in fortification, the wall with which a bastion, or any other bulwark of earth, is lined for its greater support and strength: or it is the solidity of the wall from the talus to the stone-row.

Fire-CHEMISE, a piece of linen cloth, steeped in a composition of oil of petrol, camphor, and other combustible matters, used at sea, to set fire to an enemy's vessel.

CHEMISTRY

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 his 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 metallurgical part, 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 Trismegistus; 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 of an inundation of the Nile. No less than 36,000 books are said to have been written under the name of Hermes; but, according to Jamblichus, a custom prevailed of inscribing all books of science with the name of Hermes. Some authors deny the existence of Hermes, and maintain that his history is allegorical.

As the science of chemistry is supposed to have been well known to the Egyptians, Motes, who was skilled poled to be in their wisdom, is thence ranked among the number killed 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. Dr Black, however, is of opinion, that Democritus knew nothing more of these arts than that of making a coarse kind of glass, as no mention is particularly made of his imitating any other precious stone than the emerald, whose colour is green; and the coarser the glass the greener it is.

After the time of Democritus, we may know that considerable improvements were made in chemistry, as physicians began to make use of metallic preparations, as cerule, verdigris, litharge, &c. Dioscorides describes the distillation of mercury from cinnamon by means of an alembic, from which, by adding the Arabic Al, Derivation comes the term Alembic. The art of distillation, however, at that time, was in a very rude state; the operation being performed chiefly by separating the air, and more subtle part of tar, from the rest of the matter. This was done by putting the matter to be distilled into a vessel, the mouth of which was covered with a wet cloth; and by this the streams of ascending vapour were condensed, which were afterwards procured by wringing out the cloth. No other distillation, besides this kind, is mentioned by Galen, Oribasius, Elian, or Paulus Aegineta.

The precise time is not known when the three mineral acids were first discovered; though, as no mention is made of them by Geber, Avicenna, or Roger Bacon, it is probable that they were not known in the 12th century. Raymond Lully gives some hints of his being acquainted with the marine acid; whence it is probable, that it was discovered towards the end of the 13th or beginning of the 14th century.

Several chemical facts are related by Pliny, particularly the making of glass, which he attributes to the following accident. "Some merchants in the Levant, who had nitre on board their ship, having occasion to land, lighted a fire on the sand in order to prepare their food. To support their vessels they took some of the lumps of nitre with which their ship was loaded; and the fire acting on these, melted part of them along with the sand, and thus formed the transparent substance called glass, to the great surprize of the beholders." But it is probable, that the art of glass-making was known long before; and it is by no means likely that it took its rise from such an accident.

The next traces we find of chemistry are to be extracted from the extravagant pursuits of the Alchemists, who imagined it possible to convert the baser metals into gold or silver. The first mention we find of this study is by Julius Firmicus Maternus, who lived in the beginning of the fourth century, and speaks of it as a well-known pursuit in his time. Æneas Blasius, who lived in the fifth century, likewise speaks of it; and Suidas explains the term by telling us, that it is the art of making gold and silver. He tells us, that Diocletian, when persecuting the Christians, forbade all chemical operations, lest his subjects should discover the art of making gold, and thus be induced to rebel against him. He supposes also, that the Argonautic expedition was only an attempt to procure a skin or parchment, on which was written the recipe for making gold. It is a common practice, however, in some places where gold is washed down in small particles by brooks and rivulets from the mountains, to suspend in the water the skins of animals having wool or hair upon them, in order to detain the heavier particles which contain the gold; and this probably gave rise to the fable of the golden fleece. Suidas, however, who lived as late as the tenth century, deserves very little credit, especially as alchemy is not mentioned by any ancient author.—The Arabian physicians afford the most clear and distinct evidence concerning alchemy. Avicenna, who lived in the tenth century, is said by a disciple of his to have wrote upon alchemy; he mentions also rose-water, and some other chemical preparations; and in the 12th century we find physicians advised to cultivate an acquaintance with the chemists; and another of the Arabian writers say, that the method of preparing rose-water, &c. was then well known.—From this evidence of the existence of alchemy among the Arabs, with the prefatory article Al, to denote the greatness of the science, it has been conjectured, that the doctrine of the transmutation of metals first took its rise among the Arabs, and was introduced into Europe by means of the Crusades, and by the rapid conquests of the Arabs themselves in Europe as well as in Asia and Africa. Europe at that time had been in a state of the greatest barbarity from the incursions of the northern nations; but the Arabs contributed to revive some of the sciences, and introduced alchemy among the rest, which continued till the middle of the 17th century; at which time the extravagance of its professors rose to the greatest height.

Though the pretensions of the alchemists are now universally refuted, yet from some of the discoveries due to which have been made in chemistry, we are even yet in danger of giving some credit to the possibility of the process of transmutation. When we consider that the metals are bodies compounded of parts which we can take away and restore, and that they are closely allied to one another in their external appearance, we may be inclined to think favourably even of the projects of the alchemists. The very separation of the metals from their ores, the depriving them of their ductility and malleability, and the restoration of these properties to them at pleasure, will appear very surprising to those who are unacquainted with chemistry. There are also processes of the more difficult kind, by which quicksilver may be produced from metals that quicksilver are commonly solid, as from lead. Some of these we produced in Boerhaave, Boyle, &c., authors of the greatest credit, who both speak of the operation and product as realities of which they were convinced by their own experience. These have been urged, not without some plausibility, in favour of the transmutation of the imperfect metals into gold; and hence the delusions of alchemy were not confined to the vain, the ignorant, and the ambitious part of mankind; but many ingenious and learned men, who took pleasure in the study of nature, have been seduced into this unhappy pursuit. This happened chiefly in Germany, where the variety of mines naturally turned the thoughts of chemists principally towards the metals, though the numerous failures of those who had attempted this art ought to have taught them better.

About the beginning of the 16th century, the pretenders to alchemy were very numerous, and a multitude of knaves, who had beggared themselves in the attempt, now went about to ensnare others, performing legerdemain tricks, and causing people believe that they could actually make gold and silver. A number of the tricks they made use of are to be met with in Lemeray. Many books, with the same design of imposing upon mankind, were written upon the subject of alchemy. They assumed fictitious names of the greatest antiquity, and contained rules for preparing the philosopher's stone; a small quantity of which thrown into a base metal should convert the whole into gold. They are wrote in a mysterious style, without any distinct meaning; and though sometimes processes are clearly enough described, they are found to be false and deceitful upon trial, the products not answering the pretensions of the authors. Their excuse was, that it was vain to expect plain accounts of these matters, or that the books on these subjects should be written distinctly and clearly; that the value of gold was in proportion to its scarcity, and that it might be employed to bad purposes: they wrote only for the laborious and judicious chemists, who would understand them provided they made themselves acquainted with the metals by study and experience. But in fact, no distinct meaning has ever been obtained, and the books have only served to delude and betray a great number of others into the loss of their lives.

But though the alchemists failed in the execution of their their grand project, we must still own ourselves indebted to them for many discoveries brought to light during the time they vainly spent their labour in the expectation of making gold. Some of these are the methods of preparing spirit of wine, aquafortis, volatile alkali, vitriolic acid, and gun-powder. Medicine too was indebted to them for several valuable remedies; whence also it appears that many, who had waited their time in the vain pursuit of the philosopher's stone, thought of trying some of their most elaborate preparations in the cure of diseases; and meeting with some success, they presumed that diseases were only to be cured by the assistance of chemistry; and that the most elaborate of all its preparations, the philosopher's stone, would cure all diseases. Some cures they performed did indeed awaken the attention of physicians; and they introduced the use of opium, which had formerly been accounted poisonous. They succeeded also in the cure of the venereal disease, which had lately made its appearance, and baffled the regular physicians; but the chemists, by giving mercury, put a stop to its ravages, and thus introduced this valuable article into the materia medica.

The most famous of the chemical professors was Paracelsus, well known for his arrogance, absurdity, and profigacy. He was bred to the study of medicine; but becoming acquainted with the alchemists, travelled about in the character of a physician, and was at great pains to collect powerful medicines from all quarters. These he used with great freedom and boldness. His success in some cases operated so upon the natural arrogance and self-sufficiency of his disposition, that he formed a design of overturning the whole system of medicine, and supplying a new one from chemistry; and indeed he found but very weak adversaries in the subtle theories of Galen with the refinements of the Arabian physicians, which only prevailed in his time; and he no doubt had some share in banishing that veneration which had been so long entertained for these celebrated personages.

From the time of Paracelsus, chemistry began everywhere to assume a new face. In our own country, Lord Verulam amused himself at his leisure hours with forming plans for promoting the sciences in general, especially those which related to the study of nature. He soon found that chemistry might turn out one of the most useful and comprehensive branches of natural philosophy, and pointed out the means of its improvement. A number of experiments were proposed by him; but he observed, that the views of chemists were as yet only adapted to explain their particular operations on metals; and he observed, that instead of the abstruse and barren philosophy of the times, it was necessary to make a very large collection of facts, and to compare them with each other very maturely and cautiously, in order to discover the common causes and circumstances of connection upon which they all depend. He did not, however, make any considerable discoveries, and his works are tedious and disagreeable to the reader.

A superior genius to Lord Verulam was Mr Boyle, who was born the very day that the former died. His circumstances were opulent, his manners agreeable; he was endowed by nature with a goodness of heart; and his inclination led him entirely to the study of nature, which he was best pleased with cultivating in the way of experiment. He considered the weight, spring, and qualities of the air; and wrote on hydromatics and other subjects; and was possessed of that happy penetration and ingenuity so well suited to the making of experiments in philosophy, which serves to deduce the most useful truths from the most simple and seemingly insignificant facts. As chemistry was his favourite science, he spared no pains to procure from chemists of greatest note the knowledge of curious experiments, and entertained a number of operators constantly about him. His discoveries are related in an easy style; and though rather copious, suited to the taste of the times in which he lived, and free from that absurd and mysterious air which formerly prevailed in chemical writings: nor does he betray a design of concealing anything except some particulars which were communicated to him under the notion of secrecy, or the knowledge of which might do more harm than good. It is objected indeed, that he betrays a good deal of credulity with regard to facts which are given on the faith of others, and which may seem incredible; but this proceeded from his candour, and his being little disposed to suspect others. He showed the necessary connection between philosophy and the arts; and said, that by attending the shop of a workman, he learned more philosophy than he had done in the schools for a long time. Thus his writings showed an universal taste for the study of nature, which had now made some advances in the other parts of the world.

Agricola is one of the first and best authors on the subject of metallurgy. Being born in a village in Mif. Chemistry emerges in 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. Lazarus Erker, Schinder, Schlutter, Henkel, &c. have also written on metallurgy, and described the art of assaying metals. Anthony Neri, 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; so true it is, that an obstinate and invertebrate malady never disappears at once, without leaving traces behind. In a short time, 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 Conringius a physician, who wrote with much success and reputation.

About the year 1650 the Royal Society was founded by a number of gentlemen who were unwilling to engage in the civil wars; and being struck with the extensive views of Lord Verulam and Mr Boyle, contributed to the expense of costly experiments. This example appeared so noble, and the design so good, that it has been followed by all the civilized states of Europe, and has met with the protection of their respective sovereigns; and from these chemistry has received considerable improvements. In France, Geoffroy, Lemery, Reaumur, &c. came to be distinguished; ed; and in Germany Margraaf, Pott, and others, have made a considerable figure in those societies. Kuncel, Beger, Stahl, and Hoffman, &c., have done great service to society, by introducing new arts, and the numerous improvements they have made.

The chemists who have made a figure in Germany and France are more in number than those whom our island has produced. In France, the society was encouraged by the sovereign; and in it they have developed themselves of that mysterious air which was affected in former ages. In Germany, the richness of the country, and the great variety of mines, by turning the attention of chemists to the metals, have given that alchemical air to their writings which we observe in them. The number of those who have applied themselves to chemistry is very small in England, owing to the great improvements made by Sir Isaac Newton in the sciences of astronomy and optics; which, by turning the general attention that way, has occasioned what may be called a neglect of chemistry. But if their number be inconsiderable, they are by no means inferior in merit and fame. The name of Boyle has always been held in the highest esteem, as well as that of Hales, for the analysis he has made of the air. Sir Isaac Newton alone has done more to establishing a rational chemical theory than ever was done before. Of late, the taste for the study has become more general, and many useful books have appeared; so that it is to be hoped we shall soon excel in this branch of science, as we have done in all the rest.

**Part I. Theory**

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; and these 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 so little the subject of examination, that it hath hitherto baffled the ingenuity of the greatest philosophers to understand its composition. 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 recompose them again by a proper mixture of the elements when 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 metal whatever, though but a grain of lead, the least valuable of them all; 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 generally 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; that air was generally produced from the substance of 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 shown that air may be absorbed from the external atmosphere, and fixed in a great number of solid substances. His assertion 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 drams, or ¼ths 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.

The decomposition of water, however, in another way, by the combination of one part of it with the phlogistic, and another with the earthy part of a metal, is now well ascertained, and the experiments which led to the discovery are treated of under the articles Aerology and Water.

Even the existence of earth as an element appears 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 desire in any terrestrial substance. It is white, infusible, inodorous, 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 Epson salt and felspar 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 Epson salt or felspar. 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.

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. Any common element of all bodies must therefore be a substance which has no property similar to any other in the whole system of nature, and consequently must 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. The existence of this element was first asserted by Stahl, and from him the opinion has been derived to other chemists; but of late a new doctrine was broached by M. Lavoisier, who denies the existence of phlogiston altogether. Though none of these substances therefore are properly the objects of chemistry, yet as they have so much ingrossed the attention of modern chemists, we shall here give an account of the most remarkable theories that have appeared concerning them.

Sect. I. Of the Element of Fire.

The opinions concerning the element of fire may be divided into two general classes; the one considering it as an effect, the other as a cause. The former is maintained by Lord Bacon, Mr Boyle, and Sir Isaac Newton; whose respectable names for a long time gave heat such a sanction to this theory, that it was generally looked upon as an established truth. Some learned men, however, among whom was the great Dr Boerhaave, always dissented, and insisted that fire was a fluid universally diffused, and equally present in the frozen regions of Nova Zembla as in a glas-house furnace, only that in the latter its motion made it conspicuous; and by setting it in motion in the coldest parts of the world, its previous existence there would be equally demonstrable as in the furnace above-mentioned.

Lord Bacon defines heat, which he uses as a synonymous term for fire, to be an expansive undulatory motion in the particles of a body, whereby they tend with some rapidity towards the circumference, and also a little upwards. Hence, if in any natural body you can excite a motion whereby it shall expand or dilate itself, and can repel and direct this motion upon itself in such a manner that the motion shall not proceed uniformly, but obtain in some parts and be checked in others, you will generate heat or fire.

The same opinion is supported by Mr Boyle in the following manner: "The production of heat differs opinion nothing, either in the agent or patient, but motion, and its natural effects. When a smith briskly hammers a small piece of iron, the metal thereby becomes exceedingly hot: yet there is nothing to make it so, except the motion of the hammer impressing a vehement and variously determined agitation on the small parts of the iron; which, being a cold body before, grows hot by that superinduced motion of its small parts: first, in a more loose acceptation of the word, with regard to some other bodies, in comparison of which it was cold before; then sensibly hot, because the motion in the parts of the iron is greater than that in the parts of our fingers; at the same time that the hammer and anvil, by which the percussion is communicated, may, on account of their magnitude, remain cold. It is not necessary, therefore, that a body should itself be hot in order to communicate heat to another."

The arguments made use of by Sir Isaac Newton Sentiments are not intended positively to establish any kind of theory relating to fire, but are to be found in a conjecture, published at the end of his Treatise on Optics, concerning the nature of the sun and stars. "Large bodies (he observes) preserve their heat the longest, their parts heating one another; and why may not great, dense, and fixed bodies, when heated beyond a certain degree, emit light so copiously, as, by the emission and reaction of it, and the reflections and refractions within the pores, to grow continually hotter, till they arrive at such a period of heat as is that of the sun?" Their parts... parts may be further preserved from fuming away, not only by their fixity, but by the vast weight and density of the atmosphere incumbent on them, strongly compressing them, and condensing the vapours exhaled from them. Thus we see, that warm water, in an exhausted receiver, shall boil as vehemently as the hottest water exposed to the air; the weight of the incumbent atmosphere in this latter case keeping down the vapours, and hindering the ebullition till it has received its utmost degree of heat. Thus also a mixture of tin and lead, put on a red hot iron in vacuo, emits a fume and flame; but the same mixture in the open air, by reason of the incumbent atmosphere, does not emit the least sensible flame." In consequence of these experiments, Sir Isaac conjectures, that there is no essential distinction between fire and gross bodies; but that they may be converted into one another. "Fire (he says) is a body heated so hot as to emit light copiously; for what (says he) is a red hot iron but fire?"

The hypotheses of these great men produced long and violent disputes, which were never decisively settled. The discoveries in electricity, however, furnished such additional strength to the followers of Dr Boerhaave, that fire is now believed to be an element and fluid distinct from all others, by at least as many as espouse the contrary system; but the question is not decided, Whether the fire itself is to be considered as the agent? or, Whether its action is to be derived from the principles of attraction and repulsion, the natural agents supposed to influence other material substances? This has produced two other systems of a kind of mixed nature, in which heat or fire is considered as a substance distinct from all others, but which acts in other bodies according to its quantity. These systems have been promulgated by Dr Black of Edinburgh and Dr Irvine of Glasgow. They differ from the opinions of Mr Boyle, Lord Bacon, and Sir Isaac Newton, in supposing heat to be a fluid distinct from all other material substances; and they also differ from the hypothesis of Dr Boerhaave, Lemery, and others, in supposing different terrestrial substances to be hot according to the quantity of fluid contained, and not according to the force with which it moves in them.

Dr Black is of opinion that heat, which he seems to make synonymous with fire, exists in two different states; in one of which it affects our senses and the thermometer, in the other it does not. The former therefore he calls sensible heat, the latter latent heat. On these principles he gives the only satisfactory explanation of the phenomena of evaporation and fluidity that has yet appeared, as shall afterwards be more fully explained. At present we shall only observe, that, according to the theory of Dr Black, heat or fire itself seems to be the agent; but, according to that of Dr Irvine, as far as we can gather it from the treatises of Dr Crawford and others, the principles of attraction and repulsion are the agents by which heat, as well as other bodies, is influenced. Thus, on the principles of Dr Black, we say, that water is converted into vapour by a quantity of heat entering into it in a latent state, and thereby rendering it specifically lighter than the atmosphere; according to the principles of Dr Irvine, we say, that water is converted into vapour by having its capacity for attracting heat from the atmosphere increased. So that, according to the former, the absorption of heat is the cause; according to the latter, the effect, of its conversion into vapour.

Dr Crawford, in his Treatise on Heat, published in 1788, informs us, that heat, in the philosophical sense of the word, has been used to express what is frequently called the element of fire, in the abstract, without regard to the peculiar effects which it may produce in relation to other bodies. This, with Dr Irvine, he calls absolute heat; and the Absolute external cause, as having a relation to the effects it produces, he calls relative heat. "From this view of the matter (says he), it appears, that absolute heat expresses, in the abstract, that power or element which, when it is present to a certain degree, excites in all animals the sensation of heat; and relative heat expresses Relative the same power, considered as having a relation to heat, the effects by which it is known and measured.

"The effects by which heat is known and measured are three; and therefore relative heat may admit of three subdivisions. 1. This principle is known by the peculiar sensations which it excites in animals. Considered as exciting those sensations, it is called sensible heat. 2. It is known by the effect which it produces upon an instrument that has been employed to measure it, termed a thermometer. This is called the temperature of heat in bodies. 3. It has been found by experiment, that in bodies of different kinds the quantities of absolute heat may be unequal, though the temperatures and weights be the same. When the principle of heat is considered relatively to the whole quantity of it contained in bodies of different kinds, but which have equal weights and temperatures, I shall term it comparative heat. If, for example, the temperatures and weights being the same, the whole quantity of heat in water be four times as great as that of antimony, the comparative heats of these substances are said to be as four to one."

In order to have a proper conception of what is meant by a difference in absolute heat, when the temperatures are the same, it will be necessary to relate some experiments, by which Dr Black was first led to the discovery of latent heat. He observes, that when discovery two equal masses of the same matter, heated to different degrees, are mixed together, the heat of the mixture ought to be an arithmetical mean between the two extremes. This, however, only takes place on mixing hot and cold water together; but if instead of cold water we take ice, the case is remarkably different. Here the temperature of the mixture is much below the arithmetical mean, and a quantity of heat is apparently lost. Now we know that the temperature of ice newly frozen is generally 32 degrees of Fahrenheit; supposing therefore the temperature of the water which dissolves it to be 120°, the arithmetical mean is 71°; but if the mixture indicates a temperature only of 60°, then we must suppose that the ice contained 11° of heat less than was indicated by the thermometer; consequently, that water at 32° contains 11° more of absolute heat than ice at 32°.

The same thing is made still more evident from the Great condensation of vapour. The fluid of water is not capable of sustaining a great degree of heat; and 212° of Fahrenheit is the utmost it can be made to bear, by the condensation without an extraordinary degree of pressure, as in Pan's pin's of vapour. pin's digester, or the admixture of saline substances; the temperature of the steam emitted by it therefore never can exceed $212^\circ$, except in the cases just mentioned; and it is often capable of bearing a great degree of cold without being condensed. When the condensation takes place at last, however, a very considerable degree of heat is always produced; and Dr Black has shown, that, in the condensation of steam by the refrigeratory of a common still, as much heat is communicated to the water in the refrigeratory as would be sufficient to make the water which comes over as hot as red hot-iron, were it all to exist in a sensible state. His method of making the calculation is very easy. For, supposing the refrigeratory to contain 100 pounds of water, and that one pound has been distilled; if the water in the refrigeratory has received 10 degrees of heat, we know that the distilled pound has parted with 1000. If in passing through the worm of the refrigeratory, it has been reduced to the temperature of $50^\circ$ of Fahrenheit, having been at $212^\circ$ when it entered it, then it has lost only $162^\circ$ of sensible heat; all the rest communicated to the water of the refrigeratory amounting to more than 800°, having been contained in a latent state, and such as could not then affect the thermometer. This experiment was tried by Mr Watt in a manner still more striking, by a distillation of water in vacuo. Thus the steam, freed from the pressure of the atmosphere, could not conceive such a degree of sensible heat as in the common method of distilling. It came over therefore with a very gentle warmth, scarce more than what the hand could bear; nevertheless it had absorbed as much heat as though the distillation had been performed in the common way; for the refrigeratory had 1000 degrees of heat communicated to it.

The difference of absolute heat is likewise perceptible betwixt any two bodies of different density, water and mercury for instance; and in comparing these, it will always be found that the thinnest fluids contain the greatest quantity of absolute heat; as water more than mercury, spirit of wine more than water, ether more than spirit of wine, and air more than any of them. Dr Black having brought equal bulks of mercury and water, the former to a temperature of $50^\circ$ degrees higher than the latter, found that, on mixture, there was a gain of only 20 degrees above the original; but on reversing the experiment, and heating the water 50 degrees above the mercury, there was a gain of 30 degrees on the whole. "Hence (says Dr Cleghorn in his thesis de Ignis) it appears, that the quantity of heat in water is to that in mercury, when both are of equal temperatures, as 3 to 2." Dr Crawford, however, tells us, that "the same quantity of heat which raises a pound of water one degree, will raise a pound of mercury 28 degrees; whence it follows, that the comparative heat of water is to that of mercury as 28 to 1; and consequently, the alterations which are produced in the temperatures of bodies by given quantities of absolute heat, may properly be applied as a measure of their comparative heats; the alterations of temperature and the comparative heats being reciprocally proportional to one another.

"Sensible heat (continues Dr Crawford) depends partly on the state of the temperature, and partly on that of the organ of feeling; and therefore if a variation be produced in the latter, the sensible heat will be different, though the temperature continue the same. Thus water at the temperature of $62^\circ$ of Fahrenheit appears cold to a warm hand immersed in it; but on the contrary, that fluid will appear warm if a hand be applied to it which has a lower degree of heat than $62^\circ$. For this reason, the thermometer is a much more accurate measure of heat than the senses of animals. As long, however, as the organs remain unchanged, the sensible heat is in proportion to the temperature; and therefore those terms have generally been considered as synonymous.

On this subject Dr Reid observes, that until the ratio Dr Reid's between one temperature and another be ascertained by observation experiment and induction, we ought to consider temperature as a measure which admits of degrees, but not ratios; and consequently ought not to conclude, that the temperature of one body is double or triple to that of another, unless the ratio of different temperatures were determined. Nor ought we to use the expressions of a double or triple temperature, these being expressions which convey no distinct meaning until the ratio of different temperatures be determined."

In making experiments on the comparative quantities of heat in different bodies, our author chooses rather to use equal weights than equal bulks of the substances to be compared. Thus he found the comparative heat of water to be to that of mercury as 28 to 1 by weight, and 2 to 1 by bulk; which differs very considerably from the conclusion of Dr Black, who makes it only as 3 to 2, as has been already mentioned.

From the differences observed in the quantities of absolute heat contained in different bodies, our author concludes, that "there must be certain essential differences in the nature of bodies; in consequence of which, some have the power of collecting and retaining that element in greater quantity than others." These different powers he calls the capacities for containing heat. Thus, if we find by experiment that a pound of water contains four times as much absolute heat as diaphoretic antimony, when at the same temperature, the capacity of water for containing heat is said to be to that of antimony as 4 to 1.

"The temperature, the capacity for containing heat, How the and the absolute heat contained, may be distinguished capacity, from each other in the following manner:

"The capacity for containing heat, and the absolute heat contained, are distinguished as a force distinct from the subject upon which it operates. When we speak of the capacity, we mean a power inherent in the heated body; when we speak of the absolute heat, we mean an unknown principle which is retained in the body by the operation of this power; and when we speak of the temperature, we consider the unknown principle as producing certain effects upon the thermometer.

"The capacity for containing heat may continue unchanged, while the absolute heat is varied without end. If a pound of ice, for example, be supposed to retain its solid form, the quantity of its absolute heat will be altered by every increase or diminution of its sensible heat; but as long as its form continues the same, its capacity for receiving heat is not affected by..." an alteration of temperature, and would remain unchanged though the body were wholly deprived of its heat."

In the course of his work, Dr Crawford observes, that "he has not entered into the inquiry which has been so much agitated among the English, the French, and the German philosophers. Whether heat be a fluid or a quality? In some places indeed he has used expressions which seem to favour the former opinion; but his sole motive for adopting these was, because the language seemed to be more simple and natural, and more consonant to the facts which had been established by experiment. At the same time, he is persuaded that it would be a very difficult matter to reconcile many of the phenomena with the supposition that heat is a quality. It is not easy to conceive, upon this hypothesis, how heat can be absorbed in the processes of fusion, evaporation, combustion; how the quantity of heat in the air can be diminished, and that in the blood increased, by respiration, though no sensible heat or cold be produced.

Whereas, if we adopt the opinion that heat is a distinct substance, or an element *fus generis*, the phenomena will be found to admit of a simple and obvious interpretation.

"Fire will be considered as a principle; which is distributed in various proportions throughout the different kingdoms of nature. The mode of its union with bodies will resemble that particular species of union, wherein the elements are combined by the joint forces of pressure and attraction." Of this kind is the combination of fixed air and water; for fixed air is retained in water partly by its attraction for that fluid, and partly by the pressure of the external air; and if either of these forces be diminished, a portion of the fixed air escapes. In like manner, it may be conceived that elementary fire is retained in bodies, partly by its attraction to these bodies, and partly by the action of the surrounding heat; and in that case a portion of it will be disengaged, either by diminishing the attractive force, or by lessening the temperature of the circumambient medium. If, however, fire be a substance which is subject to the laws of attraction, the mode of its union with bodies seems to be different from that which takes place in chemical combination: for, in chemical combination, the elements acquire new properties, and either wholly or in part lose those by which they were formerly characterized. But we have no sufficient evidence for believing that fire, in consequence of its union with bodies, does, in any instance, lose its distinguishing properties."

Dr Berkenhout, in his First Lines of the Theory and Practice of Philosophical Chemistry, informs us, that "heat, or the matter of heat, is by Scheele and Bergman substituted for fire, which they believe to be the action of heat when increased to a certain degree. The first of these celebrated chemists believed this matter of heat to be a compound of phlogiston and pure air. He was certainly mistaken. It seems more philosophical to consider heat as an *afflux* of which fire is the sole cause.

"Heat I consider not as a distinct substance, but as an effect of fire, fixed or volatile; in both which states fire seems to exist in all bodies, solid and fluid. Fixed fire I believe to be a constituent part of all bodies, and their specific heat to depend on the quantity of fixed fire in each. This fixed, this latent fire, cannot be separated from the other constituent parts of bodies but by their decomposition: it then becomes volatile and incorcible. If this hypothesis be true, fire exists, in all natural bodies that contain phlogiston, in three different states: 1. In that volatile state in which it perpetually fluctuates between one body and another. 2. Combined with an acid, probably in the form of fixed inflammable air or phlogiston. 3. Uncombined and fixed, as a constituent principle, determining the specific heat of bodies.

"Pure (or volatile) fire is distinguished by the following properties. 1. It is essentially fluid, invisible, latile fire and without weight. 2. It is the immediate cause of all fluidity. 3. It penetrates and pervades all bodies on the surface of the earth, and as far beneath the surface as hath hitherto been explored. Water hath never been found in a congealed state in the deepest mines. 4. It has a constant tendency to diffuse itself equally through all bodies, however different in point of density. A marble slab, a plate of iron, a decanter of water, and a lady's muff, at the same distance from the fire, and other external circumstances being equal, possess an equal degree of heat, which is precisely that of the atmosphere in which they stand. 5. It is perpetually in motion from one body to another, and from different parts of the same body, because external circumstances are continually varying. 6. In fluctuating from one body to another, it produces a constant vibration of their constituent parts; for all bodies expand and contract in proportion to the quantity of fire they contain. 7. Accumulated beyond a certain quantity, it effects the dissolution of bodies, by forcing their constituent parts beyond the sphere of mutual attraction, called the attraction of cohesion, which is the cause of solidity. Hence the sovereign agency of fire in chemical operations."

Dr Crawford, besides the opinions already quoted, tells us, that fire, in the vulgar acceptation of the word, expresses a certain degree of heat accompanied with light; and is particularly applied to that heat fire, and light which are produced by the inflammation of combustible bodies. But as heat, when accumulated in a sufficient quantity, is constantly accompanied with light; or, in other words, as fire is always produced by the increase of heat, philosophers have generally considered these phenomena as proceeding from the same cause: and have therefore used the word fire to express that unknown principle, which, when it is present to a certain degree, excites the sensation of heat alone; but, when accumulated to a greater degree, renders itself obvious both to the sight and touch, or produces heat accompanied with light. In this sense, the element of fire signifies the same thing with absolute heat.

Having premised these general definitions and remarks, he gives the properties of heat in the following words:

"I. Heat has a constant tendency to diffuse itself over all bodies till they are brought to the same temperature. Thus it is found by the thermometer, that if two bodies of different temperatures are mixed together, or placed contiguous, the heat passes from the colder one to the other till their temperatures become equal; and..." and that all inanimate bodies, when heated and placed in a cold medium, continually lose heat, till in process of time they are brought to the state of the surrounding medium.

"From this property of heat it follows, that the various classes of bodies throughout the earth, if they were not acted upon by external causes, would at length arrive at a common temperature when the heat would become quiescent; in like manner as the waters of the ocean, if not prevented by the winds and by the attractions of the sun and moon, would come to an equilibrium, and would remain in a state of rest. But as causes continually occur in nature to disturb the balance of heat as well as that of the waters of the ocean, those elements are kept in a constant fluctuation.

"II. Heat is contained in considerable quantities in all bodies when at the common temperature of the atmosphere.

"From the interesting experiments which were made on cold by Mr Wilson, we learn, that at Glasgow, in the winter of the year 1785, the thermometer on the surface of snow sunk 25 degrees below the beginning of Fahrenheit's scale.

"We are told by Dr Pallas, that in the deserts of Siberia, during a very intense frost, the mercury was found congealed in thermometers exposed to the atmosphere, and a quantity of that fluid in an open bowl placed in a similar situation, at the same time became solid. The decisive experiments of Mr Hutchins at Hudson's Bay prove, that the freezing point of mercury is very nearly 40° below the zero (or 0°) of Fahrenheit. From which it follows, that at the time of Dr Pallas's observation, the atmosphere in Siberia must have been cooled to minus 40°. By a paper lately transmitted to the Royal Society we are informed, that the spirit-of-wine thermometer in the open air at Hudson's Bay fell to — 42° in the winter of 1785; and from the same communication we learn, that by a mixture of snow and vitriolic acid, the heat was so much diminished, that the spirit of wine sunk to — 80°, which is 112 below the freezing point of water.

"Hence it is manifest, that heat is contained in considerable quantities in all bodies when at the common temperature of the atmosphere. It is plain, however, that the quantity inherent in each individual body is limited. This, I think, must be admitted, whatever be the hypothesis which we adopt concerning the nature of heat; whether we conceive it to be a force or power belonging to bodies, or an elementary principle contained in them. For those who consider heat as an element, will not suppose that an unlimited quantity of it can be contained in a finite body; and if heat be considered as a force or power, the supposition that finite bodies are actuated by forces or powers which are infinite is equally inadmissible.

"To place this in another light, we know that bodies are universally expanded by heat, excepting in a very few instances, which do not afford a just objection to the general fact; because, in those instances, by the action of heat a fluid is extricated that previously separated the particles from each other. Since, therefore, heat is found to expand bodies in the temperatures which fall within the reach of our observation, we may conclude that the same thing takes place in all temperatures."

Our author, by a set of very accurate and laborious experiments, determines that the expansions in mercury and four other fluids are proportional to the quantities of heat applied; "from which (says he) it is manifest, that the quantities of heat in bodies are limited, because an infinite heat would produce an infinite expansion.

"It is manifest, that the number of degrees of sensible heat, as measured by the thermometer, and extent of heat, mated from the beginning of the scale, must be the same in all bodies which have a common temperature; for by the first general fact it is proved, that heat has a constant tendency to diffuse itself uniformly over bodies till their temperatures become equal. From which it may be inferred, that if a quantity of heat were added to bodies absolutely cold, the same uniform diffusion would take place; and that if a thermometer, altogether deprived of its heat, were applied to such bodies, it would be equally expanded by them, the whole of the sensible heat which they had acquired being indicated by that expansion.

"III. If the parts of the same homogeneous substances have a common temperature, the quantity of absolute heat will be proportional to the bulk or quantity of matter. Thus the quantity of absolute heat in two pounds of water is double that which is contained in one pound when at the same temperature.

"IV. The dilatations and contractions of the fluid of heat portable in the mercurial thermometer are nearly proportional to those of the quantities of absolute heat which are communicated to the same homogeneous bodies, or separated from them, as long as they retain the same form. Thus the quantity of heat required to raise a body four degrees in temperature by the mercurial thermometer, is nearly double that which is required to raise it two degrees, four times that required to raise it one degree, and so in proportion."

Thus we find, that Dr Black, Dr Irvine, Dr Crawford, and Dr Berkenhout, agree in speaking of fire or heat as a fluid substance distinct from all other bodies. Mr Kirwan, in his Treatise of Philogifton, agrees in the Mr Kirwan's opinion. "Some (says he) have thought, that man's opinion should have included the matter of heat, or elementary fire, in the definition of inflammable air; but as fire is contained in all corporeal substances, to mention it is perfectly needless, except where bodies differ from each other in the quantity of it they contain." On the other hand, Mr Cavendish, Phil. Trans. Lxxiv. p. 141. tells us, that "he thinks it more likely that there is no such thing as elementary heat:" but, as he it is not a gives no reason for this opinion, it seems probable that the greater part of philosophers either positively believe that heat is an elementary fluid distinct from all others, or find themselves obliged to adopt a language which necessarily implies it. The only difficulty which now remains therefore is, to affix a proper idea to the phrase quantity of heat, which we find universally made the phrase use of, without any thing to determine our opinions concerning it.

That we cannot speak of a quantity of fire or heat in the same sense as we speak of a quantity of water or any other fluid is evident, because we can take away used in the quantity of water which any substance contains, but cannot do so with heat. Nay, in many cases we are sure, that a substance very cold to the touch does yet to fire. yet contain a very considerable quantity of heat. The vapour of water, for instance, may be made much colder than the usual temperature of the atmosphere without being condensed, when at the same time we are certain that it contains a great quantity of heat; and the same may be said of water, which, in the act of freezing, throws out a great quantity of heat without becoming colder; and in the act of melting absorbs as much without becoming warmer. It is not therefore by the mere presence or absence of this fluid that we can determine the real quantity of this fluid; nor does it appear that the word *quantity* can be at all accurately applied to the element itself, because we have no method of measuring it.

Dr Cleghorn, in his inaugural dissertation *De Igni*, throws some light on this subject, by observing, that "the thermometer shows only the quantity of heat going out of a body, not that which is really contained in it;" and he also insists, that "we can neither assent to the opinion of Dr Boerhaave, who supposed that heat was distributed among bodies in proportion to their bulk; nor to the hypotheses of others, who imagined that they were heated in proportion to their densities." But in what proportion, then, are they heated; or how are we to measure the *quantity* which they really contain, seeing the thermometer informs us only of what they part with?

As this point is by no means ascertained, we cannot form a direct idea concerning the absolute quantity of heat contained in any body; and therefore when we speak of quantities of this fluid, we must in fact, if we mean anything, think of the sensible quantity flowing out of them; and though we should suppose the whole of this sensible heat to be removed, it would still be impossible for us to know how much remained in a latent state, and could not be dissipated. This difficulty will still appear the greater, if with Dr Cleghorn and others we suppose the fluid of heat to be subject to the laws of attraction and repulsion. This gentleman supposes, that the particles of heat (like the particles of electric fluid according to the Franklinian hypothesis) are repulsive of one another, but attracted by all other substances. "If any body (says he), heated beyond the common temperature of the air, is exposed to it, the heat flows out from it into the atmosphere, and diffuses itself equally all around till the air becomes of the same temperature with itself. The same happens to bodies suspended in vacuo. Hence it is justly concluded, that there exists between the particles of heat a repulsive power, by which they mutually recede from each other. Notwithstanding this repulsive power, however, the quantities of heat contained in different substances, even of the same temperature, are found to be altogether different; and from Dr Black's experiments it now appears, that the quantity of heat is scarce ever the same in any two different bodies; and hence we may conclude, that terrestrial bodies have a power of attracting heat, and that this power is different in different substances.—From these principles it evidently follows, that heat is distributed among bodies directly in proportion to their attracting powers, and inversely according to the repulsive power between the particles of heat themselves. Such is the distribution of heat among bodies in the neighbourhood of each other, and which is called the equilibrium of heat, because the thermometer shows no difference of temperature among them. For seeing the heat is distributed according to the attracting power of each, the thermometer having also a proper attraction of its own, can show no difference in the attracting power of each; for which reason all bodies in the neighbourhood of each other are soon reduced to the same temperature."

If we assent to Dr Cleghorn's hypothesis, the quantity of heat contained in any substance depends, in the first place, on the attracting power of that substance, determined which is altogether unknown; and, in the second place, on the repulsive powers of the particles of heat themselves, which are equally unknown. To determine the quantity, therefore, must be impossible. Neither will the mixture of two different fluids, as in Dr Black's experiments, afford us in the least; for though water, heated more than mercury, communicates a greater heat to that fluid than the latter does to water; this only shows that water more readily parts with some part of the heat it contains than mercury does, but has not the least tendency to discover the quantity contained in either.

Dr Crawford, as we have already seen, calls the degree, or, if we may vary the phrase, the quantity of power or element (fluid, if we may substitute a synonymous word) existing or present in any body, its absolute heat; and lays down a rule for determining the proportional quantities of heat in different bodies. "It Dr Crawford will appear (says he) from the experiments afterwards recited, that if a pound of water and a pound of diaphoretic antimony have a common temperature, the proportion of absolute heat contained in the former quarter is nearly four times that contained in the latter."—The manner in which he illustrates this is as follows.

"If four pounds of diaphoretic antimony at 20 be mixed with one pound of ice at 32, the temperature will be nearly 26: the ice will be cooled six degrees, and the antimony heated six. If we reverse the experiment, the effect will be the same. That is, if we take six degrees of heat from four pounds of antimony, and add it to a pound of ice, the latter will be heated six degrees. The same quantity of heat, therefore, which raises a pound of ice six degrees, will raise four pounds of antimony six degrees.

"If this experiment be made at different temperatures, we shall have a similar result. If, for example, the antimony at 15, or at any given degree below the freezing point, be mixed with the ice at 32, the heat of the mixture will be the arithmetical mean between that of the warmer and colder substance. And since the capacities of bodies are permanent as long as they retain the same form, we infer, that the result would be the same if the antimony were deprived of all its heat, and were mixed with the ice at 32. But it is evident, that in this case the ice would communicate to the antimony the half of its absolute heat. For if 200 below frost be conceived to be the point of total privation, the antimony will be wholly deprived of its heat when cooled to 200 degrees below 32, and the heat contained in the ice when at 32 will be 200 degrees. If we now suppose them to be mixed together, the temperature of the mixture will be half the excess of the hotter above the colder, or the ice will be be cooled 100 degrees and the antimony heated 100. The one half of the heat, therefore, which was contained in the ice previous to the mixture will be communicated to the antimony; from which it is manifest, that after the mixture the ice and antimony must contain equal quantities of absolute heat.

"To place this in another light, it has been proved, that the same quantity of heat which raises a pound of ice six degrees will raise four pounds of antimony five degrees. And as the capacities of bodies, while they retain the same form, are not altered by a change of temperature; it follows, that the same quantity of heat which raises the ice 200 degrees, or any given number of degrees, will raise the antimony an equal number of degrees.

"A pound of ice, therefore, and four pounds of antimony, when at the same temperature, contain equal quantities of absolute heat. But it appears from the third general fact (no. 67.), that four pounds of antimony contain four times as much absolute heat as one pound of antimony; and hence the quantity of absolute heat in a pound of ice is to that in a pound of antimony as four to one."

From this quotation it is evident, that, notwithstanding all the distinctions which Dr Crawford has laid down between absolute heat and temperature, it is only the quantity of the latter that can be measured; and all that we can say concerning the matter is, that when certain bodies are mixed together, some of them part with a greater quantity of heat than others; but how much they contain must remain forever unknown, unless we can fall on some method of measuring the quantity of heat as we do that of any other fluid.

Mr Nicolson, who has collected the principal opinions on the subject of heat, seems undetermined whether to believe the doctrine of Boyle or of Boerhaave on the subject. "There are two opinions (says he) concerning heat. According to one opinion, heat consists in a vibratory motion of the parts of bodies among each other, whose greater or less intensity occasions the increase or diminution of temperature. According to the other opinion, heat is a subtle fluid that easily pervades the pores of all bodies, causing them to expand by means of its elasticity or otherwise. Each of these opinions is attended with its peculiar difficulties. The phenomena of heat may be accounted for by either of them, provided certain suppositions be allowed to each respectively; but the want of proof of the truth of such suppositions renders it very difficult, if not impossible, to decide as yet whether heat consists merely in motion or in some peculiar matter. The word quantity, applied to heat, will therefore denote either motion or matter, according to the opinion made use of, and may be used indefinitely without determining which.

"The chief advantage which the opinion that heat is caused by mere vibration possesses, is its great simplicity. It is highly probable, that all heated bodies have an intestine motion, or vibration of their parts; and it is certain that percussion, friction, and other methods of agitating the minute parts of bodies, will likewise increase their temperature. Why, then, it is demanded, should we multiply causes, by supposing the existence of an unknown fluid, when the mere vibration of parts which is known to obtain may be applied to explain the phenomena?"

To this the reply is obvious, that the vibration of parts is an effect; for matter will not begin to move itself; and if it is an effect, we must suppose a cause for answering it; which, though we should not call it a fluid, would Mr Nicholson be equally unknown and inexplicable with that whose existence is affected by those who maintain that fire is a fluid per se. Dr Cleghorn, however, in the different Dr Cleghorn already quoted, asserts, that "heat is occasioned by some fluid, and not by motion alone, as some eminent writers have imagined: because, 1. Those who have adopted the hypothesis of motion could never even prove the existence of that motion for which they contended; and though it should be granted, the phenomena could not be explained by it. 2. If heat depended on motion, it would instantaneously pass through an elastic body; but we see that heat passes through bodies slowly like a fluid. 3. If heat depended on vibration, it ought to be communicated from a given vibration in proportion to the quantity of matter; which is found not to hold true in fact. On the other hand, there are numberless arguments in favour of the opinion that heat proceeds from elementary fire. 1. Mr Locke hath observed, that when we perceive a number of qualities always existing together, we may gather from thence that there really is some substance which produces these qualities. 2. The hypothesis of elementary fire is simple and agreeable to the phenomena. 3. From some experiments made by Sir Isaac Newton, it appears, that bodies acquire heat and cold in vacuo, until they become of the same temperature with the atmosphere; so that heat exists in the absence of all other matter, and is therefore a substance by itself."

But though these and other arguments seem clearly to establish the point that fire or heat is a distinct fluid, concerning its nature and properties. If it be supposed a fluid, it is impossible to assign any limits to its extent; and we must of necessity likewise suppose that it pervades the whole creation, and consequently constitutes an absolute plenum, contrary to a fundamental principle of the received system of natural philosophy. But if this is the case, it is vain to talk of its being absorbed, accumulated, collected, or attracted by different bodies, since it is already present in all points of space; and we can conceive of terrestrial bodies no otherwise than as sponges thrown into the ocean, each of which will be as full of fluid as it can hold. The different capacities will then be similar to the differences between bits of wood, sponge, porous stones, &c., for containing water; all of which depend entirely on the structure of the bodies themselves, and which, unless we could separate the water by pressure, or by evaporation, would be forever unknown. Supposing it were impossible to collect this water in the manner we speak of, we could only judge of the quantity they contained by the degree to which they swelled by being immersed in it. It is easy to see, however, that such a method of judging would be very inadequate to the purpose, as substances might contain internal cavities or pores in which water could lodge without augmenting the external bulk. This would suggest another method of judging of the quantity, namely, the specific gravity; Theory.