning mirror, it only softens a little, and forms crystalline filaments. With glafts of bismuth, and the simple and arsenicated glafts of lead, it forms opake milky compounds. By this property it is fitted for making the basis of the imperfect glafts called enamels (see Glass, and Enamel). The author of the Chemical Dictionary relates, that "having exposed very pure tin, singly, to a fire as strong as that of a glasshouse furnace, during two hours, under a muffe, in an uncovered telft; and having then examined it, the metal was found covered with an exceedingly white calx, which appeared to have formed a vegetation; under this matter was a reddish calx, and an hyacinthine glaft; and lastly, at the bottom, was a piece of tin unaltered. The experiment was several times repeated with the same success."

Nitre deflagrates with tin, and hastens the calcination of this as well as of other imperfect metals. The vapours which rise from tin, by whatever method it is calcined, have generally an arsenical smell. Tin melted with arsenic falls in great part into a whitish calx; the part which remains uncalcined proves very brittle, appears of a white colour, and a sparkling plated texture, greatly resembling zinc. The arsenic is strongly retained by the tin, so as scarcely to be separable by any degree of fire; the tin always discovering, by its augmentation in weight, that it holds a portion of arsenic, though a very intense fire has been used. Hence, as the tin-ores abound in arsenic, the common tin is found also to participate of that mineral.

Hennel discovered a method of separating actual Arsenic from tin; namely, by slowly dissolving the tin in eight times its quantity of an aqua regia made with fai ammoniae, and setting the solution to evaporate in a gentle warmth: the arsenic begins to concrete whilst the liquor continues hot, and more plentifully on its growing cold, into white crystals. Mr Margraaf, in the Berlin Memoirs for 1747, has given a more particular account of this process. He observes, that the white sediment which at first separates during the dissolution, is chiefly arsenical: that Malacca tin, which is accounted one of the purest sorts, yielded no less than ¼ its weight of arsenical crystals; that some sorts yielded more; but that tin extracted from a particular kind of ore which contained no arsenic, afforded none. That the crystals were truly arsenical, appeared from their being totally volatile; from their subliming (a little fixed alkaline salt being added to absorb the acid) into a colourless pellucid concrete; from the sublimate, laid on a heated copper-plate, exhaling in fumes of a garlic smell; from its staining the copper white; and from its forming with sulphur, a compound similar to the yellow or sulphurated arsenic. He found that the arsenic was separable also by means of mercury: an amalgam of tin being long triturated with water, and the powder which was washed off committed to distillation, a little mercury came over, and bright arsenical flowers arose in the neck of the retort. Dr Lewis observes, that the cracking noise of tin in bending may possibly arise from its arsenic; observations as these operations which are said to separate arsenic from the metal, likewise deprives it of this property.

Tin may be alloyed, in all proportions, with all metals by fusion: but it absolutely destroys their ductility, and renders them brittle, as in bell-metal; whence this metal has obtained the name of diabolus metallicus. It is remarkable, that the most ductile metals are most injured by the addition of a small quantity of tin; the vapour of a single grain of tin being sufficient to destroy the ductility of a considerable quantity of gold.

Iron is dissolved by tin in a heat far less than that in which iron itself melts: the compound is white and brittle. Iron added to a mixture of lead and tin, takes up the tin, leaving the lead at the bottom; and, in like manner, manner, if lead, tin, and silver are melted together, the addition of iron will absorb all the tin, and the tin only. Hence an easy method of purifying silver from tin.

Not liable to rust.

Tin, notwithstanding it is, like lead, soon deprived of its lustre by exposure to the air, is nevertheless much less liable to rust than either iron, copper, or lead; and hence is advantageously used for covering over the insides of other metallic vessels. The amalgam of mercury and tin is employed to cover one of the surfaces of looking-glasses; by which they are rendered capable of reflecting the rays of light. The amalgam also, mixed with sulphur and sal ammoniac, and let to sublimate, yields a sparkling gold-coloured substance called aurum mosaicum; which is sometimes used as a pigment. This preparation is commonly made from quicksilver and tin, of each two parts, amalgamated together; and then thoroughly mixed with sulphur and sal ammoniac, of each one part and a half. The mercury and sulphur unite into a cinnabar, which sublimes along with the sal ammoniac; and, after sublimation, the aurum mosaicum remains at the bottom.

Sulphur may be united with tin by fusion; and forms with it a brittle mass, more difficultly fusible than pure tin. Sulphur has, in this respect, the same effect upon tin, as upon lead. The alloy of tin lessens the fusibility of these very fusible metals; while it increases the fusibility of other difficultly fusible metals, as iron and copper.

7. Mercury, or Quicksilver.

Mercury is a fluid metallic substance, of a bright silver colour, resembling lead or tin when melted; entirely void of taste and smell; extremely divisible; and congealable only in a degree of cold very difficultly produced, in this country, by art (see Cold, and Congelation). It is the most ponderous of all fluids, and of all known bodies, gold and platinum excepted; its specific gravity being to that of water nearly as 14 to 1. It is found to be specifically heavier in winter than in summer, by 25 grains in 11 ounces.

Neither air nor water, nor the united action of these two, seem to make any impression upon mercury: nor is it more susceptible of rust than the perfect metals. Its surface, nevertheless, is more quickly tarnished than gold or silver; because the dust which floats in the air, quickly settles on its surface. The watery vapours also, which float in the air, seem to be attracted by mercury.

From these extraneous matters, which only slightly adhere to it, mercury may be easily cleansed by passing it through a clean new cloth, and afterwards heating it; but if mixed with any other metal, no separation can be effected without distillation. In this process, a small portion of some of the metals generally arises along with the mercury. Thus, quicksilver distilled from lead, bismuth, or tin, appears less bright than before; stains paper black; sometimes exhibits a skin upon the surface; and does not run freely, or into round globules. Mr Boyle relates, that he has observed the weight of mercury sensibly increased by distillation from lead, and this when even a very moderate fire was made use of. By amalgamation with stellated regulus of antimony, and then being distilled after a few hours digestion, mercury is said to become, by a few repetitions of the process, more ponderous, and more active: the animated, or philosophic mercuries of some of the alchemists, are supposed to have been mercury thus prepared. By the curious flame, or similar processes, seem to have been obtained the curious mercuries which Boyle declared he was possessed of, and made himself; which were "considerably heavier in specie than common quicksilver,—diffused gold more readily,—grew hot with gold, so as to be offensive to the hand, and elevated gold in distillation." When quicksilver is to be distilled, it is proper to mingle it with a quantity of iron-flings; which have the property of making it much brighter than it can be otherwise obtained, probably by furnishing phlogiston.

By digestion in a strong heat for several months, mercury undergoes a considerable alteration, changing into a powder, at first air-coloured, afterwards yellow, at length of a bright-red colour, and an acid taste; and is then called mercurius precipitatus per f. In this last state it proves similar to the red precipitate, prepared from a solution of mercury in nitrous acid; the nitrous acid in the air seeming to be revivified into its proper form, by long contact with the metal, (see Air, n° 44). This calx proves less volatile in the fire than the mercury in its fluid state. It supports, for some time, even a degree of red heat. In the focus of a burning-glass, it is said, if laid upon a tile, to vitrify on a piece of charcoal, and to revive into running mercury before it exhales. Evaporated by common fire, it leaves a small portion of a light brown powder; which, Boerhaave relates, bore a bluish heat; swelled into a spongy mass; formed with borax a vitreous friable substance; but vanished in cupellation. By long continued digestion in a gentle heat, mercury suffers little change. Boerhaave digested it in low degrees of heat, both in open and close vessels, for 15 years together, without obtaining any other reward for his labour than a small quantity of black powder; which, by trituration, was quickly revived into running mercury. Constant trituration, or agitation, produce a change similar to this, in a short time. Both the black and red powders, by bare exposure to a fire sufficient to elevate them, return into fluid mercury. The red powder has been revived by simply grinding it in a glass-mortar.

In like manner, quicksilver remains unchanged by Orby distillation. Boerhaave had the patience to distil 18 ounces of mercury upwards of 300 times over, without observing any other change than that its fluidity and specific gravity were a little increased, and that some grains of a fixed matter remained. The vapours of mercury, like those of all other volatile bodies, cause violent explosions if confined. Mr Hellot gives an account of his being present at an experiment of this kind: a person pretending to fix mercury, had inclosed it in an iron box closely welded. When the mercury was heated, it burst the box, and diffused into invisible vapours.

Mercury dissolves or unites with all metallic bodies, except three, viz. iron, arsenic, and nickel: in some cases it will absorb metals, particularly gold and silver; from Practice from their solutions in acids or alkalies; but does not act upon any metal when combined with sulphur, nor on precipitates made by alkalies, nor on calces by fire. Whatever metal it is united with, it constantly preserves its own white colour. It unites with any proportion of those metallic substances with which it is capable of being combined; forming, with different quantities, amalgams of different degrees of consistence. From the fluid ones, greatest part of the quicksilver may be separated by colature. Bismuth is so far attenuated by mercury, as to pass through leather with it in considerable quantity. It also promotes the action of quicksilver upon lead to a great degree; so that mercury united with \( \frac{1}{2} \), \( \frac{1}{3} \), or \( \frac{1}{4} \) its weight of bismuth, dissolves masses of lead in a gentle warmth, without the agitation, trituration, commination, or melting heat necessary to unite pure mercury with lead. From these properties, this solution of bismuth in mercury becomes a proper solvent for pieces of lead lodged in the human body.

On triturating or digesting amalgams for a length of time, a blackish or dusky-coloured powder arises to the surface, and may be readily washed off by water. Some of the chemists have imagined, that the amalgamated metal was here reduced to its constituent parts; but pure mercury is by itself reducible to a powder of the same kind; and the metallic particles in this process, united with the mercury, are found to be no other than the metal in its entire substance. Some metals separate more difficultly than others; gold and silver the most so. Boerhaave relates, that if the powder which separates from an amalgam of lead be committed to distillation with vinegar in a tall vessel, the mercury will rise before the vinegar boils: that, by a like artifice, quicksilver may be made to distil in a less degree of heat than that of the human body; but Dr Lewis, though he made many trials, was never able to succeed.

By amalgamation with gold, mercury may become exceedingly fixed; so as not to be disipable by the greatest heat. Concerning this, Dr Brandt relates the following curious experiment: "Having amalgamated fine gold with a large proportion of quicksilver, and strained off the superfluous mercury, he digested the amalgam in a close flint vessel for two months with such a degree of heat, that a part of the quicksilver sublimed into the neck of the flask. The matter being then ground with twice its weight of sulphur, and urged with a gradual fire in a crucible, a spongy calx remained; which being melted with borax, and afterwards kept in fusion by itself for half an hour, in a very violent fire, still retained so much of the quicksilver, as to become brittle under the hammer, and appear internally of a leaden colour. The metal being again amalgamated with fresh mercury, the amalgam again ground with sulphur, and exposed to an intense fire, a spongy calx remained as before. This calx being digested in two or three fresh parcels of aqua regia, a small portion of whitish matter remained at last undissolved. The paper which covered the cylindrical flask wherein the digestion was performed, contracted, from the vapours, a deep-green circular spot in the middle, with a smaller one at the side; whereas the aqua regia digested in the same manner by itself, or with gold, or with mercury, gave no stain."

The first solution, on the addition of oil of tartar per deliquium, grew red as blood; on standing, it deposited, first, a little yellow calx, like aurum fulminans; afterwards, a bright matter like fine gold; and at last, a paler precipitate, inclining to green; its own deep red colour and transparency remaining unchanged. Being now committed to distillation, a colourless liquor arose; and the residuum, perfectly exsiccated, yielded, on edulcoration, a yellow calx of gold; which the alkaline lixivium had been unable to precipitate. The second solution turned green on the admixture of the alkaline liquor, and let fall a white precipitate, which turned black and brown. The several precipitates were calcined with twice their weight of sulphur, and then melted with four times their quantity of flint, and twelve of pot-ash, in a fire vehemently excited by bellows. The scoria appeared of a golden colour, which, on pulverization and edulcoration, vanished. At the bottom was a regulus, which looked bright like the purest gold; but was not perfectly malleable. Broke, it appeared internally white; and the white part amounted to at least one-third its bulk. Besides this lump of metal, there were several others, white like silver, and soft as lead."

In Willson's chemistry, we have a process for converting quicksilver into water, by dropping it by little and little into a tall iron vessel, heated almost to a white heat in the bottom. Over the mouth of this vessel were fitted seven alembics; and on the top, a glass alembic head, with a beak, to which was fitted a receiver. The mercury was put in so slowly, that it required 16 hours for one pound. Every time that a little quantity of mercury was put in, it made a great noise, filling the alembic's head, and receiver, with white fumes. When the vessels were cooled, a little water was found in each of the receivers; and in the first and second, some grains of crude mercury. The whole quantity amounted to 13 ounces and 6 drachms; which was expected to prove a powerful solvent of gold and silver; but, on trial, was found to be in no respect different from common water. On this experiment, Dr Lewis has the following note:

"The possibility of converting mercury into water, or at least of obtaining a great quantity of water from mercury, has not only been believed by several great men in the chemical art; but some have even ventured to assert that they have actually made this change. Yet nevertheless, they have delivered the history of this affair with such marks, as seem to make the reality of the change extremely doubtful. Mr Boyle, (in his tract of the productions of Chemical Principles, annexed to Scept. Chemist, p. 235), says, 'that he once obtained water from mercury without addition, without being able to make the like experiment succeed afterwards.' Mr Le Febure, who is generally looked upon as an honest practitioner, directs a process similar to that above (Willson's), for obtaining this mercurial water. But it is to be suspected, as Mr Hales very well observes (in his Statistical Experiments, p. 200.), that Mr Boyle, and others, were deceived by some unheeded circumstance, when they thought they obtained a water from mercury, which should seem rather to have arisen from the late Practice and earthen vessels made use of in the distillation; for Mr Hales could not find the least sign of any moisture upon distilling mercury in a retort made of an iron gun-barrel, with an intense degree of heat; although he frequently cohabited the mercury which came over into the recipient. In a course of chemical experiments, I repeated Mr Hales's process, and urged the mercury, which was let fall by little and little, through an aperture made in the gun-barrel, with a most intense degree of heat, without obtaining any water; but it being suspected by a bystander, that the mercury in this experiment came over before it had been sufficiently acted upon by the fire, by reason of the looseness of the neck of the distilling instrument, the experiment was varied in the following manner. Sixteen ounces of mercury were heated in a crucible, in order to evaporate any moisture that might have been accidentally mixed with it; and an iron gun-barrel of four feet in length, being placed perpendicularly in a good furnace, and a glass-head and recipient fitted to its upper part, the mercury was let fall by little and little into the barrel, and the fire urged with bellows. After each injection, the mercury made a considerable noise and ebullition, and arose into the head; where it soon condensed and trickled down, in the common form of running mercury, into the recipient, without the least perceptible appearance of any aqueous humidity."

Mercury is difficultly amalgamated with regulus of antimony and copper; for which some particular manoeuvres are required. Two of Dr Lewis's receipts for uniting quicksilver with copper, we have already given, (see no 376); with regulus of antimony, mercury, he says, may be perfectly united, by pouring a small stream of melted regulus into a considerable portion of mercury, made almost boiling hot. Another method directed by Henckel, is to put mercury into an iron mortar along with some water, and let the whole over the fire. When the water boils, a third or fourth part of melted regulus is to be poured in, and the mass ground with a pestle, till the amalgama is completed. The use of the water, as Dr Lewis observes, is to hinder the mercury from flying off by the heat of the regulus; but as the two are by this means not put together in so hot a state, the union is more difficult, and less perfect. The loss of the mercury, in the first process, may be prevented by using a large vessel, and covering it with a perforated iron-plate, through the hole in which the regulus is to be poured. This method is likewise applicable to the amalgamation of copper.

With sulphur, mercury unites very readily, forming by trituration, or simple fusion, a black powder, or mass, called Ethiop's mineral; which, by careful sublimation, becomes the beautiful red pigment called vermillion. (See Sulphur, sect. iv.)

8. Zinc.

This is a semi-metal of a bluish white colour. It is the least brittle of any of the semi-metals; and when amply supplied with phlogiston, which may be done by treating it in close vessels with inflammable matters, it possesses a semi-ductility, by which it may be flattened into thin plates. When broken, it appears of many flat shining plates or facets, which are larger when slowly than when hastily cooled. When heated, it is very brittle; and crackles like tin, only louder, when bent. Exposed to the air, it contracts in length of time a yellowish rust. Its specific gravity, according to Dr Lewis, is to that of water as 7½ to 1. It begins to melt as soon as red hot; but does not flow thin till the fire is raised to a white heat. Then the zinc immediately begins to burn with an exceedingly bright and beautiful flame. Kept just in fusion, it calcines slowly; not only on the upper surface, but likewise round the sides, and at the bottom of the crucible. If several pieces are just melted together, the mass, when grown cold, may be broke into the same number; their union being prevented by a yellowish calx, with which each piece is covered over.

Mr Malouin relates, in the French Memoirs for 1742, that a quantity of zinc being melted six times, and the fusion continued fifteen hours each time, it proved, on every repetition, harder, more brittle, less fusible, and less calcinable: that after the two first fusions, its colour was grey; after the third, brown; and after the fourth, black; that the fifth rendered it of a slate-blue; and the sixth of a clear violet.

So violent is the deflagration of zinc, that the whole flowers of its calx is sublimed by it, in the form of light flecks, or wool; which, however, are easily reduced to a fine powder. These are used in medicine, and reckoned an excellent remedy in epileptic cases. When once sublimed, they are by no means capable of being elevated again by the most violent heat. In a heat far greater than that in which they first arose, they suffer no alteration; in a very vehement one they melt, according to Henckel, into a semi-opaque green glass. Vitrified with borax, they give a grey, or brownish, glass. From the brightness of the flame of burning zinc, and the garlick-smell which it is said to emit, some have concluded that zinc contained the phosphoric acid; which, from some other circumstances, is not altogether improbable.

The flowers of zinc have been thought very difficultly, or not at all, reducible to their metallic form by an addition of phlogiston. But Dr Lewis observes, that this difficulty proceeds not from their unfitness to be restored into the form of zinc, but from the volatility of the semi-metal, which occasions its being dissipated in flames, if the common methods are made use of. All calces, those of iron excepted, require a greater heat for their fusion than that in which the metal itself melts; and as a full melting heat is the greatest that zinc can sustain, it burns and calcines the infant of its revival, if the air is admitted; and in close vessels escapes, in part at least, through their pores. On mixing flowers of zinc with powdered charcoal, and urging them with a strong fire in a crucible, a deflagration and fresh sublimation ensue; sufficient marks that the zinc has been reduced to its metallic form; for as long as it remains in the state of calx, neither of these effects can happen. If the vessel is so contrived as to exclude the air, and at the same to allow the reviving semi-metal to run off from the vehemence of the heat, into a receiver kept cool, the zinc will there concretize, and be preserved in its metallic state. It is still more effectually detained by certain metallic bodies, as copper, or iron; with which the zinc, when thus applied, unites more readily and perfectly than it can be made to do by any other means.

Homberg pretended to obtain an oil from the flowers of zinc, by dissolving them in distilled vinegar, and then distilling the solution in a glass retort. At first a quantity of phlegm arose; then the superfluous acid; and at last an empyreumatic oil. This last, which Homberg imagined to proceed from the flowers of zinc, Neuman very justly attributes to the distilled vinegar.

An oil of another kind was obtained by Mr Hellot from the above solution, by digesting the ash-coloured residuum, which remained after the distillation with the acidulous phlegm which came over, for eight or ten days; distilling the tincture to dryness; and repeating the extraction with the distilled liquor, till the quantity of dry extract, thus obtained, was very considerable. This resin-like matter, distilled in a retort with a stronger fire, yielded a yellowish liquor, and a white sublimate. The liquor discovered no mark of oil; but, upon being passed upon the sublimate, immediately dissolved it, and then exhibited on the surface several drops of a reddish oil. Some of this oil was taken up on the point of a pencil, and applied to gold and silver-leaf. In twenty-four hours, the parts touched appeared, in both, equally dissolved.

Zinc does not unite in fusion with bismuth, or the semi-metal called nickel. It unites difficulty with iron; less so with copper; easily with the other metals. It renders iron or copper more easily fusible; and, like itself, brittle whilst hot, though considerably malleable when cold. It brightens the colour of iron almost into a silver hue, and changes that of copper into a yellow or gold colour. It greatly debases the colour of gold; and renders near an hundred parts of that most ductile metal brittle and intractable. A mixture of equal parts of each is very hard, white, and bears a fine polish; hence it is proposed by Mr Hellot for making specula. It is not subject to rust or tarnish in the air, like those metals whose basis is copper. It improves the colour and lustre of lead and tin, renders them firmer, and consequently fitter for several mechanic uses. Tin, with a small proportion of zinc, forms a kind of pewter. Lead will bear an equal weight, without losing too much of its malleability. Malouin observes, that arsenic, which whitens all other metals, renders zinc black and friable; that when the mixture is performed in close vessels, an agreeable aromatic odour is perceived on opening them; that zinc amalgamated with mercury, and afterwards recovered, proves whiter, harder, and more brittle than before, and no longer crackles on being bent.

Mixtures of zinc with other metals, exposed to a strong fire, boil and deflagrate more violently than zinc by itself. Some globules of the mixture are usually thrown off during the ebullition, and some part of the metal calcined and volatilized by the burning zinc; hence this substance has been called metallic nitre. Gold itself does not entirely resist its action. It very difficultly volatilizes copper; and hence the sublimes obtained in the furnaces where brass is made, or mixtures of copper and zinc melted, are rarely found to participate of that metal. On melting copper and zinc separately, and then pouring them together, a violent detonation immediately ensued; and above half the mixture was thrown about in globules.

Zinc does not unite in the least with sulphur, or with crude antimony, which destroy all other substances except gold and platinum; nor with compositions of sulphur and fixed alkaline salts, which dissolve gold itself. With nitre it deflagrates violently. Its flowers do not feebly deflagrate; yet alkalize double their weight of the salt, more readily than the zinc itself. The alkaline mass appears externally greenish, internally of a purple colour. It communicates a fine purple to water, and a red to vinegar. The aceton tincture impregnated, leaves a tenacious substance which soon runs in the air into a dark red caustic liquor, the alkaline of some of the pretended adepts.

9. BISMUTH.

This semi-metal, called also tin-plaist, and by some naturalists marcasita efficiarum, is somewhat similar to the regulus of antimony. It appears to be composed of cubes formed by the application of plates upon each other. Its colour is less white than that of regulus of antimony; and has a reddish tinge, particularly when it is exposed to the air. In specific gravity it approaches to silver; being nearly ten times heavier than water. It has no degree of malleability; breaking under the hammer, and being reducible by trituration to fine powder. Its melts a little later than tin, and seems to flow the thinnest of all metallic substances. Bismuth is semi-volatile, like all other semi-metals. When exposed to the fire, flowers rise from it; it is calcined; and converted into a litharge and glass nearly as lead is: (See Glass.) It may even be employed like that metal, in the purification of gold and silver by capellation. (See Refining.) When in fusion it occupies less volume than in its solid state: a property peculiar to iron among the metals, and bismuth among the semi-metals. It emits fumes in the fire as long as it preserves its metallic form; when calcined or vitrified, it proves perfectly fixed.

Bismuth mingles in fusion with all the metallic substances, except regulus of cobalt and zinc. The addition of nickel, or regulus of antimony, renders it miscible with the former, though not with the latter. It greatly promotes the tenacity as well as facility of the fusion of all those metals with which it unites. It whitens copper and gold, and improves the colour of some of the white metals: mixed in considerable quantity, it renders them all brittle, and of a flaky structure like its own. If mixed with gold, or silver, a heat that is but just sufficient to melt the mixture, will presently vitrify a part of the bismuth; which, having then no action on those perfect metals, separates, and glazes the crucible all round.

10. REGULUS OF ANTIMONY.

This semi-metal, when pure, and well fused, is of a white shining colour, and consists of laminae applied to each other. When it has been well melted, and not too hastily cooled, and its surface is not touched by any hard body during the cooling, it exhibits the appearance of a star on its perfect surface. Regulus of antimony is moderately hard; but, like other semi-metals, it has no ductility, and breaks in small pieces under a hammer. It loses \( \frac{1}{3} \) of its weight in water. The action of air and water destroys its lustre, but does not rust it so effectively as iron or copper. It is fusible with a heat sufficient to make it red hot; but when heated to a certain degree, it fumes continuously, and is dissipated in vapours. These fumes form what are called the argentine flowers of regulus of antimony, and are nothing but the earth of this semi-metal deprived of part of its inflammable principle, and capable of being reduced to its regaline state by an union with this principle.

There are different methods of preparing the regulus of antimony; but all of them consist merely in separating the sulphur which this mineral contains, and which is united with the regulus. It is plain, therefore, that regulus of antimony may be made by an addition of any substance, to crude antimony in fusion, which has a greater attraction for sulphur than the regulus itself has. For this purpose, alkaline salts have been employed, either previously prepared, or contemporaneously produced in the process, by a deliquescence of tartar and nitre. By this means, the sulphur was indeed absorbed; but the hepatic sulphur, formed by the union of the sulphur and alkali, immediately dissolved the regulus, so that very little, sometimes none at all, was to be obtained distinct from the scoria. Metals are found to answer better than alkaline salts, but the regulus is seldom or never free from a mixture of the metal employed. The way of obtaining a very pure regulus, and in great quantity, is to calcine the antimony in order to distillate its sulphur; then to mix the calx with inflammable matters, such as oil, soft soap, etc., which are capable of retorting the principle of inflammability to it. This method was invented by Kunckel. Another, but more expensive way of procuring a large yield of very pure regulus, is by digesting antimony in aqua regis, which dissolves the reguline part, leaving the sulphur untouched, precipitating the solution, and afterwards reviving the precipitate by melting it with inflammable matters.

There are considerable differences observed in the regulus of antimony, according to the different substances made use of to absorb the sulphur. When prepared by the common methods, it is found to be very difficultly amalgamated with mercury, (see no. 426); but Mr Pott has discovered, that a regulus prepared with two, or five parts of iron, four of antimony, and one of chalk, yield reguli which readily unite with mercury into an hard amalgam, by bare trituration with water. Marble and quicklime succeed equally well with chalk; but clay, gypsum, or other earths, have no effect.

One earthly substance, found in lead-mines, and commonly called cawk, has a very remarkable effect upon antimony. This is found in whitish, moderately compact, and ponderous masses: it is commonly supposed a spar; but differs from bodies of this kind, in not being acted upon by acids. If a lump of cawk, of an ounce or two, be thrown red hot into 16 ounces of melted antimony, the fusion continued about two minutes, and the fluid matter poured off, "you will have 15 ounces like polished steel, and as the most refined quicksilver." Phil. Trans.: no. 110. Dr Lewis mentions his having repeated this experiment several times with success; but having once varied it by mixing the cawk and antimony together at the first, a part of the antimony was converted into a very dark black vitreous matter, and part seemed to have suffered little change; on the surface of the mass some yellow flowers appeared. Neither the nature of cawk, nor of the change produced on antimony by it, has been hitherto well considered.

Regulus of antimony enters into the compositions for metallic speculums for telescopes, and for printing-types. It is also the basis of many medicinal preparations; but many of these, which were formerly much esteemed, are found to be either inert, uncertain, or dangerous in their operation. When taken in substance, it is emetic and purgative, but uncertain in its operation; because it only acts in proportion to the quantity of solvent matter it meets with in the stomach; and if it meets with nothing capable of acting upon it there, the regulus will be quite inactive. For these reasons, the only two preparations of antimony now retained, at least by skilful practitioners, are the infusion of glas of antimony in wine, and emetic tartar. For making the glas of antimony we have the following process. "Take a pound of antimony; reduce it to fine powder, and let it over a gentle fire; calcine it in an unglazed earthen pan, till it comes to be of an ash colour, and cease to fume: you must keep it continually stirring; and if it should run into lumps, you must powder them again, and then proceed to finish the calcination. When that is done, put the calcined antimony into a crucible; set it upon a tile in a wind-furnace; put a thin tile on the top; and cover it all over with coals. When it is brought into fusion, keep it so in a strong fire for an hour; then put it into an iron rod; and when the melted antimony, which adheres to it, is transparent, pour it upon a smooth, hot, marble; and when it is cold, put it up for use. This is vitrum antimonii, or fibium."

This preparation is more violent in its effects than the pure regulus itself; because it contains less phlogiston, consequently is similar to a regulus partially calcined, and is more soluble. Hence it is the most proper for infusion in wine, or for making the tartar emetic. It is obviously, however, liable to great uncertainties in point of strength; for as the antimony is more or less strongly calcined, the glas will turn out stronger or weaker in its operation, and consequently all the preparations of it must be liable to much uncertainty. This uncertainty is very apparent in the strength of different parcels of emetic tartar: accordingly Mr Geoffroy found by examination of different emetic tartars, that an ounce of the weakest contained from 30 to 90 grains of regulus; an ounce of moderate strength contained about 108 grains; and an ounce of the strongest kind contained 154 grains. For these reasons, the author of the Chemical... mical Dictionary recommends the pulvis algaroth, (no 258.) as the most proper material for making emetic tartar; being perfectly soluble, and always of an equal degree of strength. Emetic tartar, as he justly observes, ought to be a metallic salt composed of cream of tartar saturated with the regulus of antimony; and Mr Beaumé has shown such a saturation to be possible, and that the neutral salt crystallizes in the form of pyramids. They are transparent while moist; but by exposure to a dry air, they lose the water of their crystallization, and become opaque. The preparation of this salt, according to Mr Beaumé, consists in mixing together equal parts of cream of tartar, and levigated glaas of antimony: these are to be thrown gradually into boiling water; and the boiling continued, till there is no longer any effervescence, and the acid is entirely saturated. The liquor is to be filtered; and upon the filter is observed a certain quantity of sulphureous matter, along with some undissolved parts of the glaas of antimony. When the filtered liquor is cooled, fine crystals will be formed in it, which are a soluble tartar perfectly saturated with glaas of antimony. He observes, that the dissolution is soon over if the glaas is well levigated, but requires a long time if it is only grossly pounded.

The trouble of levigating glaas of antimony, as well as the uncertainty of dissolving it, would render pulvis algaroth much preferable, were it not on account of its price; which would be a temptation to those in use to prepare medicines, to substitute a cheaper antimonial preparation in its place.

As regulus of antimony, like other metallic substances, is soluble in liver of sulphur, it happens, that, on boiling antimony in an alkaline ley, the salt, uniting with the sulphur contained in that mineral, forms an heap sulphur, which dissolves some of the reguline part. If the liquor is filtered, and saturated with an acid, the regulus and sulphur will fall together in form of a yellowish or reddish powder, called golden sulphur of antimony. If the ley is suffered to cool, a like precipitation of a red powder happens. This last is called kerme mineral.

Nitre deflagrates violently with antimony, consuming not only its sulphureous part, but also the phlogiston of the regulus; and thus reduces the whole to an inert calx, called antimoniana diaphoretica. If equal parts of nitre and antimony are deflagrated together, the sulphureous part is consumed, as well as part of the inflammable principle of the regulus. The metallic part melts, and forms a semi-vitreous mass, of a reddish colour, called crocus metallicum, or liver of antimony. It is a violent emetic, and was formerly used for making infusions in wine similar to those of glaas of antimony; but is now disfavored on account of its uncertainty in strength. It is still used by the farriers: but the substance sold for it is prepared with a far less proportion of nitre; and sometimes even without any alkaline salt being added to absorb part of the antimonial sulphur. This crocus is of a dull red colour; and when powdered, affumes a dark purple.

11. Arsenic.

This mineral, when in its pure state, has no appearance of a metallic substance. It is moderately heavy, compact, brittle, and of a crystalline and vitreous appearance. Exposed to the air, it changes to a milky hue like that of porcelain; and at length to the opaque whiteness of white enamel. The larger masses preserve their transparency longer than the small, and longer in a dry than in a moist air. In the fire it totally exhales before melting, with a strong smell of garlic. The fumes, caught in proper vessels, condense either into a white powder, or into crystalline masses, as the receiver is more or less removed from the fire.

Arsenic easily unites with all metals and semi-metals, all of which it renders brittle. It renders gold of a greyish colour in its broken surface; silver, of a deep grey; and copper, white. Tin becomes, by mixture with arsenic, much harder, and more unyielding; lead becomes hard and very brittle; and iron is changed into a blackish mass. It volatilizes, vitrifies, and corrosifies, all solid bodies; gold, silver, and platinum, excepted.

We have already taken notice (no 183.) that arsenic neutral salt is capable of decomposing nitre, and uniting with its acid basis. It then forms a singular sort of salt which cannot be decomposed by any acid; because the arsenic, when deprived of phlogiston, (which it perfectly is by the decomposition of the nitre,) seems to have a greater attraction for alkalies than acids have. Was it possible therefore to deprive arsenic of all its phlogiston, we should no doubt find it capable of expelling the marine, or vitriolic, as well as the nitrous acid. It readily unites with alkaline salts, in the common way of fusion: but is then easily separable by an acid; because, not being deprived of its phlogiston, the union between it and the alkali is very weak. This neutral May be decomposed by phlogi-arsenical salt may itself be decomposed, by melting it with inflammable matters, or adding a solution of it to any metallic solution. A double decomposition and combination then take place; the acid unites with the alkali, and the arsenic with the metal. Concerning the uses of this neutral salt, we find the following paragraph in the Chemical Dictionary.

"The uses of the neutral arsenical salt are not yet well determined; yet, as the arsenic seems to be strictly combined with the fixed alkali, this salt may probably be usefully employed. 1. For the preparation of regulus of arsenic. 2. To combine arsenic conveniently with metallic matters. 3. In the composition of many glaases. 4. As the corrosive mineral acids, when saturated with fixed alkali, form very mild salts, we may be induced to believe that arsenic completely saturated with a fixed alkali, as it is in the neutral arsenical salt, might form a very mild salt, which may be powerful in medicine; but the name of arsenic is so terrible, that it will probably never be tried: but if it should, very numerous and long trials ought previously to be made on animals.

"This salt might probably be useful in arts; for Mr Beaumé prepares large quantities of it for different manufactures; but the uses to which it is applied are kept a secret."

In his prediction, however, our author has been mistaken; for a treatise has appeared, recommending, not the neutral arsenical salt, or any milder preparation of this substance, but pure white arsenic itself, as a specific in cancers. This treatise is published by... by M. le Febure, a French physician, who directs the arsenic to be taken in the following manner: "Take four grains* of arsenic, of a clear, white, thinning appearance, and in small crystals; dissolve it by boiling in a pint† of distilled water; let the patient take a table spoonful of this solution, with an equal quantity of milk, and half an ounce of syrup of poppies, every morning fasting, and taking care to taste nothing for an hour after. This course must be continued for eight days, after which a dose is to be taken in the same manner twice every day; the first in the morning, the second towards eight at night. At the end of a fortnight three doses are to be given in a day, the third being taken at mid-day.

"In this manner women of a weakly constitution may continue till the cure is completed. But, with an adult of a good constitution, the dose may be augmented, by degrees, every eight days, till he take five spoonfuls of the solution every day; two tablespoonfuls being taken for each dose, with as much milk, and half an ounce of syrup of poppies. For children, tea-spoons must be used; and the dose should, on no account, exceed three of these, with a proportional quantity of syrup of poppies.

"But besides that the solution of arsenic is thus to be increased to a certain height, in point of quantity, the strength is also to be augmented. Six grains of arsenic may be dissolved in the second bottle of the solution, and eight in the third. But beyond this, our author thinks it unadvisable to proceed. He has, in general, found six bottles of the solution sufficient for the cure of an open cancer. In one case, however, eight were necessary.

"He informs us, that this remedy, taken with the above precautions, never occasions any unlucky accident; and is not disagreeable to the taste. It does not act in any certain manner upon the secretions or excretions. Some, indeed, discharge their urine more freely than usual; with some the belly is more loose; and with others the perspiration is more copious; but these effects are neither general nor constant.

"A purgative compounded of manna, rhubarb, and sal reginae, is to be given every eight or twelve days. Whey, with twelve grains of nitre to the bottle, or a weak decoction of the root of senna, with an equal quantity of nitre, is to be used for common drink. With respect to regimen, it is necessary to abstain from wine and fermented liquors. Broth made with a little veal, beef, or chicken, &c., are proper.

"M. le Febure has sometimes been obliged to give the Peruvian bark, and to open an issue, when the humours were either very alkaline, or in very great quantity. He even considers an issue as useful in every case.

"Besides this treatment by internal medicines, he recommends, that the tumour, if not ulcerated, should be washed with a solution of arsenic, in the proportion of eight grains to a pint; and he advises the following cataplasm. Take of carrot-juice, one pound; of sugar of lead, half an ounce; of arsenic, dissolved in distilled vinegar, half an ounce; of liquid laudanum, a drachm and an half: form the whole into a mass, with as much powder of hemlock as is necessary. With part of this cataplasm the tumour is to be covered to a tolerable thickness, and the whole kept on with a diachylon platter.

"If the cancer is of the ulcerated kind, he advises that the ichorous ferocity be taken away at each dressing, by means of dry charpee. He then directs the ulcer to be fomented with the arsenical solution, having the chill taken off it, and having about a third part of red wine added to it. If the sore is of a very bad kind, he proposes that the arsenic be dissolved in a decoction of bark, for fomenting the ulcer. After this, the cataplasm mentioned above, and the plaster, are to be applied. This treatment must be renewed every twelve hours.

"Mr. le Febure, before he concludes this treatise, affirms his readers, that, in more than two hundred instances, he has had proofs of the efficacy of the medicine here proposed. He does not, however, pretend that it is infallible in every case. He considers the disease to be incurable, if, in its progress, a considerable hemorrhage has happened, from the erosion of large blood-vessels; also when it attacks hectic or phthisitical patients. To judge of the efficacy of any remedy, he observes, that the patient with whom it is tried should at least enjoy an ordinary good constitution, and be free from a complication of diseases. And he considers the exhibition of a new remedy to a patient, in some measure, breathing his last, as serving no other purpose but to bring it into discredit."

Edin. Med. Comment. vol. IV. p. 56.—61.

Whatever good effects arsenic may produce when given in this way, certain it is, that this substance is, of all others, the most poisonous, and most certainly fatal, if taken into the human body, even in a small quantity. It seems to act not only upon the stomach and intestines, but to produce also a very great tendency to dissolution in the blood itself; for those who die poisoned with arsenic, are generally covered over with red or purple spots. When arsenic is swallowed, a nausea, sickness, and reaching, commonly ensue in about half an hour. These are followed by violent vomitings, hiccups, and pains in the stomach and bowels. Convulsions and pallies of the limbs prefently succeed, with intense heats, cold sweats, palpitations of the heart, extreme anxiety, restlessness, prostration of strength, thirst, and dryness of the mouth and throat, loss of reason, and at last death. If the quantity taken was considerable, the patient dies in seven or eight hours after taking it; and the stomach and intestines are found, upon dissection, to be corroded or perforated.

Arsenic is a poison the most certainly fatal and most difficult to be cured of any, (if we except, perhaps, large doses of antimonial emetics,) on account of its being difficultly soluble in water, and incapable of decomposition. Corrosive sublimate mercury, solution of mercury in aqua fortis, &c., will as certainly poison as arsenic itself; but they are by no means so dangerous; because, being compounded of quicksilver united with an acid, any alkaline substance will infallibly decompose. Practice compound and destroy the poison, so that scarcely any bad effects can happen but what arise from its first action on the stomach. Arsenic, on the contrary, cannot be decomposed, nor united with any known substance, without a considerable degree of heat. It therefore remains in the stomach, continually exerting its mischievous qualities, till it is all discharged by vomiting.

Many antidotes have been proposed against this pernicious mineral; but, that they might be rationally put confidence in, it should first be demonstrated, that they can make a change upon arsenic in substance, or in solution, in a heat no greater than that of the human body. Alkalies, which have been directed, cannot unite with arsenic when in the stomach. Acids, which have been ordered on a contrary supposition, will indeed dissolve arsenic; but the solution, in all probability, would prove a more violent poison than the arsenic itself. Oils, fats, warm fat broths, fresh butter, or milk, are recommended as the most proper means of obviating the poison, and promoting its discharge by vomit; and indeed in such deplorable cases they are the only remedies to which we can apply; though it is evident the efficacy even of these must be exceedingly uncertain; and for this plain reason, that the arsenic is already in contact with the stomach; and though they might prevent the action of the poison if they had been first swallowed, their operation must be exceedingly less efficacious after the poison has had access to the stomach, and begun to exert its virulent effects upon it.

The best method of giving arsenic the metallic form, or changing it into a regulus, as it is called, is by mixing it, when powdered, with oil-olive, so as to form a patte; the mixture is to be put into a retort, or glass matrais, and to be distilled, or sublimed, with a fire at first very moderate, and sufficient to raise only the oil. After the oil has penetrated the arsenic, its more fluid parts exhale, and it remains in form of a charred coal. Then the fire is to be increased, and the metallized arsenic soon sublimes to the top. When no more sublimes, the vessel is to be broke, and the adhering crust of regulus of arsenic separated. The regulus must be sublimed a second, or even a third time, in order to give it as perfect a metallic form as it can receive. The oil, which arises during this operation, is more fetid than any empyreumatic oil, and almost intolerable.

Regulus of Cobalt and Nickel. See Metallurgy.

Sect. IV. Inflammable Substances.

These may be divided into the following classes:

1. Sulphurs. 2. Ardent spirits. 3. Oils and fats. 4. Resins. 5. Bitumens; and, 6. Charcoal.

I. Sulphurs.

1. Common sulphur. For the extraction of this substance from its ores, see Sulphur. The artificial composition of it we have already related, n° 165.; and have now only to take notice of a very few of its properties, which come more properly under this section.

Sulphur, as commonly used in commerce and the arts, is of a pale yellow colour, of a disagreeable and peculiar smell, which is rendered more sensible when it is heated or rubbed. By rubbing, it receives very curious electrical qualities: (See Electricity.) Its specific gravity is considerably greater than that of water, though less than earths or stones. In close vessels, sulphur is incapable of receiving any alteration. It melts with a very gentle heat; and then is sublimed, adhering to the capital in small, very fine, needle-like crystals, called flowers of sulphur. It may thus be sublimed many times without alteration. If sulphur is exposed to a heat barely sufficient to melt it, and, very slowly cooled, it crystallizes in form of many needles crossing one another. Some of these pointed crystals may also be observed in the interior parts of the lumps of sulphur which have been melted, and cast into cylindrical moulds, as they are commonly sold; because the center of these cylindrical rolls is more slowly cooled than the surface. Sulphur also gives this needle-like form to cinnamon, antimony, and many other minerals containing it. Sulphur may be decomposed in several ways. The most simple is by burning; which we have already taken notice of, n° 118. It may also be very effectually decomposed by mixing it with iron filings and water. In this case the phlogiston is dissipated, and the acid uniting with the iron forms a green vitriol.

It is very remarkable, that though sulphur is composed of vitriolic acid and phlogiston, yet the addition of more inflammable matter, so far from making the union stronger, weakens it to a great degree: and hence we have another method of decomposing this substance; namely, by combining it with a large quantity of oil, and distilling the compound.

Sulphur is capable of being easily dissolved in expressed oils, but very difficultly in essential ones. These compositions are called balsams of sulphur; and are sometimes employed in medicine, but are found to be of a very heating nature. They are much used by farriers. According to Mr Beaumé, sulphur cannot be dissolved in oil, without a heat sufficient to melt it. A larger quantity is kept dissolved when the mixture is hot, than when cold; and consequently, the sulphur, especially if it has been dissolved in a thin essential oil, crystallizes on cooling the mixture. The sulphur, thus separated from the oil, is found not to be altered in any respect from what it formerly was; but if the mixture is exposed to a degree of heat capable of entirely decomposing the oil, the sulphur is decomposed along with it, and the same products are obtained by distilling this mixture to dryness, as if a mixture of pure oil of vitriol and oil were distilled. These products are, first a portion of oil, when an essential oil was made use of in the composition of the balsam; then some volatile sulphureous acid, which is at first watery, and afterwards becomes stronger; along with this acid more oil arises, which becomes more and more thick towards the end of the distillation; and lastly, when the retort has been made red hot, nothing remains but a fixed coal.

In this process we find, that both the sulphur and oil are decomposed. The acid of the sulphur seems to attack the watery principle of the oil, while its phlogiston remains confounded with that of the oil, or is dissipated in vapours.—Hence, though the vitriolic acid in sul- phur is concentrated to the utmost degree, and perfectly free from water, what rises in this distillation is very aqueous, by reason of the water which it attracts from the oil.

Spirit of wine does not sensibly act upon sulphur in its liquid state; but if both the spirit of wine and sulphur meet in the state of vapour, they will then unite, and a perfect solution will take place. By methods of this kind, many combinations might be effected, which have been hitherto thought impossible.

Pure sulphur unites easily with all metals; gold, platinum, and zinc, excepted. The compounds, except that with mercury, possess a metallic lustre without any ductility. The sulphur may be separated by exposing the mixture to a strong fire, (see Metallurgy,) or by dissolving the metallic part in acids. The sulphur, however, defends several of the metals from the action of acids; so that this dissolution succeeds but imperfectly. The reguline part of antimony is more easily separated from sulphur by means of acids, than any other metallic substance. Alkaline salts will separate the sulphur from all metals in fusion, but they unite with them themselves, and form a compound equally capable of dissolving the metal. (See Alkaline Salts.)

Sulphur united with quicksilver, forms the beautiful pigment called cinnabar, or vermillion; which is so much used in painting, that the making of it is become a distinct trade. Neuman relates, that in the making of cinnabar by the Dutch method, six or eight parts of quicksilver are made use of to one of sulphur. The sulphur is first melted; and then the quicksilver is flung into it; upon which they unite into a black mass. In this part of the process the mixture is very apt to take fire; of which it gives notice by swelling up to a great degree. The vessel must then be immediately covered. The mass being beaten to powder, is afterwards to be sublimed in large earthen jars almost of an equal wideness from end to end; these are hung in a furnace by a strong rim of iron. When the matter is put in, the mouth of the vessel is covered, the fire increased by degrees, and continued for several hours, till all the cinnabar has sublimed; care being taken to introduce at times an iron-rod to keep the middle clear; otherwise the cinnabar concreting there, and stopping up the passage, would infallibly burst the vessels.

The quantity of sulphur directed in the common receipts for making cinnabar is greatly larger than the above; being no less than one-third of the quantity of quicksilver employed: accordingly it has been found, that the sublimate, with such a large quantity of sulphur, turned out of a blackish colour, and required to be several times sublimed before it became perfectly red; but we cannot help thinking, that by one gentle sublimation the superfluous sulphur might be separated, and the cinnabar become perfectly pure the second time. Hoffman gives a curious method of making cinnabar without sublimation; by shaking, or digesting a little mercury with volatile tincture of sulphur: the mercury readily imbibes the sulphur from the volatile spirit, and forms with it a deep red powder, not inferior in colour to the cinnabar prepared in the common manner. Dr Lewis has found the common solutions of sulphur by alkalies, or quicklime, to have a similar effect. This cinnabar will likewise be of a darker or lighter colour, according as the solution contains more or less sulphur.

Sulphur is a principal ingredient in gun-powder, Pulvis fulvus (see Gun-powder.) It also enters the composition of the pulvis fulminans. This consists of three parts of nitre, two of the dry alkali of tartar, and one part of sulphur, well ground together. If a little quantity of this powder is laid on an iron-spoon, or shovel, and slowly heated, it will explode, when it arrives at a certain degree of heat, with astonishing violence and noise. The most probable opinion concerning this is, that the fixed air contained in the alkali is, by the acid vapours acting upon and endeavouring to expel it all at once, driven off with such force, that a loud explosion is produced.

2. Phosphorus of Urine. This is a very inflammable substance, composed of phlogiston united with a certain acid, the properties of which we have already taken notice of, n° 307—310. The preparation of it was long a secret; and only perfectly discovered by Mr Margraaff, who published it in the Berlin Memoirs in 1743. This process being by far the best, and most practicable, we shall content ourselves with inferring it alone.

Two pounds of sal ammoniac are to be accurately mixed with four pounds of muriatic, and the mixture distilled in a glass-retort; by which means a very penetrating, caustic alkaline spirit will be obtained. The residuum, after the distillation, is a kind of plumbum corrosive; n° 149. This is to be mixed with nine or ten pounds of extract of urine, evaporated to the consistence of honey. (Seventy or eighty gallons of urine are required to produce this quantity of extract.) The mixture is to be made slowly in an iron pot set over the fire, and the matter frequently stirred. Half a pound of powdered charcoal is then to be added, and the evaporation continued till the whole is reduced to a black powder. This powder is to be put into a retort; and urged with a graduated heat, till it becomes red hot, in order to expel all the volatile alkali, fetid oil, and ammoniacal salt, that may be contained in the mixture. After the distillation, a black friable residuum remains, from which the phosphorus is to be extracted by a second distillation, and a stronger heat. Before it is subjected to another distillation, it may be tried by throwing some of it upon hot coals. If the matter has been well prepared, a smell of garlic exhales from it, and a blue phosphorical flame is seen undulating along the surface of the coals.

The matter is to be put into a good earthen retort, capable of sustaining a violent fire. Three quarters of the retort are to be filled with the matter which is to yield the phosphorus, and it is to be placed in a furnace capable of giving a strong heat. Mr Margraaff divides the matter among five retorts, so that if any accident happens to one, the whole matter is not lost. The retorts ought to be well fitted to a receiver of moderate size, pierced with a small hole, and half full of water; and a small wall of bricks must be raised between the furnace and receiver, in order to guard this vessel against heat, as much as possible. The retorts are to be heated by slow degrees for an hour and a half; then the heat is to be increased till the vessels are... are red hot, when the phosphorus ascends in luminous vapours. When the retort is heated till between a red and white, the phosphorus palls in drops, which fall and congeal in the water at the bottom of the receiver. This degree of heat is to be continued till no more comes over. When a retort contains eight pints or more, this operation continues about five hours.

In the first distillation, phosphorus never palls pure, but is always of a blackish colour, by reason of its carrying along with it some part of the coal. From this, however, it may be purified, by rectification in a small glass-retort, to which is fitted a receiver half full of water. A very gentle heat is sufficient; because phosphorus, once formed, is very volatile; and as the fuliginous matter was raised probably by the fixed air emitted by the charcoal, in the infant of its union with the phosphoric acid, none of it can arise in a second distillation.

The phosphorus is then to be divided into small cylindrical rolls, which is done by putting it in glass-tubes immersed in warm water; for the phosphorus is almost as fusible as fat. It takes the form of the glass-tubes; from which it may be taken out, when it is cold and hardened. This must be done under water, lest the phosphorus should take fire.

This concrete continually appears luminous in a dark place; and by a very slight heat takes fire, and burns far more vehemently than any other known substance. Hence, it is necessary to be very cautious in the distillation of it; for if the receiver should happen to break while the phosphorus is distilling, and a little flaming phosphorus fall upon the operator's legs or hands it would burn its way to the bone, in less than three minutes. In this case, according to Mr Hellot, nothing but urine will stop its progress.

Though phosphorus takes fire very readily by itself, it does not inflame at all by grinding it with other inflammable bodies, as camphor, gun-powder, or essential oils. In grinding it with nitre, some luminous flashes are observed; but the mixture never burns, unless the quantity of phosphorus be large in proportion to the nitre; rubbed pretty hard on a piece of paper or linen, it sets them on fire if they are rough, but not if they are smooth. It fires written paper more readily than such as is white, probably from the former having more aliphanes. On grinding with iron-filings, it presently takes fire.

Oils ground with phosphorus, appear, like itself, luminous in a temperately warm place; and thus become a liquid phosphorus, which may be rubbed on the hands, &c., without danger. Liquid phosphorus is commonly prepared by grinding a little of the solid phosphorus with oil of cloves, or rubbing it first with camphor, and this mixture with the oil. A luminous amalgam, as it is called, may be obtained, by digesting a scruple of solid phosphorus with half an ounce of oil of lavender, and, when the phosphorus begins to dissolve and the liquor to boil, adding a drachm of pure quicksilver; then briskly shaking the glass for five or six minutes, till they unite.

Rectified spirit of wine, digested on phosphorus, extracts a part of it, so as to emit luminous flashes on being dropped into water. It is computed that one part of phosphorus will communicate this property to practice 600,000 parts of spirit. The liquor is never observed to become luminous of itself, nor in any other circumstance except that above mentioned. By digestion for some months, the undissolved phosphorus is reduced to a transparent oil, which neither emits light, nor concretes in the cold. By washing with water, it is in some measure revived; acquiring a thicker consistence, and becoming again luminous, though in a less degree than at first. During this digestion, the glass is very apt to burst.

Phosphorus is partially dissolved by expressed oils; with essential oils and acids. When essential oils are saturated with it by heat, a part of the phosphorus separates, on standing in the cold, in a crystalline form. Concentrated spirit of salt has no action on it. In distillation, the spirit rises first, and the phosphorus after it unchanged. Spirit of nitre dissolves it, and the dissolution is attended with great heat and copious red fumes, so that great part of the spirit distils without the application of any external heat, and the phosphorus at last takes fire, explodes, and bursts the vessels. Oil of vitriol, likewise, dissolves phosphorus, but not without a heat sufficient to make the acid distil. The distilled liquor is white, thick, and turbid; the residuum is a whitish tenacious mass, which deliquesces, but not totally, in the air. Phosphorus itself is resolved into an acid liquor on being exposed two or three weeks to the air, its inflammable principle seeming by degrees to be dissipated.

Phosphorus has been reported to produce extraordinary effects in the resolution of metallic bodies; but from the experiments that have been made with this view, it does not appear to have any remarkable action on them; at least on the precious ones, gold and silver, for the resolution or sublimation of which it has been chiefly recommended. The following experiments were made by Mr Margraaff.

1. A scruple of filings of gold were digested with a drachm of phosphorus for a month, and then committed to distillation. Part of the phosphorus arose, and part remained above the gold, in appearance resembling glass; this grew moist on the admission of air, and dissolved in water, leaving the gold unaltered. Half a drachm of fine silver, precipitated by copper, being digested with a drachm of phosphorus for three hours, and the fire then increased to distillation, greatest part of the phosphorus arose pure, and the silver remained unchanged. Copper filings being treated in the same manner, and with the same quantity of phosphorus, the phosphorus sublimed as before; but the remaining copper was found to have lost its metallic brightness, and to take fire on the contact of flame. Iron filings suffered no change. Tin filings run into granules, which appeared to be perfect tin. Filings of lead did the same. The red calx of mercury, called precipitate per se, treated in the same manner, was totally converted into running quicksilver.

2. Regulus of antimony suffered no change itself, but occasioned a change in the consistence of the phosphorus, which, after being distilled from this semi-metal, refused to congeal, and continued under water, fluid like oil-olive. With bismuth there was no alteration. A drachm of phosphorus Practice phosphorous being distilled and cohabited with an equal quantity of zinc, greatest part of the zinc sublimed in form of very light pointed flowers, of a red-dish-yellow colour; these flowers, injected into a red hot crucible, took fire, and run into a glass resembling that of borax. White arsenic, sublimed with phosphorus, arose along with it in form of a mixed red sublimate. Sulphur readily unites with phosphorus, into a mass which smells like baper sulphuris. This does not easily take fire on being rubbed; but, exposed to a moderate dry heat, it flames violently, and emits a strong sulphureous fume. If phosphorus is burnt in an open vessel, a quantity of acid remains behind; and if a glass bell is held over it, an acid likewise sublimes in the form of white flowers.

3. Mr Canton's phosphorus. This is a composition of quicklime and common sulphur. The receipt for making it is as follows. "Calcine some common oyster-shells, by keeping them in a good coal-fire for half an hour; let the purest part of the calc be pulverized and sifted. Mix with three parts of this powder, one part of flowers of sulphur. Let this mixture be rammed into a crucible of about an inch and a half in depth, till it be almost full; and let it be placed in the middle of the fire, where it must be kept red hot for one hour at least; and then set by to cool: when cold, turn it out of the crucible; and cutting or breaking it to pieces, scrape off, upon trial, the brightest parts; which, if good phosphorus, will be a white powder." This kind of phosphorus shines on being exposed to the light of the sun, or on receiving an electrical stroke.

4. Phosphorus of Homberg. This substance, which has the singular property of kindling spontaneously when exposed to the air, was accidentally discovered by Mr Homberg, as he was endeavouring to distil a clear flavourless oil from human excrements. Having mixed the excrement with alum, and distilled over as much as he could with a red heat, he was much surprised at seeing the matters left in the retort take fire upon being exposed to the air, some days after the distillation was over. This induced him to repeat the operation, in which he met with the same success; and he then published a process wherein he recommended alum and human excrement for the preparation of the phosphorus. Since his time, however, the process has been much improved; and it is discovered, that almost every vitriolic salt may be substituted for the alum, and most other inflammable substances for the excrement; but though alum is not absolutely necessary for the success, it is one of the vitriolic salts that succeed best. The following process is recommended in the Chemical Dictionary.

Let three parts of alum and one of sugar be mixed together. This mixture must be dried in an iron shovel, over a moderate fire, till it be almost reduced to a blackish powder or coal; during which time it must be stirred with an iron spatula. Any large masses must be bruised into powder; and then it must be put into a glass matras, the mouth of which is rather frail than wide, and seven or eight inches long. This matras is to be placed in a crucible, or other earthen vessel, large enough to contain the belly of the matras, with about a space equal to that of a finger all round it. This space is to be filled with sand, so that the matras shall not touch the earthen vessel. The apparatus is then to be put into a furnace, and the whole to be made red hot. The fire must be applied gradually, that any oily or fuliginous matter may be expelled; after which, when the matras is made red hot, sulphurous vapours exhale: this degree of heat is to be continued, till a truly sulphureous flame, which appears at the end of the operation, has been seen nearly a quarter of an hour: the fire is then to be extinguished, and the matras left to cool, without taking it out of the crucible; when it ceases to be red hot, it must be stopped with a cork. Before the matras is perfectly cold, it must be taken out of the crucible, and the powder it contains poured as quickly as possible into a very dry glass vial, with a glass stopper. If we would preserve this phosphorus a long time, the bottle containing it must be opened as seldom as possible. Sometimes it kindles while it is pouring into the glass vial; but it may be then extinguished by closing the vial expeditiously. A small quantity of this pyrophorus laid on paper, and exposed to the air, immediately takes fire, becomes red like burning coals, and emits a strong sulphureous vapour greatly resembling that which arises on decomposing liver of sulphur.

The most plausible theory of this strange appearance is, that, during the operation, part of the vitriolic acid combines with the phlogiston of the coal, into perfect sulphur; while part remains imperfectly combined either with the phlogiston, or the earthy basis of the alum. This last part, which is also exceedingly concentrated, probably attracts the moisture of the air so strongly, as to produce the heat requisite for kindling the coaly matter.

II. ARDENT SPIRITS.

See Fermentation and Distillation.

III. OILS.

1. Essential Oils. Those oils are called essential which have evidently the smell of the vegetable from which they are drawn. For the method of procuring them, see Distillation. They are distinguished from all others by their superior volatility, which is so great as to cause them rise with the heat of boiling water. All these have a strong aromatic smell, and an acid, caustic taste; in which respect also they differ from other oils. This taste is thought to proceed from a supposed copious and disengaged acid, with which they are all alike penetrated. The presence of this disengaged acid in essential oils, appears from the impression they make upon the corks of bottles in which they are kept. These corks are always stained of a yellow colour, and a little corroded, nearly as they are by nitrous acid. The vapour of these oils also reddens blue paper, and converts alkalies into neutral salts.

This acid is likewise supposed to be the cause of their solubility in spirit of wine. They are not all equally soluble in this menstruum, because they do not all contain an equal quantity of acid. As this acid is much disengaged, they lose a great deal of it by repeated distillations, and therefore they become less and less soluble on being frequently distilled. By evaporation they lose their most volatile and thin part, in which the specific smell of the vegetable from which which they are extracted resides; by which loss they become thick, and acquire the smell and consistence of turpentine, and even of resins. In this state they are no longer volatile with the heat of boiling water; and, if distilled with a stronger fire, they give over an oil which has neither smell nor taste of the vegetable whence they were extracted, but is entirely empyreumatic, and similar to those oils procured by distilling vegetable or animal substances with a strong fire.

See Distillation.

To the class of essential oils, the volatile concrete called camphor seems most properly to belong. With them it agrees in its properties of inflammability, solubility in spirit of wine, and a strong aromatic flavour. The only differences between them are, that camphor is always in a solid state, and is incapable of decomposition by any number of sublimations.

According to Neuman, all the camphor made use of is the produce of two species of trees; the one growing in Sumatra and Borneo, the other in Japan. Of these, the Japan kind is the only one brought into Europe. The tree is about the size of a large lime, the flowers white, and the fruit a small red berry. All parts of the tree are impregnated with camphor; but the roots contain most, and therefore are chiefly made use of for the preparation of this commodity; though, in want of them, the wood and leaves are sometimes mixed.

The camphor is extracted by distillation with water in large iron pots filled with earthen heads stuffed with straw; greatest part of the camphor concretes among the straw, but part passes down into the receiver among the water. In this state it is found in small bits like gray salt-petre, or common bay-fall; and requires to be purified either by a second sublimation, or by distillation in spirit of wine, filtration, and evaporation. If the first method is followed, there will be some difficulty in giving it the form of a perfect transparent cake. A difficulty of this kind indeed always occurs in sublimations, and the only way is to keep the upper part of the glass of such a degree of heat as may keep the sublimate in a half-melted state. Dr Lewis recommends the depuration of camphor by spirit of wine, and then melting it into a cake in the bottom of a glass.

Camphor possesses considerable antiseptic virtues; and is a good diaphoretic without heating the constitution, with which intention it is often used in medicine. It is likewise employed in fire-works and several other arts, particularly in making varnishes. See Varnish.

This substance dissolves easily and plentifully in various spirits and in oils; four ounces of spirit of wine will dissolve three of camphor. On distilling the mixture, the spirit rises first, very little camphor coming over with it. This shows that camphor, however volatile it may seem by its smell, is very far from having the volatility of ether, and consequently is improperly classified with substances of that kind. It is dissolved, but not altered in the least, by the strongest mineral acids; always separating from them in its proper form, on the addition of water. It may however be changed into a fluid oil by repeated distillations from bone or other loamy earths.

2. Empyreumatic Oils. Under this name are comprehended all those oils, from whatever substance obtained, which require a greater heat for their distillation than that of boiling water. These are partially soluble in spirit of wine, and become more and more so by repeated distillations. The empyreumatic oils obtained from animal substances are at first more fetid than those procured from vegetables; but by repeated distillations, they become exceedingly attenuated and volatile, becoming almost as white, thin, and volatile, as ether. They then acquire a property of acting upon the brain and nervous system, and of allaying its irregular movements, which is common to them with all other inflammable matters when highly attenuated and very volatile; but this kind of oil is particularly recommended in epileptic and convulsive affections. It is given from four to ten or twelve drops; but, though prepared with the utmost care, it is very susceptible of losing its whiteness, and even its thinness, by a short exposure to air; which proceeds from the almost instantaneous evaporation of its more thin and volatile parts, and from the property which the less volatile remainder has of acquiring colour. To avoid this inconvenience, it must be put, as soon as it is made, into very clean glass bottles with glass stoppers, and exposed to the air as little as possible.

The most important observations concerning the method of making the pure animal oil are, first to change the vessels at each distillation, or at least to make them perfectly clean; for a very small quantity of the thicker and less volatile part is sufficient to spoil a large quantity of that which is more rectified. In the second place, Mr Beaumé has observed, that this operation may be greatly abridged, by taking care to receive none but the most volatile part in each distillation, and to leave a large residuum, which is to be neglected, and only the more volatile part to be further rectified. By this method a considerable quantity of fine oil may be obtained at three or four distillations, which could not otherwise be obtained at fifty or sixty.

3. Animal Fats. Though these differ considerably from one another in their external appearance, and probably in their medicinal qualities, they afford, on a chemical analysis, products similar in quality, and differing but inconsiderably in quantity. They all yield a large proportion of oil, and no volatile salts; in which respect they differ from all other animal substances. Two ounces of hog's lard yielded, according to Neuman, two drachms of an empyreumatic liquor, and one ounce five drachms and thirty grains of a clear brown-coloured oil of a volatile smell, somewhat like horseradish. The caput mortuum was of a shining black colour, and weighed ten grains.

Tallow being distilled in the same manner, two drachms of empyreumatic liquor were obtained from two ounces of it; of a clear brown oil, smelling like horseradish, one ounce six drachms and twelve grains. The remaining coal was of a shining black colour, and weighed eighteen grains.

The marrow of bones differs a little from fats; Marrow when chemically examined. Four ounces of fresh marrow, distilled in the usual manner, gave over three drachms and a scruple of a liquor which smelled like tal- low; two scruples and an half of a liquor which had more of an empyreumatic and a fourill finell; two ounces and an half of a yellowish-brown, butyraceous oil, which finelled like horse-radish; and fix drachms and an half of a blackish-brown oil of the same finell. The caput mortuum weighted four scruples.

All animal fats, when perfectly pure, burn totally away without leaving any feces, and have no particular finell. In the flate in which we commonly find them, however, they are exceedingly apt to turn rancid, and emit a most disagreeable and noxious finell; and to this they are peculiarly liable, when long kept in a gentle degree of heat. If this flate, too, an inflammable vapour arises from them, which when on fire is capable of producing explosions. Hence, in those works where large bellows are used, they have been often suddenly burst by the inflammable vapours arising from the rancid oil employed for softening the leather. The expressed unctuous oils of vegetables are subject to the same changes: but from this rancidity they may all be freed most effectually, by the simple process of agitating them well with water; which is to be drawn off, and fresh quantities added, till it comes off, at last, clear and insipid, without any ill finell. The proper instrument for performing this operation in large, is a barrel-churn, having in it four rows of narrow split deals, from the center to the circumference, each piece set at obtuse angles to the other, in order to give different directions to the oil and water as the churn turns round, thereby to mix them more intimately. The churn is to be swiftly turned round for a few minutes; and must then be left at rest, till the oil and water have fully separated; which will be in 15 or 20 minutes, more or less, according to the size of the churn. When this water is drawn off, fresh water is to be put in, and the churn again turned round, and this continued till the oil is perfectly sweet. If the oil and water are allowed to stand together for some days, a gelatinous substance is found between them, which is not very easily miscible either with oil or water. Chalk, quicklime, and alkaline salts, are found also capable of taking off the rancidity from oils and fats; but have the inconvenience of destroying a part of their substance.

IV. RESINS AND BALSAMS.

These are commonly reckoned to be composed of an essential oil thickened by an acid; as the essential oils themselves are found to be convertible into a similar substance, by the exhalation of their more volatile parts. True resins are generally transparent in a considerable degree; soluble in spirit of wine; and possessed of a considerable degree of flavour.

Resins are originally produced by insufflating the natural juices which flow from incisions made in the stems of growing vegetables, and are in that state called balsams. The balsams may be considered as essential oils thickened by losing some of their odoriferous principle, and of their finest and most volatile part. There are several kinds of balsams; which, however, differ from each other only in the finell, and degree of consistence; and, therefore, all yield similar products on distillation. An analysis of turpentine therefore will be sufficient as an example of the analysis and natural properties of all the rest.

The true turpentine-tree is found in Spain and the southern parts of France, as well as in the island of Chio and in the Indies. It is a middling sized evergreen tree, with leaves like those of the bay, bearing paraphyll, imperfect flowers; and, on separate pedicles, hard, unctuous berries, like those of juniper. It is extremely resinous; and, unless the resin is discharged, decays, produces fungous excrescences, swells, bursts, and dies; the prevention of which consists wholly in plentiful bleeding, both in the trunk and branches. The juice is the Chio or Cyprus turpentine of the shops. This fort is quite of a thick consistence, of a greenish white colour, clear and transparent, and of scarcely any taste or smell.

The kind now called Venice turpentine, is no other than a mixture of eight parts of common yellow or black rosin with five parts of oil of turpentine. What was originally Venice turpentine is now unknown. Neuman relates, that the Venice turpentine sold in his country was no other than that prepared from the larix tree, which grows plentifully in some parts of France, as also in Austria, Tyrol, Italy, Spain, &c. Of this there are two kinds; the young trees yielding a thin limpid juice, resembling balm of copaiba; the older, a yellower and thicker one.

The Straßburg turpentine is extracted from the Straßburg silver-fir. Dr Lewis takes notice that some of the exotic firs afford balms, or resins, superior to those obtained from the native European ones; as particularly that called balm of Gilead fir, which is now naturalized to our own climate. A large quantity of an elegant resinous juice may be collected from the cones of this tree: the leaves also, when rubbed, emit a fragrant smell; and yield, with rectified spirit, an agreeable resinous extract.

The common turpentine is prepared from different forts of the pine; and is quite thick, white, and opaque. Even this is often counterfeited by mixtures of rosin and common expressed oils.

All the turpentines yield a considerable proportion of essential oil. From sixteen ounces of Venice turpentine, Neuman obtained, by distillation with water, four ounces, and three drachms of oil. The same quantity distilled, without addition, in the heat of a water bath, gave but two ounces and an half; and from the residuum treated with water, only an ounce could be obtained. The water remaining in the still is found to have imbibed nothing from the turpentine: on the contrary, the turpentine is found to imbibe part of the water; the residuum and the oil amounting to a full ounce on the pound more than the turpentine employed. When turpentine is distilled, or boiled with water till it becomes solid, it appears yellowish; when the process is further continued, of a reddish brown colour: in the first state, it is called boiled turpentine; and in the latter, colophony, or resin.

On distilling sixteen ounces of turpentine in a retort with an open fire, increased by degrees, we obtain first four ounces of a limpid colourless oil; then two ounces and two drachms of a yellowish one; four ounces and three drachms of a thicker yellow oil; and two ounces and one drachm of a dark brownish This red empyreumatic oil, of the consistence of balsam, and commonly called balsam of turpentine.

The limpid essential oil called spirit of turpentine, is exceedingly difficult of solution in spirit of wine; though turpentine itself dissolves with great ease. One part of the oil may indeed be dissolved in seven parts of rectified spirit; but, on standing for some time, the greatest part of the oil subsides to the bottom, a much greater proportion of spirit being requisite to keep it dissolved.

2. Benzoin. This is a very brittle brownish resin, of an exceedingly fragrant smell. The tree which produces benzoin is a native of the East Indies; particularly of Siam, and the island of Sumatra. It is never permitted to exceed the fifth year; being, after this time, unfit for producing the benzoin. It is then cut down, and its place supplied by a young tree raised commonly from the fruit. One tree does not yield above three pounds of benzoin.

A tree supposed to be the same with that which affords benzoin in the East Indies, is plentiful also in Virginia and Carolina; from whence it has been brought into England, where it grows with vigour in the open ground. The bark and the leaves have the smell of benzoin; and yield with rectified spirit a resin of the same smell: but no resin has been observed to issue from it naturally in this climate; nor has any benzoin been collected from it in America.

Benzoin dissolves totally in spirit of wine into a blood-red liquor, leaving only the impurities, which commonly amount to no more than a scruple on an ounce. To water, it gives out a portion of saline matter of a peculiar kind, volatile and sublimable in the fire; and which is most effectually freed by sublimation.

This substance, called flowers of benzoin, is best prepared by moistening benzoin, grossly powdered, with spirit of wine; and then proceeding to distillation with a very gentle heat, in a wide-necked glass-retort. The flowers arise immediately after the spirit, partly in a concrete saline form, and partly in that of a white butter. The receiver being now changed, and the fire increased, a small portion of brown-coloured flowers sublimes; followed, first, by a subtile oil; afterwards by a brownish oil; and last of all by a black, thick, empyreumatic one, together with an acid spirit. If the flowers and butter be dissolved in distilled water, over a gentle fire, the solution filtered, and set in the cold, the saline matter floats into crystalline concretions of a fine silver whiteness; this salt-like tartar, being difficult of solution; and, when dissolved in hot water, separating again as the liquor cools. The distillation and filtration should be performed as expeditiously, and the vessel kept as much covered, as possible, to prevent any considerable dissipation of the volatile matter. The salt still retains, even after this purification, a portion of oil; as appears from its penetrating smell, and from its burning in the fire. The spirit of wine, which arises at first in the distillation, is impregnated with a little of the salt. The oil which follows the flowers, re-distilled from earthy powders, or with water, may be used as an essential oil of benzoin; for it has little or nothing of an empyreumatic taint. From 16 ounces of benzoin are obtained two ounces of rough flowers, nine ounces of oil, and seven scruples of an acid spirit. The residuum weighs two ounces and an half.

The principal use of resins is in the making of lacquers, varnishes, &c. See Varnish.

V. BITUMENS.

These are inflammable mineral bodies, not sulphurous, or only casually impregnated with sulphur. They are of various degrees of consistence; and seem, in the mineral kingdom, to correspond with the oils and resins in the vegetable.

Concerning the origin of bitumens, chemists are not at all agreed. Some chemical writers, particularly Mr Macquer, imagine bitumens to be no other than vegetable resins altered in a particular manner by the admixture of some of the mineral acids in the earth; but Dr Lewis is of a contrary opinion, for the following reasons.

"Mineral bitumens are very different in their qualities from vegetable resins; and, in the mineral kingdom, we find a fluid oil very different from vegetable oils. The mineral oil is changed by mineral acids into a substance greatly resembling bitumens; and the vegetable oils are changed by the same acids into substances greatly resembling the natural resins." (Here we cannot help differing from the Doctor, as we have never seen or heard of any instance of such a change taking place, on mixing a mineral acid with any vegetable oil, either expressed or distilled.)

"From bitumens we obtain, by distillation, the mineral oil, and from resins the vegetable oil, distinct in their qualities as at first. Vegetable oils and resins have been treated with all the known mineral acids; but have never yielded anything similar to the mineral bitumens. It seems, therefore, as if the oily products of the two kingdoms were essentially and specifically different. The laws of chemical inquiries at least demand, that we do not look upon them any otherwise, till we are able to produce from one a substance similar to the other. When this shall be done, and not before, the presumption that nature effects the same change in the bowels of the earth, will be of some weight."

There is a perfectly fluid, thin bitumen, or mineral oil, called naphtha, clear and colourless as crystal; of a strong smell; extremely subtile; so light as to swim on all known liquors, ether perhaps excepted; spreading to a vast surface on water, and exhibiting rainbow-colours; highly inflammable; formerly made use of in the composition of the supposed inextinguishable greek fire.

Next to this in consistence is the oleum petrae, or petroleum; which is groser and thicker than naphtha, of a yellowish, reddish, or brownish colour; but very light, so as to swim even on spirit of wine. By distillation, the petroleum becomes thinner and more subtile, a grose matter being left behind; it does not, however, easily arise, nor does it totally lose its colour by this process, without particular management or additions.

Both naphtha and petroleum are found plentifully in some parts of Persia, trickling through rocks, or swimming on the surface of waters. Kempfer gives an account of two springs near Baku; one affording naphtha, naphtha, which it receives in drops from subterranean veins; the other, a blackish and more fetid petroleum, which comes from Mount Caucasus. The naphtha is collected for making varnishes; the petroleum is collected in pits, and sent to different places for lamps and torches.

Native petrolea are likewise found in many different places, but are not to be had in the shops; what is sold there for petroleum, being generally oil of turpentine coloured with alkanet root. The true naphtha is recommended against disorders of the nerves, pains, cramps, and contractions of the limbs, &c.; but genuine naphtha is rarely or never brought to this country.

There are some bitumens, such as amber, ambergrise, pit-coal, and jet, perfectly solid; others, such as Barbadoes tar, of a middle consistence between fluid and solid. Turf and peat are likewise thought to belong to this class.

1. Amber. This substance melts, and burns in the fire, emitting a strong peculiar smell. Distilled in a strong heat, it yields a phlegm; an oil; and a particular species of acid salt, (no 313.—315). The distillation is performed in earthen or glas-retorts, frequently with the addition of sand, sea-salt, coals, &c., which may break the tenacity of the melted mass, so as to keep it from swelling up, which it is apt to do by itself. These additions, however, make a perceptible difference in the produce of the distillation: with some, the salt proves yellowish and dry; with others, brownish or blackish, and unctuous or soft like an extract: with some, the oil is throughout of a dark-brown colour; with others, it proves externally green, or greenish; with elixated ashes, in particular, it is of a fine green. The quantity of oil and phlegm is greatest when coals are used, and that of salt when sea-salt is used.

The most advantageous method of distilling amber, however, is without any addition; and this is the method used in Prussia, where the greatest quantities of salt and oil of amber are made. At first a phlegmatic liquor distils; then a fluid oil; afterwards one that is thicker and more ponderous; and last of all, an oil still more ponderous along with the salt. In order to collect the salt more perfectly, the receiver is frequently changed; and the phlegm, and light oil, which arise at first, are kept by themselves. The salt is purified, by being kept some time on bibulous paper, which absorbs a part of the oil; and changing the paper as long as it receives any oily stain. For the further depuration as well as the nature of this salt, see Succinum.

2. Ambergrise. This concrete, which is only used as a perfume, yields, on distillation, products of a similar nature to that of amber, excepting that the volatile salt is in much less quantity. See Ambergrise.

3. Pit-coal. [See the articles Coaleries and Lithanthrax.] This substance yields by distillation, according to the translator of the Chemical Dictionary, 1. a phlegm, or water; 2. a very acid liquor; 3. a thin oil, like naphtha; 4. a thicker oil, resembling petroleum, which falls to the bottom of the former, and which rises with a violent fire; 5. an acid, concrete salt; 6. an inflammable earth, (we suppose he means a piece of charred coal, or cinder), remains in the retort. The fluid oil obtained from coals is said to be exceedingly inflammable, so as to burn upon the surface of water, like naphtha itself.

4. Peat. There are very considerable differences in this substance, proceeding probably from the admixture of different minerals: for the substance of peat is plainly of vegetable origin; whence it is found to answer for the melting of ores, and the reduction of metallic calces, nearly in the same manner as coals of wood. Some sorts yield, in burning, a very disagreeable smell, which extends to a great distance; whilst others are inoffensive. Some burn into grey or white, and others into red, ferruginous ashes. The ashes yield, on elixiation, a small quantity of alkaline, and some neutral salt.

The smoke of peat does not preserve or harden flesh like that of wood; and the food into which it is condensed is more apt to liquefy in moist weather. On distilling peat in close vessels, there arises a clear insipid phlegm; an acid liquor; which is succeeded by an alkaline one, and a dark-coloured oil. The oil has a very pungent taste, and an empyreumatic smell; less fetid than that of animal substances, but more so than that of mineral bitumens. It congeals, in the cold, into a pitchy mass, which liquefies in a small heat: it readily catches fire from a candle; but burns less vehemently than other oils, and immediately goes out upon removing the external flame. It dissolves almost totally in rectified spirit of wine, into a dark, brownish-red, liquor.

VI. Charcoal.

This is the form to which all inflammable matters are reducible, by being subjected to the most vehement action of fire in close vessels; but though all the coals of different substances are nearly similar to one another in appearance, there is nevertheless a very considerable difference among them as to their qualities. Thus the charcoal of vegetables parts with its phlogiston very readily, and is easily reducible to white ashes; charred pit-coal, or, as it is commonly called, coak, much more difficultly; and the coals of burnt animal-substances, far more difficulty than either of the two. Mr Macquer acquaints us, that the coal of bullock's blood parts with its phlogiston with the utmost difficulty. He keeps it very red, in a shallow crucible, surrounded with charcoal, for six hours and more, stirring it constantly that it might be all exposed to the air, without being able to reduce it to white, or even grey ashes. It still remained very black, and full of phlogiston. The coals of pure oils, or concrete oily substances, and foot, which is a kind of coal raised during the inflammation of oils, are as difficultly burnt as animal coals. These coals contain very little saline matter, and their ashes furnish no alkali. These coals, which are so difficultly burnt, are also less capable of inflaming with nitre than others more combustible; and some of them, in a great measure, resist even the action of nitre itself.

Charcoal is the most refractory substance in nature; charcoal no instance having been known of its ever being melted, or shewing the least disposition to fusion, either by itself, or with additions: hence, charcoal is found to be the most proper support for such bodies as are to be exposed to the focus of a large burning glass. The only true solvent of charcoal is hepar sulphuris. See no. 325.

The different quantities of phlogiston contained in different coals, and perhaps some other circumstances, render some kinds of charcoal much less fit to be used in reviving metals from their calces, or in smelting them originally from their ores. The coals of vegetable substances are found to answer best for this purpose. See Metallurgy.

Sect. V. Vegetable and Animal Substances.

The only substances afforded by vegetables or animals, which we have not yet examined, are the mucilaginous, or gummy; and the colouring parts obtained by infusion, or boiling in water. The last of these are treated of under the article Colour-Making, to which we refer; and in this section shall only consider the nature of mucilage, or gum.

The mucilage of vegetables is a clear transparent substance, which has little or no taste or smell, the consistence of which is thick, rosy, and tenacious, when united with a certain quantity of superabundant water. It is entirely and intimately fusible in water, and contains no disengaged acid or alkali.

When mucilage is dissolved in a large quantity of water, it does not sensibly alter the consistence of the liquor; but, by evaporation, the water grows more and more thick; and, at last, the matter acquires the consistence of gum-arabic, or glue; and this without losing its transparency, provided a heat not exceeding that of boiling water has been used.

Gums, and solid mucilages, when well dried and very hard, are not liquefied in the fire like resins, but swell, and emit many watery fumes; which are, at first, watery; then oily, fuliginous, and acrid. Distilled in close vessels, an aqueous acid liquor comes over, along with an empyreumatic oil, as from other vegetable substances; a considerable quantity of coal remains, which burns to ashes with difficulty.

Mucilages and gums are not fusible either by oils, spirit of wine, alkalies, or acids, except in so far as they dissolve in these liquors by means of the water in which the alkali or acid are dissolved. They are, however, the most effectual means of uniting oil with water. Three parts of mucilage, poured upon one part of oil, will incorporate with it by trituration or agitation; and the compound will be fusible in water. Vegetable gums are used in medicine, as well as the mechanic arts; but the particular uses to which each of them is applicable, will be mentioned under the name of each particular gum.

The mucilage obtained from animal substances, when not too thick, is called jelly, or gelatinous matter; when further insipidated, the matter becomes quite solid in the cold, and is called glue. If the evaporation is still further continued, the matter acquires the consistence of horn.

This gelatinous substance seems to be the only true animal one; for all parts of the body, by long continued boiling, are reducible to a jelly, the hardest bones not excepted. Animal jelly, as well as vegetable mucilage, is almost insipid and inodorous; but, though it is difficult to describe the difference between them when apart, it is very easily perceived when they are both together. Acids and alkalies, particularly the latter, dissolve animal jellies with great ease; but the nature of these combinations is not yet understood. The other properties of this substance are common to it with the vegetable gums, except only that the animal mucilage forms a much stronger cement than any vegetable gum; and is therefore much employed for mechanical purposes, under the name of glue. See Glue, and Isinglass.