in physiology, has a double meaning; being put either for that peculiar sensation which is felt on the approach of burning bodies, or for the cause of that sensation; in which last sense it is synonymous with FIRE. This mode of speaking, however, is inaccurate, and, by confounding the effect with the cause, sometimes produces obscurity: it were to be wished therefore that the word heat was used only to denote the effect; and fire, or some other term, to denote the cause of that effect.

The disputes which formerly were so much agitated in the learned world concerning the nature of heat, viz. whether it consisted merely in the motion of the terrestrial particles of bodies, or in that of a subtile fluid, are now mostly ceased, and it is almost universally believed to be the effect of a fluid. Unluckily, however, from the promiscuous use of the words fire and heat, an opinion seems to have gained ground, that there is in nature a fluid essentially hot; and that wherever the opposite sensation prevails, the former fluid is in part absent. Hence have arisen numberless speculations concerning the attraction, absorption, and capacities of bodies for heat; all of which being built on a false principle, have served no other purpose but to involve this part of natural philosophy in obscurity and confusion. Under the articles Chemistry, Combustion, Electricity, &c. it is so fully shown that heat properly so called is not a fluid, but the modification of a fluid, that it is superfluous to say more on the subject at present. This being admitted, it will evidently follow, that heat can neither be absorbed nor attracted; neither can any body have a greater capacity for it than another, except in proportion to its bulk, which allows a larger quantity of the fluid to enter and to assume the particular motion which constitutes heat. From some of Dr Black's experiments indeed it would appear at first view, that heat was absorbed, or attracted in the strictest sense of the word; but this must be attributed merely to the transferring of the modification of the fluid from one substance to another, without regarding whether it is the identical quantity of fluid which acts as heat in one substance that is transferred to the other, or whether only by some unknown means a similar motion is produced in another portion of the same. At any rate, however, some word must be made use of to express this operation; and absorption or attraction will answer the purpose. purpose as well as any others; but still we ought to remember, that these are inaccurate; and when we begin to argue from them as if they fully and exactly determined the mode in which the fluid acts, or rather is acted upon (for both these words suppose heat to be passive, and not active), we must certainly err. As to the phrase capacity for containing heat, absolute heat, &c., they are still more inaccurate than the words absorption and attraction, and cannot convey any distinct idea; whence the systems founded upon the explanations of these terms, assumed gratis dictum without the least proof, have never been able to support themselves, but are liable to endless and insuperable objections.

It is by no means indeed easy, nay we may boldly say that it is absolutely impossible, for human genius to investigate all the phenomena of this subtle and invisible element. All that can be done is, to discover a few general rules according to which the fluid acts in certain cases. From these we can only reason analogically to cases where its action is less obvious. But we are not to expect that by reasoning in this manner we can solve every phenomenon; nor can it be any recommendation to an hypothesis, merely that it solves some phenomena, unless we were able by its means to solve them all; but this no wise man will pretend to do, nay, not even to know them all. It appears exceedingly erroneous therefore to invent solutions of certain phenomena, and then to argue for the truth of the hypothesis from the facility with which the phenomena are explained by it. The true and proper method of proceeding in this case is to lay down certain principles established from the obvious phenomena of nature, and to reason from them fairly as far as we can; but where this ends, our knowledge must stop, and we cannot by any means proceed farther upon a sure foundation.

The only general principles as yet certainly established from obvious phenomena upon this subject are the following: 1. Heat and cold are found to expel one another. Hence we ought to conclude, that heat and cold are both positives; for a negative can neither be expelled nor accumulated. 2. Heat is visibly occasioned by the rays of the sun concentrated, and likewise by the fluid of electricity concentrated. If fire, therefore, properly so called, be the cause of heat, than which nothing can be more evident to our senses, we are certainly intitled to conclude, that both the light of the sun and the electric fluid are elementary fire. Hence also we conclude their identity; for two different substances cannot by any means produce constantly the same effect when put in the same circumstances, which both light and electricity do in this case, merely by concentration, or discharging a great quantity of the fluid upon a small portion of any terrestrial body. 3. Heat expands bodies in every direction: whence we conclude, that the fluid, when producing heat, acts from a centre towards a circumference; and by analogy, that when it produces cold it acts from a circumference towards a centre. 4. It appears from undeniable experiments, that heat, somehow or other, is the cause of fluidity. As the action of the fluid has already been shown, when it produces heat, to be from a centre to a circumference, it follows, that when the expansive action of the fluid is confined within the surface of any body, this may be called its latent heat; because it extends not beyond the surface, and therefore cannot affect the thermometer, or be known to us as heat by the sense of feeling. But when this expansive action is transferred from the internal parts of the substance to the surface, it then affects the thermometer, and the body is said to become hotter at the same time that it cools or is said to be frozen. This is what some philosophers call the conversion of the latent into sensible heat; others, the alteration of the capacity; but whatever term we give to the effect, the cause must remain the same, viz. the opposite actions of the same fluid; the expansive power in some cases counteracting or overcoming the condensing one, and vice versa. 5. Though sometimes the expansive action is sufficiently strong to produce fluidity naturally, and in most cases may be made to strong artificially, as to make bodies fluid, yet in all cases it is not so. A certain degree of expansive power exists in all bodies whatever, and this by philosophers is called the specific heat of the body. 6. Whatever is called the cooling of any body is only the diminution of the expansive action upon its surface, or, if we may use the expression, on the surface of its particles. This is accomplished by an opposite power or modification of the fluid taking place on the outside; but when this becomes sufficiently strong to penetrate the whole substance, it then expels part of the fluid acting in the opposite direction, and then some change takes place in the texture of the body. It is, however, impossible to speak very perspicuously upon this subject, as the futility and indiscernibility of the fluid render all reasonings upon it very precarious. 7. It is altogether impossible to calculate the quantity of absolute heat contained in any substance, because this depends on the proportion between the quantity of fluid acting expansively and that acting in the opposite direction in the same. These two must some way or other counterbalance each other throughout the whole system of nature; and we may say with certainty, that any substance in which the one exists without the other, is none of those subject to the investigation of our senses, and all speculations concerning it must be vain. 8. When the fluid contained in any substance is vehemently agitated, this naturally produces an expansion in it; and therefore bodies become hot by violent friction, percussion, &c. In these cases, however, we have no right to say that the fluid is expelled, but only that its mode of action is altered; for this is constantly sufficient to produce heat, and in this indeed the very essence of heat consists. 9. When the expansive action of elementary fire within any substance becomes greater than is consistent with the cohesion of that substance, it is dissipated or resolved into vapour. This, however, may be done in such a manner that the heat still acts upon the separated parts of the body without spending any of its force upon external substances. Hence vapour continues to exist in a temperature much below that in which it was originally produced; nay, will sometimes be excessively cold to the touch, when it really contains as much heat, though in a latent state, as before. 10. When this latent heat is transferred to external bodies, the vapour then ceases to be vapour, or is condensed, and in some cases returns to its original state; in others, it is productive of light and vehement sensible heat; whence all the phenomena of Distillation, Evaporation, Flame, Ignition, Combustion, &c.

These are the principal facts which can be looked upon as established with regard to heat considered in a philosophical view. In common discourse it is always spoken of as a certain substance distinct from all others, and may properly enough be reckoned so with regard to all the purposes of life. In this sense, heat is accumulated by certain bodies in a much greater proportion than others. Dr Franklin made the experiment with pieces of cloth of various colours laid upon snow and exposed to the sunshine, and in all cases found that the pieces dyed with the darkest colours sunk deepest in the snow. Mr Cavallo examined the matter more accurately; first by observing the height to which a thermometer with a blackened bulb rose in comparison with one of clear glass, and then by comparing the heights of different thermometers whose bulbs were painted of various colours. Having therefore constructed two thermometers whose scales exactly corresponded with each other, he fixed them both upon the same frame, about an inch apart, having the balls quite detached from the frame; and in this manner exposed them to the light of the sun or of a lamp. When these were exposed, to the sun or kept in the shade, with the glass of both bulbs clear, they showed precisely the same degree; and the difference between the degree shown by the thermometers when exposed to the sun and when kept in the shade, at about the same time of the day, was very trifling.

The ball of one of the thermometers being painted black, and that of the other left clean, they showed different degrees of temperature on being exposed to the sun; the difference sometimes amounting to 10°; but was never constant; varying according to the cleanness of the sun's light as well as of the air, and likewise according to the different degrees of temperature in the atmosphere.

On keeping the thermometer with the painted ball on the inside of a window, Mr Cavallo observed that strong daylight had an effect in raising the mercury as well as the sun's light. To ascertain this, he cleaned the bulb of the painted thermometer, and blackened that of the other; but the effect was constant, viz., the quicksilver in the tube of the thermometer, whose ball was painted black, was constantly higher than the other whenever they were exposed to the strong daylight. The difference was commonly about one-third of a degree, but sometimes it amounted to three-fourths, or even to a whole degree; and the experiment answered even when the sun was hid by clouds, which seems to indicate that every degree of light is accompanied with a corresponding one of heat.

By this consideration Mr Cavallo was induced to try whether, by directing the concentrated light of the moon upon the blackened bulb of a thermometer, it would be raised higher than a clean one standing in the same. The experiment was several times tried with a large lens, and afterwards with a burning mirror of 18 inches diameter; yet sometimes for want of proper means of observing the height of the mercury in the tubes of the thermometers, sometimes for want of a continued clear light of the moon, or in short from some unfavourable circumstance or other, he was never able to make a fair and decisive trial of this experiment.

Making trial of the heat of a lamp, he found that it also had a considerable effect. The ball of one being blackened, and both set at two inches distance from the flame of a lamp, they both rose from 58° to 65°; deg., and the thermometer which was blackened to 67°. Another time the uncoloured thermometer rose to 67°, and the coloured one to 68°. From a number of trials it at last appeared, that the difference at this distance from the lamp amounted generally to about a degree. When the thermometers were removed farther than two inches from the lamp, the difference decreased; and at the distance of about 14 or 15 inches it vanished entirely.

On this occasion Mr Cavallo had an opportunity of making a curious observation concerning the decrease of heat at different distances from the centre. "It is mathematically true, that emanations which proceed from a centre, and expand in a sphere, must become more and more rare in proportion to the squares of the distances from the centre. Thus it is said, that the intensity of light proceeding from a luminous body, at the double, treble, quadruple, &c. distance from that body, must be respectively four, nine, sixteen, times, &c. less dense. The same thing may be said of heat; but with respect to the latter, it appeared, that its intensity did not decrease exactly in the duplicate proportion of the distances from the flame of the lamp, but showed a very odd irregularity. It seemed to decrease faster than the duplicate proportion of the distances for the space of two inches and a half or three inches, after which it decreased much slower; but whether this proceeded from some different state of the air's purity at different distances from the flame of the lamp, or from the vapours coming from the flame, I cannot take upon me to determine."

Mr Cavallo next made some experiments upon thermometers, the balls of which were painted of various colours. His view was to examine with precision the degrees of heat imbibed by differently coloured substances, in order to determine whether they kept any proportion to the spaces occupied by the prismatic colours in the prismatic spectrum, or if they followed any other law. In these experiments he met with considerable difficulties, chiefly arising from the different nature of the colours with which the bulbs were painted. By reason of this diversity the bulbs could not be made equally smooth, which occasioned a considerable difference in the effect; as he found by painting two bulbs of thermometers with the same colour, only making the one smooth and the other rough.

To avoid this inconvenience, he attempted to make thermometers with tubes of differently coloured glasses; but when a ball was formed with any of these, the glass of the ball was so thin, that it differed very little from that which was entirely colourless. He then included the thermometers in boxes, where the rays entered through coloured glasses; but here the rays were not only far from being homogeneous, but there was such a difference in the transparency of some of the coloured glasses, that this method proved also ineffectual. The least ambiguous method, therefore, was that of painting the balls of the thermometers with water-colours, taking care to lay them on as equally and smooth as possible. In this manner the experiments were repeated, using sometimes a dozen of thermometers. mometers at once, whose balls were painted with various colours, and were exposed to the sun; but from a vast number of experiments, and some weeks observation, it could only be deduced, that if the colours with which the balls of the thermometers were painted had any considerable resemblance to those of the prism, those which were nearest to the violet showed a greater degree of heat than the others; but they were all, even that painted with white lead, in some intermediate degree between the blackened thermometer and that which was left quite clear. If the colours had not the proper density, the effects were different; thus, a thermometer painted with a light blue flood lower than another painted with good carmine.

In the course of his thermometrical experiments, Mr Cavallo likewise discovered a new method of determining the expansion of mercury by weight, which seemed capable of being carried to a greater degree of exactness than any other hitherto proposed. Having first blown a ball to a capillary tube, such as are commonly used for thermometers, he weighed it, and found the weight when empty to be 79.25 grains; and he observes, that in this experiment it is a precaution absolutely necessary to have the glass as accurately cleaned as possible. Some mercury was then introduced into the item of the thermometer, taking care that none of it entered the ball; and by adapting a scale of inches to the tube, observed that 4.3 inches of it were filled with the mercury. The thermometer was now weighed again; and from this the weight of the glass being subtracted, the remainder, viz. 0.24 gr., showed the weight of that quantity of quicksilver which filled the 4.3 inches of the tube. Now the ball of the thermometer, and also part of the tube, were entirely filled with quicksilver; and in order to find out the weight of the mercury contained in it, the thermometer was weighed for the last time; and the weight of the glass being subtracted from this, the remainder, viz. 3205 grains, showed the weight of the whole quantity of quicksilver contained in the thermometer.

By comparing this instrument with a graduated thermometer of Fahrenheit, and by applying a scale of inches, he found, that 20° on the new thermometer was equal to 1.37 inches. But 0.24 grains was the weight of as much mercury as filled 4.3 inches of the tube. Therefore, by the rule of proportion, it will be found, that the weight of as much quicksilver as fills 1.33 inches of the tube, viz. the length of 20°, is equal to 0.0742 of a grain nearly; and that the weight of as much quicksilver as fills a length of the tube equivalent to one degree, is equal to 0.00371 grains. Now it is clear, that the weight of the whole quantity of quicksilver contained in the thermometer is to the weight of as much as fills the length of one degree of the tube, as the bulk of the whole quantity of quicksilver in a given degree of heat to the increase of bulk that the same whole quantity of quicksilver acquires when heated but one degree; viz. 32.05 grains is to 0.00371 grains as 1 to 0.0011+. By which experiment it appears, that one degree of Fahrenheit's thermometer increases the bulk of mercury not above eleven hundredth thousandth parts. A small deviation from mathematical exactness is indeed produced by the difference of weight between the quicksilver of the tube when first weighed and when it is afterwards heated to one degree; but by an easy calculation it will be found, that this difference is so exceedingly small that it cannot be perceived with our most exact instruments either of weight or measure.

On repeating this experiment with other thermometers, each proceeds varied a little from the other; which irregularity, Mr Cavallo thinks, was certainly owing to the imperfection of his scales: but by taking a mean of various experiments, it appears, that one degree of heat, according to Fahrenheit's thermometer, increases the bulk of a quantity of quicksilver in the temperature of 50° by about nine parts in 100,000; that is, if the bulk of any quantity of quicksilver in the temperature of 50° be 100,000, it will be 100,009 in the temperature of 51°.

In making experiments of this kind, it is necessary to have the bores of the tubes absolutely cylindrical; and the scales should be so exact as to turn with half the hundredth part of a grain when charged with half an ounce weight.

Heat of Burning Bodies. See Combustion.

Heat of Chemical Mixtures. This is a phenomenon necessarily resulting from the change of form produced in the different substances which are mixed together; and the manner in which it happens may be easily understood from the example of oil of vitriol and water. If equal quantities of concentrated vitriolic acid and water are mixed together, a very great degree of heat immediately takes place; inasmuch, that if the vessel which contains the mixture is made of glass it will probably break; and after it is cold, the mixture will be found to have shrunk in its dimensions, or will occupy less space than the bulk of the water and acid taken separately. In this case we know that the water, while in its fluid state, hath as much latent heat as it can contain; i.e. the elementary fire within it expands or separates its parts from each other, as much as is consistent with the constitution of the body. If any more is added, it cannot be absorbed, or direct its force upon the particles of the water without raising them in vapour: of consequence, part of this additional expansive power will be employed in the formation of vapour, and the rest will be discharged upon the neighbouring bodies, i.e. will be converted into sensible heat. The vitriolic acid, in its concentrated state, contains a great quantity of latent heat, which is necessary to preserve its fluidity. But when it is mixed with the fluid water, the latent heat contained in the latter is abundantly sufficient for both: of consequence, the great expansive power in the oil of vitriol itself becomes now totally useless, and therefore exerts its force upon the neighbouring bodies; and when the mixture returns to the original temperature of the oil of vitriol and water, it shows a loss of substance by its diminution in bulk. This may serve to explain all cases in chemistry where heat or cold is produced; and it will generally be found, that where bodies, by being mixed together, produce heat, they shrink in their dimensions; but when they produce cold, they are enlarged.

Methods of Measuring Heat. See Thermometer.

Expansion of Metals by Heat. See Pyrometer.

Degrees of Heat which Animals are capable of bearing. —The ancients were of opinion, that all countries lying lying within the tropics were uninhabitable by reason of their heat; but time has discovered their mistake; and it is now found, that no part of the world is too hot for mankind to live in. The learned professor Boerhaave, in his chemistry, relates certain experiments made with great accuracy by the celebrated Fahrenheit, and others, at his desire, on this subject, in a sugar-baker's office; where the heat, at the time of making the experiments, was up to 146 degrees of Fahrenheit's thermometer. A sparrow, subjected to air thus heated, died, after breathing very laboriously, in less than seven minutes. A cat resisted this great heat somewhat above a quarter of an hour; and a dog about 28 minutes, discharging before his death a considerable quantity of a ruddy-coloured foam, and exhaled a stench so peculiarly offensive, as to throw one of the assistants into a fainting fit. This dissolution of the humours, or great change from a natural state, the professor attributes not to the heat of the stove alone, which would not have produced any such effect on the flesh of a dead animal; but likewise to the vital motion, by which a still greater degree of heat, he supposes, was produced in the fluids circulating through the lungs, in consequence of which the oils, fats, and spirits of the animal became so highly exalted.

Médecins Du-Hamel and Tillet having been sent into the province of Augonnois, in the years 1760 and 1761, with a view of endeavouring to destroy an insect which corrupted the grain of that province, effected the same in the manner related in the Memoirs for 1761, by exposing the affected corn, with the insects included in it, in an oven, where the heat was sufficient to kill them without injuring the grain. This operation was performed at Rochefoucault, in a large public oven, where, for economical views, their first step was to assure themselves of the heat remaining in it on the day after bread had been baked in it. This they did, by conveying in a thermometer on the end of a shovel, which, on its being withdrawn, indicated a degree of heat considerably above that of boiling water; but M. Tillet, convinced that the thermometer had fallen several degrees in drawing to the mouth of the oven, and appearing under some embarrassment on that head, a girl, one of the attendants on the oven, offered to enter, and mark with a pencil the height at which the thermometer stood within the oven. The girl smiled on M. Tillet's appearing to hesitate at this strange proposition; and entering the oven, with a pencil given her for that purpose, marked the thermometer, after staying two or three minutes, standing at 100 degrees of Réaumur's scale, or, to make use of a scale better known in this country, at near 260 degrees of Fahrenheit's. M. Tillet began to express an anxiety for the welfare of his female assistant, and to press her return. This female salamander, however, assuring him that she felt no inconvenience from her situation, remained there 10 minutes longer; that is, near the time when Boerhaave's cat parted with her nine lives under a much less degree of heat; when the thermometer standing at 288 degrees, or 76 degrees above that of boiling water, she came out of the oven, her complexion indeed considerably heightened, but her respiration by no means quick or laborious. After M. Tillet's return to Paris, these experiments were repeated by Mons. Marantin, commissaire de guerre, at Rochefoucault, an intelligent and accurate observer, on a second girl belonging to the oven, who remained in it, without much inconvenience, under the same degree of heat, as long as her predecessor; and even breathed in air heated to about 325 degrees for the space of five minutes.

M. Tillet endeavoured to clear up the very apparent contrariety between these experiments and those made under the direction of Boerhaave, by subjecting various animals, under different circumstances, to great degrees of heat. From his experiments, in some of which the animals were swaddled with clothes, and were thereby enabled to resist for a much longer time the effects of the extraordinary heat, he infers, that the heat of the air received into the lungs was not, as was supposed by Boerhaave, the only or principal cause of the anxiety, laborious breathing, and death, of the animals on whom his experiments were made; but that the hot air, which had free and immediate access to every part of the surface of their bodies, penetrated the substance on all sides, and brought on a fever, from whence proceeded all the symptoms: on the contrary, the girls at Rochefoucault, leaving their bodies in great measure protected from this action by their clothes, were enabled to breathe the air, thus violently heated, for a long time without great inconvenience. In fact, we should think too, that the bulk of their bodies, though not thought of much consequence by M. Tillet, appears to have contributed not a little to their security. In common respiration, the blood, in its passage through the lungs, is cooled by being brought into contact with the external inspired air: In the present experiments, on the contrary, the vessels and vessels of the lungs receiving at each inspiration an air heated to 300 degrees, must have been continually cooled and refreshed, as well as the subcutaneous vessels, by the successive arrival of the whole mass of blood contained in the interior parts of the body, whose heat might be supposed at the beginning of the experiment not to exceed 100 degrees. Not to mention, that M. Tillet's two girls may not possibly have been subjected to so great a degree of heat as that indicated by the thermometer; which appears to us to have always remained on the shovel, in contact with the earth.

These experiments soon excited other philosophers to make similar ones, of which some very remarkable ones are those of Dr. Dobson at Liverpool, who gives the following account of them in the Philosophical Transactions, vol. lxv.

"I. The sweating-room of our public hospital at Liverpool, which is nearly a cube of nine feet, lighted from the top, was heated till the quicksilver stood at 224° on Fahrenheit's scale, nor would the tube of the thermometer indeed admit the heat to be raised higher. The thermometer was suspended by a string fixed to the wooden frame of the sky-light, and hung down about the centre of the room. Myself and several others were at this time inclosed in the stove, without experiencing any oppressive or painful sensation of heat proportioned to the degree pointed out by the thermometer. Every metallic about us soon became very hot.

"II. My friend Mr. Park, an ingenious surgeon of this place, went into the stove heated to 202°. Af..." ter ten minutes, I found the pulse quickened to 120. And to determine the increase of the animal-heat, another thermometer was handed to him, in which the quicksilver already stood at 98°; but it rose only to 99½, whether the bulb of the thermometer was inclosed in the palms of the hands or received in the mouth (a). The natural state of this gentleman's pulse is about 65.

"III. Another gentleman went through the same experiment in the same circumstances, and with the same effects.

"IV. One of the porters to the hospital, a healthy young man, and the pulse 75, was inclosed in the stove when the quicksilver stood at 210°; and he remained there, with little inconvenience, for 20 minutes. The pulse, now 164, and the animal-heat, determined by another thermometer as in the former experiments, was 101½.

"V. A young gentleman of a delicate and irritable habit, whose natural pulse is about 80, remained in the stove ten minutes when heated to 224°. The pulse rose to 145, and the animal heat to 102°. This gentleman, who had been frequently in the stove during the course of the day, found himself feeble, and disposed to break out into sweats for 24 hours after the experiment.

"VI. Two small tin vessels, containing each the white of an egg, were put into the stove heated to 224°. One of them was placed on a wooden seat near the wall, and the other suspended by a string about the middle of the stove. After ten minutes, they began to coagulate; but the coagulation was sensibly quicker and firmer in that which was suspended, than in that which was placed on the wooden seat. The progress of the coagulation was as follows: it was first formed on the sides, and gradually extended itself; the whole of the bottom was next coagulated; and last of all, the middle part of the top.

"VII. Part of the shell of an egg was peeled away, leaving only the film which surrounds the white; and part of the white being drawn out, the film sunk so as to form a little cup. This cup was filled with some of the albumen ovi, which was consequently detached as much as possible from every thing but the contact of the air and of the film which formed the cup. The lower part of the egg stood upon some light tow in a common gallipot, and was placed on the wooden seat in the stove. The quicksilver in the thermometer still continued at 224°. After remaining in the stove for an hour, the lower part of the egg which was covered with the shell was firmly coagulated, but that which was in the little cup was fluid and transparent. At the end of another hour it was still fluid, except on the edges where it was thinnest; and here it was still transparent; a sufficient proof that it was dried, not coagulated.

"VIII. A piece of bees-wax, placed in the same situation with the albumen ovi of the preceding experiment, and exposed to the same degree of heat in the stove, began to melt in five minutes; another piece suspended by a string, and a third piece put into the tin vessel and suspended, began likewise to liquefy in five minutes."

Even these experiments, though more accurate than the former, do not show the utmost degrees of heat which the human body is capable of enduring. Some others, still more remarkable (as in them the body was exposed to the heat without clothes), by Drs Fordyce and Blagden, are also recorded in the Philosophical Transactions. They were made in rooms heated by fires in the floor, and by pouring upon it boiling water. There was no chimney in them, nor any vent for the air, excepting through crevices at the door. In the first room were placed three thermometers, one in the hottest part of it, another in the coolest part, and a third on the table, to be used occasionally in the course of the experiment. Of these experiments, the two following may be taken as a specimen.

"About three hours after breakfast, Dr Fordyce having taken off all his clothes, except his shirt, and being furnished with wooden shoes tied on with laces, went into one of the rooms, where he stood five minutes in a heat of 90°, and begun to sweat gently. He then entered another room, and stood in a part of it heated to 110°. In about half a minute his shirt became so wet that he was obliged to throw it aside, and then the water poured down in streams over his whole body. Having remained in this heat for ten minutes, he removed to a part of the room heated 120°; and after staying there 20 minutes, found that the thermometer placed under his tongue, and held in his hand, stood just at 100°, and that his urine was of the same temperature. His pulse had gradually risen to 145 pulsations in a minute. The external circulation was greatly increased, the veins had become very large, and an universal redness had diffused itself all over the body, attended with a strong feeling of heat; his respiration, however, was little affected. He concluded this experiment by plunging in water heated to 100°; and after being wiped dry, was carried home in a chair; but the circulation did not subside for two hours.

"Dr Blagden took off his coat, waistcoat, and shirt, and went into one of the rooms, as soon as the thermometer had indicated a degree of heat above that of boiling water. The first impression of this hot air upon his body was exceedingly disagreeable, but in a few minutes all his uneasiness was removed by the breaking out of a sweat. At the end of 12 minutes he left the room very much fatigued, but no otherwise disordered. His pulse beat 136 in a minute, and the thermometer had risen to 220 degrees.

"In others of these experiments it was found, that a heat even of 260° of Fahrenheit's thermometer could be submitted to with tolerable ease. But it must be observed, that in these great heats every piece of metal they carried about with them became intolerably hot.

(a) The scale of the thermometer, which was suspended by the string about the middle of the room, was of metal; this was the only one I could then procure on which the degrees ran so high as to give any scope to the experiment. The scale of the other thermometer, which was employed for ascertaining the variations in the animal-heat, was of ivory. Small quantities of water placed in metallic vessels quickly boiled; but in a common earthen vessel it required an hour and a half to arrive at a temperature of 140°, nor could it ever be brought near the boiling point. Neither durst the people, who with impunity breathed the air of this very hot room at 264 degrees, bear to put their fingers into the boiling water, which indicated only a heat of 212°. So far from this, they could not bear the touch of quicksilver heated only to 120°, and could but just bear spirit of wine at 130°.

Animal Heat. Of this there are various degrees; some animals preferring a heat of 100° or more in all the different temperatures of the atmosphere; others keep only a few degrees warmer than the medium which surrounds them; and in some of the more imperfect animals, the heat is scarcely one degree above the air or water in which they live.

The phenomenon of animal-heat hath, from the earliest ages, been the subject of philosophical discussion; and, like most other subjects of this nature, its cause is not yet ascertained. The best treatises that have appeared on the subject are those of Dr. Dugald Leslie, published in 1778; and Mr Adair Crawford, in 1779. From the first of these performances, the following account of the different opinions on this subject is extracted.

"The ancients possessed not the requisites for minutely investigating the science of nature; and, prone to superstition, attributed every phenomenon which eluded their investigation, to the influence of a supernatural power. Hippocrates, the father and founder of medicine, accounted animal heat a mystery, and bestowed on it many attributes of the Deity. In treating of that subject, he says in express terms, 'what we call heat, appears to me to be something immortal, which understands, feels, hears, and knows everything present and to come.'—Aristotle seems to have considered the subject particularly, but nothing is to be met with in his works that can be said to throw light upon it.—Galen tells us that the dispute between the philosophers and physicians of his time was, 'whether animal-heat depended on the motion of the heart and arteries; or whether, as the motion of the heart and arteries was innate, the heat was not also innate.' Both these opinions, however, he rejects; and attempts a solution of the question on his favourite system, namely, the peripatetic philosophy; but his leading principles being erroneous, his deductions are of course inadmissible.

"To enter into a minute detail of all the opinions offered by the moderns on the cause of animal-heat, would far exceed our limits. Most of them, however, may be referred to one or other of the three general causes of heat, viz. mixture, fermentation, and mechanical means, each of which we shall particularly consider.

"1. Chemical mixture. When chemical philosophy first came into vogue, and prevailed in the theory as well as practice of medicine, almost every operation in the animal machine was said to be the effect of ferment or mixture. From observing, that on the mixing of certain bodies far below the temperature of the human body, a degree of heat sometimes rising to actual inflammation was produced; they, without further investigation, pronounced mixture the sole cause of animal heat. Various, however, were the opinions, not only respecting the place where the mixture happened, but also concerning the nature of the fluids of which it consisted. Van Helmont, Sylvius, and several others, supposed that the mixture took place in the intestinal tube; and ascribed it to an effervescence between the pancreatic juice and the bile. Others discovered acids in one place, and alkalies in another; but the general opinion for near two centuries was, that acidulous fluids taken in, meeting with others of an alkaline nature already prepared in the body, gave rise to the degree of heat peculiar to animals. But those who are in the least acquainted with the laws of the animal economy, need not be told that these opinions are mere conjectures, founded on facts gratuitously assumed. No experiments have shown either an acidification or alkalification in the bile that is sufficient to unite with the other animal juices, and generate the heat of animals. But though we should admit the supposition in its full extent, still it would by no means be sufficient to account for the stability of animal heat in different climates and seasons; its equability all over the body when in health; its partial increase in topical inflammations; or hardly indeed for any one phenomenon attending its production.

"Since, then, it appears that the fluids supposed to be mixed, the place in which the mixture is made, and every other circumstance relating to it, are neither ascertained nor seconded by analogy, none will, we presume, hesitate to reject every hypothesis of the cause of animal heat founded on the effects of mixture.

"2. Fermentation. When a more accurate and extensive knowledge of the various operations of nature had convinced physiologists of the absurdity of explaining the vital functions of animals, and the several changes which take place in the living body by the effects of chemical mixture, fermentation was substituted in its stead. All had observed, that fermentation was generally accompanied by heat; and few were ignorant, that that identical process, or one extremely similar to it, was constantly going forward in living animals; and it was not without some appearance of truth, that physiologists attributed animal heat to that cause.

"Formerly there were various modifications of this opinion; but of late it has been chiefly confined to one species of fermentation, viz. the putrefactive, which indeed is more convenient to experience and found philosophy. For although animal substances are either directly or indirectly produced from vegetables, as all animals live on vegetables, or on animals that have lived on them; and though they may be ultimately resolved into the same principles; yet they are certainly combined in a different manner; for they constitute compounds, the natures of which are essentially different; and of the three stages of fermentation, the vinous, acetous, and putrid, the last is the only one to which they show a tendency. Milk indeed tends to the acetous, and even to the vinous fermentation; but as it can hardly be considered as perfectly animalized, it ought not to be considered as an exception to the general position. And though it be readily admitted, that animal matter is extremely apt to putrefy, and that even in the living body there is a tendency to that process; yet it may be shown, that the degree to which it takes place can have little or no share in generating the heat of animals. In the first place, the effect of any degree of putrefaction in producing heat, is to this day so ill ascertained, that, with many ingenious philosophers, it is altogether problematical, whether or not animal substances, during the putrefactive process, do ever generate heat. Neither M. Beaume nor Dr Pearson, who made several accurate experiments with a view to ascertain this point, could, by the assistance of the most sensible thermometers, discover the least difference between the temperature of the putrefying mixtures and the surrounding medium; and were the putrefaction of animal substances readily attended with the generation of heat, we might expect to find it greater in proportion to the bulk of the putrefying mass. This, however, is not the case; for it has often been found, that the largest masses of animal matter, such as the carcase of a large whale, laid out and exposed to the air in such a putrid condition as to affect all the neighbourhood with an intolerable stench, did not to the persons handling it feel sensibly hotter than the circumambient air. But what at once overturns everything that can be advanced in favour of the generation of animal heat on the principles of putrefaction is, that heat is far more considerable in a living than in a dead body; and no rational physiologist will deny, that the putrid fermentation is going forward more rapidly in the latter than in the former.

"The mechanical generation of heat. This opinion first took its rise from an observation, that animal heat generally keeps pace with the state of the circulation: while the action of the heart and arteries continues unimpaired, a high degree of animal heat is produced; but when that action becomes more languid, the heat of the animal is diminished also. This, till very lately, was the favourite opinion of physicians, and was introduced immediately after Harvey had discovered the circulation of the blood, and indeed seems to be supported by many striking facts. Physiologists looked upon it as a matter almost capable of mathematical demonstration; yet they could not agree whether the heat of animals is occasioned by the friction of the blood against the vessels which contain it, or by the internal friction and agitation of the particles among one another. Various hypotheses accordingly were framed, and many ingenious arguments brought in support of them: but all suppositions of the mechanical kind are overthrown by some thermometrical observations of De Haen and others, from which it appeared, that the heat of the body was sometimes greater than is usual with healthy people, at the time the person was just expiring, when the action of the vessels was very weak; nay, even after he was dead, when it had entirely ceased. The abovementioned physician relates two very remarkable cases of this kind. In the one, he found that the temperature of his patient, which during the course of an inflammatory fever had never exceeded 103 degrees, at the time he expired, and for two minutes after, stood at 106. From the other it appeared, that the heat of a person who was dying of a lingering distemper, rose in the last agony from 100 to 101, and continued there stationary for two hours; and, even at the expiration of 15 hours, had only fallen to 85°, though the surrounding medium did not exceed 60°. The examples also of those who are suffocated by fixed air, entirely overturn not only the mechanical system, but almost every other which hath yet appeared on the subject. [See the article Blood, p. 31.]

"One or other of the abovementioned hypotheses Dr Cullen's continued to be adopted by physicians, till Dr Cullen attempted a solution on a new set of principles; but, attentive to the diffidence with which novel opinions ought to be broached, he delivered his as little more than a mere conjecture. 'May it not (says he) be supposed, that there is some circumstance in the vital principle of animals, which is in common to those of the same class, and of like economy; and which determines the effect of motion upon the vital principle to be the same, though the motion acting upon it may be in different circumstances?'—The doctor was driven to this supposition from the difficulty he found in explaining how so many animals of a different age, size, and temperament, should possess very nearly the same degree of heat; and in which it is impossible to show, that the motion of the blood in all its circumstances is exactly the same; or that in the different animals in which the degree of heat is considerably different, the motion of the circulating mass is in each correspondent to the difference of temperature. But, granting that the degree of heat does not always obtain in an exact ratio with the motion of the blood, and that this is an insuperable objection to its mechanical generation; yet there appear no plausible grounds for supposing that the effect of motion may be the same, while the motion acting upon it is in different circumstances. By this Dr Cullen means, that the different temperature of different animals is owing to a difference of the vital principle, infomuch that the velocity of the blood may be the same in a frog as in a man; and yet, in consequence of the different vital principle, the heat produced may be different. The facts upon which he seems to lay the greatest stress are, That neither where the surrounding medium considerably surpasses the temperature of the living body, nor where it is far below it, is there any sensible change in the heat of animals. These, and some similar facts, in appearance countenance his hypothesis; yet we have no solid reason for imagining the principle of life to be different in different animals. And how are we to conceive, that the same degree of motion should in one class of animals always produce a certain degree of heat, and in another class as regularly a different one? A proposition of such a nature should, no doubt, require the most obvious facts and conclusive arguments to establish it; but, in the present instance, we do not perceive any probable reason, even from analogy. Besides, to say that the principle of life can generate heat or cold, independent of chemical or mechanical means, is contrary to experience, and seems in itself absurd.

"In the 66th volume of the Philosophical Transactions, Dr Hunter, after reciting some experiments concerning animal heat, affirms, That certain animals entirely destitute of nerves, are endowed with a power of generating their own heat; and this he brings as an argumentum crucis against those who account the nervous system the seat of animal-heat. If this is really a fact, it must, no doubt, have all the weight he ascribes to it; but it is plain that no stress can be laid upon it, unless it was better ascertained, which it is evident it never can be. For though we can positively affirm that nerves exist where we see them, yet we cannot affirm with equal certainty that they do not also exist where we are not able to discover them. For all anatomists allow, that there are thousands of nervous filaments so finely interwoven into the composition of the more perfect animals of every size, that they elude not only the knife and naked eye, but even the best optical instruments hitherto invented. Since then we admit the presence of nerves in one tribe of animals, though we can only perceive them in their effects; what solid reason have we to deny them in another, in which we have the very same evidence, viz. certain indication, of sense and motion?

Another theory, and perhaps the best supported which hath yet appeared on the subject, is that of Dr Black. That excellent chemist having observed, that not only breathing animals are of all others the warmest, but also that there subsists such a close and striking connection between the state of respiration and the degree of heat in animals, that they appear to be in an exact proportion to one another, was led to believe, that animal heat depends on the state of respiration; that it is all generated in the lungs by the action of the air upon the principle of inflammability, in a manner little dissimilar to what he supposed to occur in actual inflammation; and that it is thence diffused by means of the circulation over the rest of the vital system.

This opinion is supported by many forcible arguments. 1. It is pretty generally known to naturalists, that a quantity of mephitic phlogisticated air is constantly exhaling from the lungs of living animals.—Since, therefore, atmospheric air, by passing through the lungs, acquires the very same properties as by passing through burning fuel, or by being exposed to any other process of phlogistication, it is obvious, that the change which the common air undergoes in both cases must be attributed to one and the same cause, viz. its combination with phlogiston. 2. It has likewise been urged in favour of the same hypothesis, that the celerity with which the principle of inflammability is separated in respiration, is very closely connected with the degree of heat peculiar to each animal. Thus, man, birds, and quadrupeds, vitiate air very fast; serpents, and all the amphibious kind, very slowly; and the latter are of a temperature inferior to the former, and breathe less frequently. 3. The most cogent arguments that have been brought in support of this opinion are, that no heat is generated till the function of respiration is established; and that the fetus in utero derives all its heat from the mother."

Upon this theory our author makes the following observations, which we shall give in his own words.

These arguments may, perhaps, on a superficial view of the question, appear conclusive; but a sound reasoner, who shall coolly and impartially weigh every circumstance, will, I am confident, allow that they only afford a very ambiguous and imperfect evidence of the doctrine they are meant to establish; and the subsequent animadversions on Dr Black's theory at large, will, it is hoped, suffice to show, that it is not only founded on dubious and controvertible principles, but that it is, in every point of light, clogged with unsurmountable difficulties.

I. Many and various are the proofs which evince the improbability of the lungs being the source or laboratory of animal heat: for, though it be granted, that there subsists a very striking connection between the state of respiration and the degree of heat in animals, and that they are even in proportion to one another; yet it by no means ensues, that the former is positively the cause of the latter. For, were that really the case, it is obvious, that those animals which are destitute of the organs of respiration would generate no heat. That, however, is not true in fact: for those fishes which are even destitute of gills, appear from various experiments to be warmer than the ordinary temperature of the element in which they live; an irrefragable proof that the function of respiration is not absolutely necessary to the production of heat in animals.

II. If the heat of living animals be generated solely in the lungs, two things necessarily follow: the first, That it can only be communicated to the other parts of the body through the channel of the arterial system; the second, That the heat must decrease as it recedes from its supposed centre. And a clear and satisfactory evidence of both these points will, no doubt, be deemed requisite to render Dr Black's opinion in any degree probable. So far, however, are we from meeting with those positive and convincing proofs which we had reason to expect, that we are not presented with a single plausible argument in favour of either of the points. On the contrary, it is more conformable to facts, that the venous blood is, if not warmer, at least as warm as the arterial. Dr Stevenon, an ingenious and accurate physiologist, with a view to ascertain this matter, laid bare the jugular vein and carotid artery of a calf, and then tied and cut them off at once, in order to let equal quantities of blood flow, in a given time, into vessels of an equal capacity, in each of which he had placed a well-adjusted thermometer; the result of the experiment was, that the thermometer immersed in the venous blood rose several degrees above that placed in the arterial. But though it is probable that there is not such a difference as that experiment seems to make, yet several reasons incline me to think, that the venous blood, instead of being colder, as Dr Black maintains, is in fact somewhat warmer, than the arterial; and what entirely overturns his opinion is, that no experiment, though many have been made, has ever shown that the temperature of the blood is higher in the left ventricle of the heart than in the right, which must necessarily be the case, were all the heat of the animal body generated in the lungs.

III. Having thus rendered it improbable that the generation of animal-heat should be entirely confined to the lungs, we shall venture a step farther, and endeavour to show, that the vital fluid, so far from acquiring all its heat in the pulmonary system, communicates no inconsiderable portion of what it had received in the course of the circulation to the air alternately ternately entering into that organ and issuing from it.

Various are the arguments which tend to evince this opinion. Were the blood heated in the lungs, we should certainly need less of their function in a warm than in a cold atmosphere; but we are taught by experience, that when the air is extremely hot, and we wish to be cooled, we breathe full and quick; and that when it is intensely cold, our respiration is slow and languid; which, were the blood heated in the lungs by the action of the air upon it, surely should not be the case. It is therefore more consonant with reason and experience, that the air which we inspire, by carrying off a quantity of evolved phlogiston from the lungs, rather contributes to diminish than increase the heat of breathing animals. Respiration, for this reason, has been very properly compared, by an ingenious physiologist, Dr Duncan of Edinburgh, to the blowing of bellows on a hot body. In both cases a considerable degree of heat is communicated to the air; but in neither can the air be said to generate any heat; for if it did, the heat of breathing animals should increase in proportion to the quantity of air inhaled, and a piece of inert matter heated to a certain degree should become hotter by ventilation.

"IV. The fetus in utero, according to Dr Black's hypothesis, generates no heat. The arguments by which he supports that position, how ingenious soever they may be, seem not sufficiently cogent to produce conviction; and as the question from its nature hardly admits of any direct experiment, our reasoning upon it must necessarily be analogical. Hence arises our embarrassment; for, as the discovering of analogies depends on the quickness and fertility of fancy, and the truth of all analogical ratiocination on the acuteness and nicety of judgment, two powers of the soul seldom united in an eminent degree, we cannot wonder that arguments of this kind, which to one man seem unanswerable, should to another appear futile.

"The only plausible objection to the generation of heat in the fetus, is, the supposition that it would in a short time accumulate in such a manner as to become incompatible with life.

"This argument, however, is more specious than solid; for, granting that the circulation which is carried on between the fetus and the mother, transmits very nearly the temperature of her blood, that by no means entirely supercedes the necessity of heat being generated in it. Various reasons lead to this opinion.—It is an axiom, that heat decreases as it recedes from the source from which it sprang. Now, if we admit for a moment Dr Black's opinion, and believe the heat of animals to be generated solely in the lungs, is it not obvious, that before it reaches the uterus, passes through the very minute tubes by which that organ is connected to the placenta, circulates through the umbilical vessels, and pervades the extreme parts of the fetus, it must be too much diminished to support that equilibrium which obtains in every part of the living system. Besides, as the fetus in utero may properly enough be accounted a part of the mother, the same objections that are brought against the generation of heat in it would hold equally good against the production of heat in any part or organ of her body, except the lungs. But such a multitude of accurate thermometrical observations have evinced the partial increase of heat in local inflammations, that no room is left to doubt, that in every individual part of the vital frame heat is generated; and if the fetus be, from any cause whatever, liable to topical inflammation, a thing which no physiologist has ever pretended to deny, what shadow of reason is there for doubting that such affections are accompanied with the same effects before as after birth, and consequently with a partial increase of heat?"

Our author having now, as he supposes, refuted the opinions of others, after showing that heat, though generated, cannot accumulate in the fetus, proceeds to lay down his own theory, which depends on the following principles:

1. That the blood does contain phlogiston. 2. That this phlogiston is evolved, extricated, or brought into a state of activity and motion by the action of the blood-vessels to which it is subjected in the course of circulation. 3. That the evolution of phlogiston is a cause which throughout nature produces heat, whether that heat be apparently excited by mixture, fermentation, concussion, friction, inflammation, ignition, or any similar cause. 4. That this heat, which must be produced in consequence of the evolution of the phlogiston from the blood of different animals, is in all probability equal to the highest degree of heat which these animals in any case possess (b).

The first and second of these propositions will readily be granted; but the third is liable to a very great objection, namely, that from putrefying bodies, phlogiston is evolved in quantity sufficient to reduce to their metallic form the calces of some metals exposed to the vapour, as Dr Dugald had acknowledged; yet he himself affirms, that no sensible heat is produced by putrefying animal substances. To this he is obliged to reply, that phlogiston is extricated more slowly from mixtures undergoing the putrid fermentation, than from such as are undergoing the vinous and acetous ones; and that the volatile alkali produced from putrefying substances likewise hinders the action of the phlogiston. But the first part of this answer is not proved, and is what he himself calls only a probable conjecture. Neither doth the second appear to be well founded: for putrefying substances, urine excepted, afford but little volatile alkali; and even putrid urine itself, which affords such a large portion, is not colder than other putrid matters.

It is however needless to insist farther on this theory, since his fundamental principle, namely, That the venous blood is warmer than the arterial, hath been shown to be false by Mr Adair Crawford, of whose hypothesis we must now give an account.

This gentleman, who, in his general doctrine of heat, Mr Crawford seems to agree with Dr Irvin of Glasgow, begins with the theory.

(b) These theories, inserted in the last edition of this work, we thought it proper to retain, as there seems still a possibility of the phlogistic doctrine regaining its ground, though now threatened with being expelled from the system of nature. A particular account of the dispute concerning Phlogiston is given under that article. Heat is contained in great quantities in all bodies when at the common temperature of the atmosphere.

2. Heat has a constant tendency to diffuse itself over all bodies, till they are brought to the same degree of sensible heat.

3. If the parts of the same homogeneous body have the same degree of sensible heat, the quantities of absolute heat will be proportional to the bulk or quantity of matter. Thus the quantity of absolute heat contained in two pounds of water, must be conceived to be double of that which is contained in one pound, when at the same temperature.

4. The mercurial thermometer is an accurate measure of the comparative quantities of absolute heat which are communicated to the same homogeneous bodies or separated from them, as long as such bodies continue in the same form. If therefore the sensible heat of a body, as measured by the mercurial thermometer, were to be diminished the one half, or the one third, or in any given proportion, the absolute heat would be diminished in the same proportion.

5. The comparative quantities of absolute heat which are communicated to different bodies, or separated from them, cannot be determined in a direct manner by the thermometer. Thus, if the temperature of a pound of mercury be raised one degree, and that of a pound of water one degree, as indicated by the thermometer, it does not by any means follow, that equal quantities of absolute heat have been communicated to the water and the mercury. [See Heat and Thermometer.]—If a pint of mercury at 100° be mixed with an equal bulk of water at 50°, the change produced in the heat of the mercury will be to that produced in the water as three to two; from which it may be inferred, that the absolute heat of a pint of mercury is to that of an equal bulk of water as two to three; or, in other words, that the comparative quantities of their absolute heats are reciprocally proportional to the changes which are produced in their sensible heats, when they are mixed together at different temperatures. This rule, however, does not apply to those mixtures which generate sensible heat or cold by chemical action.

From the above position, says Mr Crawford, it follows, that equal weights of heterogeneous substances, as air and water, having the same temperature, may contain unequal quantities of absolute heat. There must, therefore, be certain essential differences in the nature of bodies, in consequence of which some have the power of collecting and retaining the element of fire in greater quantities than others, and these differences he calls throughout his treatise the capacities of bodies for containing heat.

Having premised these general facts, our author gives an account of a number of experiments made, No 149.

In order to ascertain the quantity of absolute heat contained in different bodies. These experiments were made by mixing the bodies to be examined with water, heated to different degrees; and by the temperature of the mixture, he found the proportion of the capacity of the bodies for containing heat, to water, and, of consequence, to one another. Thus he found the capacity of wheat for containing heat to be to that of water as 1 to 2.0; and, of consequence, the absolute heats of the two substances to be in the same proportion. The absolute heat of oats to that of water he found as 1 to 2.7; of barley, as 1 to 2.4; of beans, as 1 to 1.6; of flesh, as 1 to 1.3; of milk, as 1 to 1.1; and of a mixture of venous and arterial blood from a sheep, as 25.4 to 24.4. By other experiments he determined, that the absolute heat of venous blood was to that of water only as 100 to 112, whereas the absolute heat of arterial blood was to that of water as 100 to 97.08.

By experiments made with air of different kinds contained in bladders, and immersed in water, he found that the absolute heat of atmospherical air was exceedingly great, being to that of water as 18.6 to 1; that of dephlogisticated air was still greater, being to the heat of common atmospherical air as 4.6 to 1. The heat of phlogisticated and fixed air was much less; that of the latter, particularly, being to the heat of atmospherical air only as 1 to 67.

From other experiments made on metals, Mr Crawford concludes, that the absolute heat of tin, in its metallic state, is to that of water as 1 to 14.7; but the heat of calcined tin is to that of water as 1 to 10.4. In like manner, the heat of iron was to that of water only as 1 to 8; but that of the calx of iron was to the heat of water as 1 to 3.1, &c. And from these experiments he is of opinion, that the more phlogiston that is added to any body, the less is its capacity for containing heat.

From these experiments our author deduces the following theory of animal heat.—"It has been proved, that the air, which is expired from the lungs of animals, contains less absolute heat than that which is inhaled in inspiration. It has been shown, particularly, that in the process of respiration, atmospherical air is converted into fixed air; and that the absolute heat of the former is to that of the latter as 67 to 1.

"Since therefore the fixed air which is exhaled by expiration is found to contain only the one sixty-seventh part of the heat which was contained in the atmospherical air previous to inspiration, it follows, that the latter must necessarily deposit a very great proportion of its absolute heat in the lungs. It has moreover been shown, that the absolute heat of florid arterial blood is to that of venous as 11.5 to 10. And hence, as the blood, which is returned by the pulmonary vein to the heart, has the quantity of its absolute heat increased, it is evident that it must have acquired this heat in its passage through the lungs. We may conclude, therefore, that in the process of respiration, a quantity of absolute heat is separated from the air and absorbed by the blood.

"That heat is separated from the air in respiration, is farther confirmed by the experiment with phlogisticated air; from which, compared with Dr Priestley's diff- discoveries, it is manifest, that the power of any species of air in supporting animal life, is nearly in proportion to the quantity of absolute heat which it contains, and is consequently proportional to the quantity which it is capable of depositing in the lungs.

"The truth of this conclusion will perhaps appear in a clearer light from the following calculation, by which we may form some idea of the quantity of heat yielded by atmospherical air when it is converted into fixed air, and also of that which is absorbed during the conversion of venous into arterial blood.

"We have seen, that the same heat which raises atmospherical air one degree, will raise fixed air nearly 67 degrees; and consequently that the same heat which raises atmospherical air any given number of degrees, will raise fixed air the same number of degrees multiplied by 67. In the Pittsburgh experiment of freezing quicksilver, the heat was diminished 200 degrees below the common temperature of the atmosphere. We are therefore certain, that atmospherical air, when at the common temperature of the atmosphere, contains at least 200 degrees of heat. Hence, if a certain quantity of atmospherical air, not in contact with any body that would immediately carry off the heat, should suddenly be converted into fixed air, the heat which was contained in the former would raise the latter 200 degrees multiplied by 67, or 13400 degrees. And the heat of red-hot iron being 1050, it follows that the quantity of heat, which is yielded by atmospherical air when it is converted into fixed air, is such (if it were not dissipated) as would raise the air so changed to more than 12 times the heat of red-hot iron.

"If therefore the absolute heat which is disengaged from the air in respiration, were not absorbed by the blood, a very great degree of sensible heat would be produced in the lungs.

"Again, it has been proved, that the same heat which raises venous blood 115 degrees, will raise arterial only 100 degrees; and consequently, that the same heat which raises venous blood any given number of degrees, will raise arterial a less number, in the proportion of 100 to 115, or 20 to 23. But we know that venous blood contains at least 230 degrees of heat. Hence, if a certain quantity of venous blood, not in contact with any body that would immediately supply it with heat, should suddenly be converted into arterial, the heat which was contained in the former would raise the latter only 115 or 230 degrees, or 200 degrees; and consequently the sensible heat would suffer a diminution equal to the difference between 230 and 200, or 30 degrees. But the common temperature of blood is 96; when, therefore, venous blood is converted into arterial in the lungs, if it were not supplied by the air with a quantity of heat proportionable to the change which it undergoes, its sensible heat would be diminished 30 degrees, or it would fall from 96 to 66.

"That a quantity of heat is detached from the air, and communicated to the blood, in respiration, is moreover supported by the experiments with metals and their calces: from which it appears, that when bodies are joined to phlogiston, they lose a portion of their absolute heat; and that, when the phlogiston is again disengaged, they reabsorb an equal portion of heat from the surrounding bodies.

"Now it has been demonstrated by Dr Priestley, that in respiration, phlogiston is separated from the blood, and combined with the air. During this process, therefore, a quantity of absolute heat must necessarily be disengaged from the air by the action of the phlogiston; the blood, at the same moment, being left at liberty to unite with that portion of heat which the air had deposited.

"And hence animal heat seems to depend upon a process similar to a chemical elective attraction. The air is received into the lungs, containing a great quantity of absolute heat. The blood is returned from the extremities, highly impregnated with phlogiston. The attraction of the air to the phlogiston is greater than that of the blood. This principle will, therefore, leave the blood to combine with the air. By the addition of the phlogiston, the air is obliged to deposit a part of its absolute heat; and as the capacity of the blood is at the same moment increased by the separation of the phlogiston, it will instantly unite with that portion of heat which had been detached from the air.

"We learn from Dr Pr... experiments with respect to respiration, that arterial blood has a strong attraction to phlogiston: it will consequently, during the circulation, imbibe this principle from those parts which retain it with least force, or from the putrefactive parts of the system: and hence the venous blood, when it returns to the lungs, is found to be highly impregnated with phlogiston. By this impregnation, its capacity for containing heat is diminished. In proportion, therefore, as the blood, which had been dephlogisticated by the process of respiration, becomes again combined with phlogiston in the course of the circulation, it will gradually give out that heat which it had received in the lungs, and diffuse it over the whole system.

"Therefore, in respiration, the blood is continually discharging phlogiston and absorbing heat; and that, in the course of circulation, it is continually imbibing phlogiston and emitting heat.

"It may be proper to add, that as the blood, by its impregnation with phlogiston, has its capacity for containing heat diminished; so, on the contrary, those parts of the system from which it receives this principle, will have their capacity for containing heat increased, and will consequently absorb heat.

"Now if the changes in the capacities, and the quantities of matter changed in a given time, were such, that the whole of the absolute heat separated from the blood were absorbed, it is manifest that no part of the heat which is received in the lungs would become sensible in the course of the circulation.

"That this, however, is not the case, will, I think, be evident from the following considerations:

"We know that sensible heat is produced by the circulation of the blood; and we have proved by experiment, that a quantity of absolute heat is communicated to that fluid in the lungs, and is again disengaged from it in its progress through the system. If, therefore, the whole of the absolute heat, which is separated from the blood, were absorbed by those parts..." of the system from which it receives the phlogiston, it would be necessary to have recourse to some other cause, to account for the sensible heat which is produced in the circulation. But, by the rules of philosophising, we are to admit no more causes of natural things than such as are both true and sufficient to explain the appearances; for nature delights in simplicity, and affects not the pomp of superfluous causes.

"We may, therefore, safely conclude, that the absolute heat which is separated from the air in respiration, and absorbed by the blood, is the true cause of animal-heat.

"It must nevertheless be granted, that those parts of the system which communicate phlogiston to the blood, will have their capacity for containing heat increased; and therefore, that a part of the absolute heat which is separated from the blood will be absorbed.

"But from the quantity of heat, which becomes sensible in the course of the circulation, it is manifest that the portion of heat which is thus absorbed is very inconsiderable.

"It appears, therefore, that the blood, in its progress through the system, gives out the heat which it had received from the air in the lungs: a small portion of this heat is absorbed by those particles which impart the phlogiston to the blood; the rest becomes redundant, or is converted into moving and sensible heat."

Mr Crawford's theory, which doth not essentially differ from Dr Black's, seems to be the best that hath yet appeared. There is, however, one difficulty which seems common to them all, and which, even on Mr Crawford's principles, seems not to admit of solution. If animal heat entirely depends on something peculiar to a living body, why doth it sometimes continue after life hath ceased? If heat depends on the evolution of phlogiston by the action of the blood-vessels, according to Dr Duguid, why should it remain when these vessels cease to act, as, according to Dr Duguid himself, it sometimes doth? If, according to Mr Crawford, it is every moment attracted from the air, why is it not always in proportion to the respiration? Or, if fixed air contains such a small proportion of absolute heat as, by Mr Crawford's experiments, it seems to do, why doth it impart such a strong and lasting degree of heat to the bodies of those who are killed by it? See Blood, n° 31.

Other objections have been made by Mr Pearson, which are related in the Medical Journal. They are founded on some appearances found on the dissection of morbid bodies; where it has been found that the pulmonary artery, and even the lungs themselves, have been totally destroyed by disease, and yet the person has survived for some time. In these cases, however, it is probable, that the blood had still an opportunity of absorbing the vital principle from the air, which might make those produce heat also by some mechanism unknown to us. The whole of Dr Crawford's doctrine of latent heat has also been attacked in a Treatise by Mr Leopold Vacca Berlinghieri. His objections are derived from the calculations of Dr Crawford himself; but our limits will not admit of our entering into this dispute.

Internal Heat of the Earth. That there is a very considerable degree of heat always felt in digging to great depths in the earth, is agreed upon by all naturalists: but the quantity of this heat hath seldom been measured in any part; much less is it known, whether in digging to an equal depth in different parts of the earth, the heat is found always the same. In digging mines, wells, &c. they find that at a little depth below the surface it feels cold. A little lower it is colder still, as being beyond any immediate influence of the sun's rays; inasmuch that water will freeze almost at any season of the year; but when we go to the depth of 40 or 50 feet, it begins to grow warm, so that no ice can bear it; and then the deeper we go, still the greater the heat, until at last respiration grows difficult, and the candles go out.

This heat of the earth hath been variously explained. Some have had recourse to an immense body of fire lodged in the centre of the earth, which they consider as a central sun, and the great principle of the generation, vegetation, nutrition, &c. of fossil and vegetable bodies. But Mr Boyle, who had been at the bottom of some mines himself, infers that this degree of heat, at least in some of them, may arise from the peculiar nature of the minerals generated therein. To confirm this, he instances a mineral of a vitriolic kind, dug up in large quantities in many parts of England, which by the bare affusion of common water will grow so hot, that it will almost take fire.—These hypotheses are liable to the following objections. 1. If there is within the earth a body of actual fire, it seems difficult to show why that fire should not consume and moulder away the outer shell of earth, till either the earth was totally destroyed, or the fire extinguished. 2. If the internal heat of the earth is owing to the action of water upon mineral substances, that action through time must have ceased, and the heat have totally vanished; but we have no reason to think that the heat of the earth is anything less just now than it was a thousand years ago. The phenomenon is easily explained by the propositions laid down under the article HEAT. If heat is nothing else than a certain mode of action in the ethereal fluid, or the matter of light, by which it flows out from a body in all directions as radius drawn from the centre to the circumference of a circle; it will then follow, that if an opaque body absorbs any considerable quantity of light, it must necessarily grow hot. The reason of this is plain. The body can hold no more than a certain quantity of ethereal matter; if more is continually forcing itself in, that which has already entered must go out. But it cannot easily get out, because it is hindered by the particles of the body among which it is detained. It makes an effort therefore in all directions to separate these particles from each other; and hence the body expands, and the effort of the fluid to escape is felt when we put our hands on the body, which we then say is hot. Now, as the earth is perpetually absorbing the ethereal matter, which comes from the sun in an immense stream, and which we call his light, it is plain, that every pore of it must have been filled with this matter long ago. The quantity that is lodged in the earth, therefore, must be continually endeavouring to separate its particles from each other, and consequently must make it hot. The atmosphere, which is perpetually receiving that portion of the ethereal matter which which issues from the earth, counteracts the force of the internal heat, and cools the external surface of the earth, and for a considerable way down; and hence the earth for 20 or 30 feet down, shows none of that heat which is felt at greater depths. See Heat.

medicine. Great heats are not so much the immediate, as the remote, cause of a general sickness, by relaxing the fibres, and disposing the juices to putrefaction; especially among soldiers and persons exposed the whole day to the sun: for the greatest heats are seldom found to produce epidemic diseases, till the perspiration is stopped by wet clothes, fogs, dews, damps, &c. and then some bilious or putrid distemper is the certain consequence, as fluxes and ardent intermittent fevers. Nevertheless, it must be allowed, that heats have sometimes been so great as to prove the more immediate cause of particular disorders; as when centinels have been placed without cover or frequent reliefs in scorching heats; or when troops march or are exercised in the heat of the day; or when people imprudently lie down and sleep in the sun. All these circumstances are apt to bring on distempers, varying according to the season of the year. In the beginning of summer, these errors produce inflammatory fevers; and in autumn, a remitting fever or dysentery. To prevent, therefore, the effects of immoderate heats, commanders have found it expedient so to order the marches, that the men come to their ground before the heat of the day; and to give strict orders, that none of them sleep out of their tents, which, in fixed encampments, may be covered with boughs to shade them from the sun. It is likewise a rule of great importance to have the soldiers exercised before the cool of the morning is over; for by that means not only the sultry heats are avoided, but the blood being cooled, and the fibres braced, the body will be better prepared to bear the heat of the day. Lastly, in very hot weather, it has often been found proper to shorten the centinels duty, when obliged to stand in the sun.