ing from the different nature of the colours with which the bulbs were painted; and the refult was not decifive.

whose ball was painted black, was conftantly higher than the other whenever they were expofed to the ftrong day-light. The difference was commonly about one 3d of a degree, but fometimes it amounted to three 4ths, or even to a whole degree; and the experiment answered even when the fun was hid by clouds, which feems to indicate that every degree of light is accompanied with a correfponding one of heat. By this confideration 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 fame. The experiment was feveral times tried with a large lens, and afterwards with a burning mirror of 18 inches diameter; yet fometimes, for want of proper means of obferving the height of the mercury in the tubes of the thermometers, fometimes for want of a continued clear light of the moon, or, in fhort, from fome unfavourable circumftance or other, he was never able to make a fair and decifive trial of this experiment. Upon trying the heat of a lamp, he found that it alfo had a confiderable effect. The ball of one being blackened, and both fet at two inches diftance from the flame of a lamp, they both rofe 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 683. From a number of trials it at laft appeared, that the difference at this diftance from the lamp amounted generally to about a degree. When the thermometers were removed farther than two inches from the lamp, the difference decreafed; and at the diftance of about 14 or 15 inches it vanifhed entirely. On this occafion Mr Cavallo had an opportunity of making a curious obfervation concerning the decrease of heat at different diftances from the centre. "It is mathematically true, that emanations which proceed from a centre, and expand in a fphere, muft become more and more rare in proportion to the fquares of the distances from the centre. Thus it is faid, that the intensity of light proceeding from a luminous body, at the double, treble, quadruple, &c. diftance from that body, must be refpectively 4, 9, 16 times, &c. lefs denfe. The fame thing may be faid of heat; but with refpect to the latter, it appeared, that its intenfity did not decrease exactly in the duplicate proportion of the distances from the flame of the lamp, but fhowed a very odd irregularity. It feemed to decrease fafter than the duplicate proportion of the diftances for the space of 2 or 3 inches, after which it decreased much flower; but whether this proceeded from fome different ftate 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 fome 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 fubtances, to determine whether they kept any proportion to the fpaces occupied by the prifmatic colours in the prifmatic spectrum, or if they followed any other law: But in thefe experiments he met with confiderable difficulties, chiefly arif

(6.) HEAT, EXPERIMENTS RESPECTING THE DEGREES OF, WHICH ANIMALS CAN BEAR. The ancients were of opinion, that all countries lying within the tropics were uninhabitable by reafon of their heat: but time has difcovered their mistake; and it is now found, that no part of the world is too hot for mankind to live in. The learned Dr Boerhaave, in his chemistry, relates certain expe riments made with great accuracy by the celebrated Fahrenheit, and others at his defire, on this fubject, 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 fparrow fubjected to air thus heated died, after breathing very laboriously, in lefs than 7 minutes. A cat refifted this great heat fomewhat above a quarter of an hour; and a dog about 28 minutes, difcharging before his death a confiderable quantity of a ruddy-coloured foam, and exhaling a ftench, fo peculiarly offenfive, as to throw one of the affiftants into a fainting fit. This diffolution of the humours, or great change from a natural ftate, the profeffor attributes not to the heat of the ftove alone, which would not have produced any fuch effect on the flesh of a dead animal; but also, to the vital motion, by which a ftill greater degree of heat, he fuppofes, was produced in the fluids circulating through the lungs, in confequence of which the oils, falts, and fpirits of the animal became fo highly exalted. Meffieurs Du Hamel and Tillet having been fent into the province of Augomois, in 1760 and 1761, with a view to destroy an infect which confumed the grain of that pro❤ vince, effected the fame in the manner related in the Memoirs for 1761, by expofing the affected corn, with the infects included in it, in an oven, where the heat was fufficient 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 affure 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 fhovel, which, on its being withdrawn, indicated a degree of heat confiderably above that of boiling water: but M. Tillet, convinced that the thermometer had fallen feveral degrees in drawing to the mouth of the oven, and appearing under fome embarrafiment 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 ftood within the oven. M. Tillet appearing to hefitate at this ftrange propofition, the girl failed, and entering the oven, marked the thermometer with a pencil, after staying two or three minutes, ftanding at 100° of Reaumur's scale, or at near 260° of Fahrenheit's. M. Tillet began to exprefs an anxiety for the welfare of his female affiftant, and to prefs her return. This female falamander, however, affuring him that the felt no inconvenience from her fituation, remained there 10 minutes longer; that is, near the time when Boerhaave's cat parted with her nine lives, under a much lefs degree of heat; when, the thermometer ftanding at 288°, or 76° above that of boiling

S 2

water,

The first impreffion of this hot air upon his body was exceedingly disagreeable, but in a few minutes all his uneafinefs was removed by the breaking out of a fweat. At the end of 12 minutes he left the room very much fatigued, but no otherwise difordered. His pulfe beat 136 in a minute, and the thermometer had risen to 220°. In other experiments it was found, that a heat even of 260° of Fahrenheit could be submitted to with tolerable ease. But it must be observed, that inthefe great heats every piece of metal they carried about with them became intolerably hot. Small quantities of water placed in metalline veffels quickly boiled; but in a common earthen veffel it required an hour and an half to arrive at a temperature of 140°, nor could it ever be brought near the boiling point. Neither durft the people, who with impunity breathed the air of this very hot room at 264°, 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 quickfilver heated only to 120°, and could but just bear spirit of wine at 130°.

(7.) HEAT, HYPOTHESES AND EXPERIMENTS RESPECTING. See CHEMISTRY, Index; COMBUSTION, III-VI; ELECTRICITY, Index; FIRE, 2, 6, 7; FLAME, § 3; &c.

(8.) HEAT, INTERNAL, OF THE EARTH. That there is a very confiderable degree of heat always felt in digging to great depths in the earth, is a greed upon by all naturalifts: but the quantity of this beat hath feldom been measured in any part; nor is it known, whether in digging to an equal depth in different parts of the earth, the heat is found always the fame. In digging mines, wells, &c. they find that at a little depth below the furface it is cold. A little lower it is colder ftill, as being beyond any immediate influence of the fun's rays; infomuch that water will freeze almost at any feafon of the year: but when we go to the depth of 40 or 30 feet, it begins to grow warm, fo that no ice can bear it; and then the deeper we go, ftill the greater the heat, until at last refpiration grows difficult, and the candles go out. This heat of the earth has been variously explained. Some fuppofe an immenfe body of fire lod. ged in the centre of the earth, which they confider as a central fun, and the great principle of the generation, vegetation, nutrition, &c. of foffile and vegetable bodies. But Mr Boyle, who had been at the bottom of fome mines himself, fufpects that this degree of heat, at leaft in fome of them, may arise from the peculiar nature of the minerals generated therein. In proof of this, he inftances a mineral of a vitriolic kind, dug up in large quantities in many parts of England, which by the bare affufion of common water will grow fo hot, that it will almoft take fire.-Thefe hypothefes are liable to the following objections: 1. If, there is within the earth a body of actual fire, it feems difficult to fhow why that fire fhould not confume the outer fhell of earth, till either the earth was totally destroyed, or the fire extinguifhed. 2. If the internal heat of the earth is owing to the action of water upon mineral fubftances that action through time must have ceafed, and the heat have totally vanished; but we have po reafon to think that the heat of the earth is

any thing lefs now than it was 1000 years ago. The phenomenon is eafily explained by the propofitions above laid down. (§ 2, 3.) 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 abforbs any confiderable quantity of light, it must neceffarily grow hot. The reafon 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 eafily 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 thefe particles from each other; and hence the body expands, and the effort of the fluid to efcape is felt when we put our bands on the body, which we then fay is hot. Now, as the earth is perpetually absorbing the ethereal matter, which comes from the fun 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 confequently muft make it hot. The atmosphere, which is perpetually receiving that portion of the ethereal matter which iffues from the earth, counteracts the force of the internal heat, and cools the external surface of the earth, and for a confiderable way down; and hence the earth for 20 or 30 feet down, shows none of that heat which is felt at greater depths.

(9.) HEAT, METHODS OF MEASURING. THERMOMETER.

See

(10.) HEAT, NOXIOUS EFFECTS OF IMMODERATE. Great heats are rather the remote, than the immediate caufe of epidemical diseases, by relaxing the fibres, and difpofing the juices to putrefaction, efpecially among foldiers and perfons expofed the whole day to the fun; for the greateft heats are feldom found to produce epidemic diseases, till the perfpiration is ftopped by wet clothes, fogs, dews, damps, &c. and then fome bilious or putrid dif temper is the certain confequence, as fluxes and ardent intermitting fevers. Sometimes, however, heats have been fo great as to prove the more immediate caufe of particular disorders; as when centinels have been placed without cover or frequent reliefs in feorching heats; or when troops march or are exercifed in the heat of the day; or when people imprudently lie down and fleep in the fun. All these circumstances are apt to bring on diftempers, varying according to the season of the year. In the beginning of fummer, they produce inflammatory fevers; and in autumn, remitting fevers or dyfenteries. To prevent, therefore, the effects of immoderate heats, commanders have found it expedient fo 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 fleep out of their tents, which, in fixed encampments, may be covered with boughs to fhade them from the fun. It is likewife a rule of great importance to have the foldiers exercised before the cool of the morning is over; as thus not only the fultry heats

are

are avoided, but the blood being cooled, and the fibres braced, the body will be better prepared to bear the heat of the day. Laftly, in very hot weather, it has often been found proper to fhorten the centinel's duty, when obliged to ftand in the fun.

(11.) HEAT OF ANIMALS. Of the natural heat of animals, there are various degrees; fome preferving a heat of 100° or more in all the different temperatures of the atmosphere; others keeping only a few degrees warmer than the medium which furrounds them; and in fome of the more imperfect animals, the heat is fcarcely one degree above that of the air or water in which they live. The phenomenon of animal heat has, from the earlieft ages, been the fubject of philofophical difcuffion; and like most other fubjects of this nature, its caufe is not yet afcertained. "The ancients (fays Dr Duguid Lellie,) poffeffed not the requisites for minutely inveftigating the fcience of nature; and, prone to fuperftition, attributed every phe. nomenon which eluded their investigation, to the influence of a fupernatural power. Hippocrates, the father and founder of medicine, accounted animal heat a mystery, and beftow. ed on it many attributes of the Deity. In treating of the fubject, he says, in exprefs terms, 'what we call heat appears to me to be fomething immortal, which understands, fees, hears, and knows every thing prefent and to come.' A riftotle feems to have confidered it particularly, but nothing is to be met with in his works that can be faid to throw light upon it. Galen tells us that the difpute between the philofophers and phyficians 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 alfo innate? Both thefe opinions, however, he rejects; and attempts a folution of the queftion on his favourite fyftem, namely, the peripatetic philofophy; but his leading principles being erroneous, his deductions are of courte inadmiffible." The heat of the human body in its natural fate, according to Dr Boerhaave, is such as to raise the mercury in the thermometer to 92°, or at moft to 94°; and Dr Pitcairn makes the heat of the human skin the fame. Indeed it is evident that different parts of the human body, and its different ftates, as well as the different feafons, will make it thow of different temperatures. Thus, by various experiments at different times, the heat of the human body is varioully eftimated by the following authors: Boerhaave and Pitcairn, 92°; Amontons, 91, 92, or 93; Sir Ifaac Newton, 95, Fahrenheit and Mufchenbroek; the blood, 96; Dr Martine, the skin, 97 or 98; the urine, 99; Dr Hales, the skin, 97; the urine, 103; Mr John Hunter, his tongue, 97; in his rectum, 984; his urethra at inch, 92; at 2 inches, 93; at 4 inches 94; the ball of the thermometer at the bulb of the urethra, 97.

(12.) HEAT OF ANIMALS, HYPOTHESES RESPECTING THE CAUSE OF THE. There is hardly any fubject of philofophical inveftiga tion that has afforded a greater variety of hy pothefes, conjectures, and experiments, than the cause of animal heat. The first opinion, which

has very generally obtained, is, that the heat of animal bodies is owing to the attrition between the arteries and the blood. All the obfervations and reasoning brought in favour of this opinion, however, only fhow, that the heat and the motion of the arteries are generally proportional to each other; without fhewing which is the cause, and which the effect; or indeed that either is the cause or effect of the other, fince both may be the effects of some other cause. Dr Douglas, in his EfJay on the Generation of Heat in Animals, afcribes it folely to the friction of the globules, of blood in their circulation through the capillary veffels. Another opinion is, that the lungs are the fountain of heat in the human body; and this opinion is fupported by much the fame fort of arguments as the former, and feemingly to little better purpose. A 3d opinion is, that the caufe of animal heat is owing to the action of the folid parts upon one another. And as the heart and arteries move most, it has been thought natural to expect that the heat fhould be owing to this motion. But even this does not feem very plautible, from the following confiderations: 1ft, The moving parts, however we term them folid, are neither hard nor dry; which two conditions are abfolutely requifite to make them fit to generate heat by attrition. 2d, None of their motions are fwift enough to promife heat in this way. 3d, They have but a little change of furface in their attritions. And, 4thly, The moveable fibres have fat, mucilage, or liquors, every way furrounding them, to prevent their being destroyed, or heated by attrition. A 4th caufe affigned for the heat of our bodies, is that procefs by which our aliment and fluids are perpetually undergoing fome alteration. And this opinion is chiefly fupported by Dr Stevenson, in the Edinburgh Medical Effuys, vol. 5, art. 77. The late ingenions Dr Frankun inclines to this opinion, when he fays, that the fluid fire, as well as the fluid air, is attracted by plants in their growth, and becomes confolidated with the other materials of which they are formed, and makes a great part of their fubitance; that when they come to be digested, and to undergo a kind of fermentation in the vellels, part of the fire, as well as part of the air, recovers its fluid active state again, and diffufes itfelf on the body, digefting and separating it, &c. Exper. and Obf. on Electr. p. 346. Dr Mortimer thinks the heat of animals explicable from the phofphorus and air they contain. Phosphorus exifts, at leaft in a dormant ftate, in quimal fluids and it is also known that they all contain air: it is therefore only neceflary to bring the phosphoreal and aerial particles into contact, and heat muit be generated; and were it not for the quantity of aqueous humours in animals, fatal accenfions would frequently happen. See Philof. Tranf. N° 476. Dr Black fuppofes, that animal heat is generated altogether in the lungs, by the action of the air on the principle of inflamma bility, and is thence diffufed over the reft of the body by means of the circulation. But Dr Leflie urges feveral arguments againft this hypothefis, tending to thow that it is repugnant to the known laws of the animal machine; and he advances another hypothefis, viz. that the pilogifton which enters into the compofition of natural bodies is,

in