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. See CHEMISTRY Index.

HEAT of Burning Bodies. See COMBUSTION, HEAT of Chemical Mixtures. CHEMISTRY Index.

Method of Measuring HEAT. See THERMOMETER and PYROMETER, CHEMISTRY Index.

Degrees of HEAT which Animals are capable of bearing.—The ancients were of opinion, that all countries 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 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, salts, and spirits of the animal became so highly exalted.

Messieurs Du Hamel and Tillet having been sent into the province of Angamois, in the years 1760 and 1761, with a view of endeavouring to destroy an insect which consumed 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 Rocheboucault, 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 Reaumur's scale, or, to make use of a scale better known in this country, at near 265 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 Rocheboucault, 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 an 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 Rocheboucault, having 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 vesicles 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° 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 substance about us soon became very hot.

"II. My friend Mr Park, an ingenious surgeon of this place, went into the stove heated to 202°. After 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 99°, 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

(a) The scale of the thermometer, which was suspended by the string about the middle of the room, was of metal; 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 101°.

"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 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 flues in the floor, and by pouring upon it boiling water. There was no chimney in them, or 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 staid five minutes in a heat of 92°, 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 to 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. 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 from this, they could not bear the touch of quicksilver heated only to $120^\circ$, and could but just bear spirit of wine at $130^\circ$.

**Animal Heat.** Of this there are various degrees; some animals preserving a heat of $100^\circ$ 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 Dugud 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, sees, 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, or friction. See Chemistry."

**Internal Heat of the Earth.** It was formerly supposed that the heat of the earth increased in proportion to the depth from the surface; but this hypothesis proceeded from imperfect and inaccurate observation, or from the preconceived notion of the existence of central fires. At great depths, it seems not impossible that the temperature of the earth is uniformly and invariably the same; that is, at depths beyond the more immediate influence of the sun's rays. But at moderate depths, so far as observation and experiment go, the temperature of the earth is precisely the same as the average temperature of the climate where the observation is made. This fact, which is established by the uniform temperature of springs corresponding exactly with the average temperature of the climate, seems to be an irresistible argument against the opinion of the existence of central fires.

This heat of the earth has 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, suspects 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 effusion 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 any thing less just now than it was a thousand years ago. 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 radii 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 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, it is supposed, the earth for 20 or 30 feet down shows none of that heat, which is felt at greater depths. See Heat.

**Heat, in 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 sentinels...