experiments were made. As to what regards Mr Leflie's pecu. liar claims to priority, it is fufficient to remark, in justice to him, that the whole of his book was printed before the Count's paper was communicated to the Royal Society; that the experiments were performed in 1801, whereas the Count only began to ope rate in 1803; and that, in 1802, Mr Leflie's first chapters were all printed off. Thefe circumftances, which are partly ftated in the preface, before the prefent part of the Philofophical Tranfactions appeared, and partly appear from comparing the dates of the two works, throw the whole fufpicion, in our mind, upon Count Rumford, and render it incumbent on Mr Lef lie only to bring forward fuch facts as he may be in poffef on of, to fhew how the knowledge of his experiments may have got abroad and reached Count Rumford while his work was preparing for publication. We fhall difrifs this part of the fubject with expreffing our high admiration of that very important and elegant modification of the air-thermometer which Mr Leflie, not Count Rumford, has happily contrived; an improve ment, calculated to introduce as much accuracy, and to open as wide a field of discovery in the fcience of heat, as the combination of glaffes did in the fciences of aftronomy and optics. By it, we are enabled to weigh, with the utmoft nicety, all proportions of caloric, and to eftimate, as correctly as by a delicate balance, every variation of temperature. In reviewing the application of this happy invention to the purposes of investigation, as detailed by the difcoverer himfelf, we shall have an opportunity of doing juftice to its merits. At prefent, we haften to sketch the ufes which the borrower of the idea has made of it, and in which we shall again be fatisfied how unwilling he has been to deviate from his original.

The mode in which Count Rumford operated with what he calls his thermofcope, was by expofing it to brafs cylinders like thofe formerly defcribed, but fixed horizontally, fo as to prefent their circular end to the ball of the inftrument. After afcertaining the exactness of the inftrument, by finding that the liquor remained stationary when two cylinders uncoated and filled with the fame hot water were placed at equal diftances from the balls, but that a flight variation in the temperature or diftance of either cylinder caufed the liquor in the tube to move, our author proceeded to verify by this delicate teft his former experiments on flow cooling. He found that the circular end of one cylinder being coated with candle fmoke, while the other remained clear; the bubble inftantly receded from the ball expofed to the former, and did not regain its equilibrium until that cylinder had been reved to four times the diftance of the other. He allo found

that

that linen, glue, fpirit-varnish and paint, produced the fame effects in the experiments of the thermofcope as they had formerly done in those of the mercurial thermometer. He then repeated his former experiments of flow cooling with veffels of lead and tinned iron, and with the brafs cylinders coated with gold and filver leaf. The refults entitled him to believe that no difference whatever was produced by any change of the metal containing the radiating body. Our author now refumed his trials with the thermofcope, and, fubftituting a cold for a hot body in the cylin der, found that the liquor in the tube was attracted towards the the bulb which was near the cold cylinder, in proportion to its degree of cold and to its proximity to the bulb; and that if two cylinders equally cold, but one coated with candle fmoke, were prefented to the balls, at equal distances, the liquor moved towards the blackened cylinder. One of the cylinders being coated with animal membrane was found to radiate both heat and cold (according to our author's theory of frigorific radiation) five times more copiously than the naked cylinder. He alfo found that if one ball remains at its natural temperature, while to the other there are prefented on oppofite fides two bodies, the one as much above that temperature as the other is below it, no change whatever takes place in the pofition of the liquid. The fame refult follows from varying this experiment by coating both the cylinders with candle-fmoke. Previous to fome ingenious fpeculations on the practical application of the foregoing facts, we are prefented with an experiment to prove that both calorific and frigorific radiation is much more copious from animal membrane of a black colour, than of any other hue. As a fpecimen of the author's ingenuity in applying his facts, we shall extract the following paffage.

It is evident, that the greater the power is which an animal poffeffes of throwing off heat from the furface of his body, independently of that which the furrounding air takes off, the lefs will his temperature be affected by the occafional changes of temperature which take place in the air, and the lefs will he be oppreffed by the intenfe heats of hot climates.

It is well known that negroes and people of colour fupport the heats of tropical climates much better than white people. Is it not probable that their colour may enable them to throw off calorific rays with great facility and in great abundance, and that it is to this circumftance they owe the advantage they poffefs over white people in fupporting heat? And even thould it be true, that bodies are cooled, not in confequence of the rays they emit, but by the action of thofe frigorific rays they receive from other colder bodies (which I much fufpect to be the cafe), yet as it has been found by experiment, that thofe bodies which emit calor fic rays in the greate!l abundance are allo molt affect

ed by the frigorific rays of colder bodies, it is evident that, in a very hot country, where the air and all other furrounding bodies are but very little colder than the surface of the skin, those who by their colour are prepared and disposed to be cooled with the greatest facility, will be the least likely to be oppreffed by the accumulation of the heat generated in them by refpiration, or of that excited by the fun's rays.' P. 129.

We have here announced to us, not merely the exiftence, but the operation of cold, and, it would appear, the banishment of heat. But the paffage is full of contradictions. For, do not all the experiments formerly analyfed prove that the existence of heat and cold is uniformly correlative-that the quantity of the one is inversely as the quantity of the other-and that when a certain portion of heat has radiated from a body, it ceases to give out any more? But can it be made to confift with all this, that bodies can only be cooled by abforbing cold, and, confequently, that they can only be heated by abforbing heat? If thefe two fubftances have each a real and feparate existence, how should it happen that equal quantities of them, when mixed, exactly go for nothing, instead of forming a third body compounded of the other two? Befides, let it be remembered, that we have the very fame evidence to prove the radiation of heat from the thermofcope to the cold body, that we have to prove the radiation of heat from the hot body to the thermofcope. Confequently, when the negro's fkin is expofed to the atmofphere of tropical climates, its colour and confistency operates in heating or in cooling him more rapidly than a white man, precifely as the atmosphere is hotter or colder than his body. If the air is cold, then he is cooler than other men; but if, which is the cafe to be explained, the air is hot, then he is much hotter than other men. It is in vain to say that he radiates heat, and receives cold more abundantly. The experiments of the thermoscope prove, that as long as he is at all cooler than the climate, he must be receiving heat more copioufly than a white man; and if he has any frigorific particles to radiate (as he muft, according to Count Rumford's theory coupled with his facts) when the air is hotter, he gives out these much more copiously than a white man.

The ingenuity of the following paffage is rather pleasing, though it is liable to fome of the foregoing objections.

Several of the favage tribes which inhabit very cold countries befmear their fkins with oil; which gives them a fhining appearance. The rays of light are reflected copiously from the furface of their bodies. May not the frigorific rays which arrive at the furface of their kin, be alfo reflected by the highly polished furface of the oil with achich it is covered?

If that fhould be the cafe, inftead of defpifing thefe poor crea, tures for their attachment to a useless and loathsome habit, we should be difpofed to admire their ingenuity, or rather to admire and adore the goodness of their invifible Guardian and Inftructor, who teaches them to like and to practise what he knows to be useful to them.

The Hottentots befmear themselves, and cover their bodies in a manner ftill more difgufting. They think themfelves fine when they are befmeared and dreffed out according to the loathfome cuftom of their country. But who knows whether they may not in fact be more comfortable, and better able to fupport the exceffive heats to which they are expofed? From feveral experiments which I made with a view to elucidate this point, (of which an account will be given to this Society at fome future period), I have been induced to conclude, that the Hottentots derive advantages from that practice exactly fimilar to those which negroes derive from their black colour.

It cannot furely be fuppofed that I could ever think of recom. mending seriously to polifhed nations the filthy practices of these favages. This is very far indeed from being my intention: For I have ever confidered cleanliness as being fo indifpenfably neceffary to comfort and happiness, that we can have no real enjoyment without it: But ftill I think, that a knowledge of the phyfical advantages which those favages derive from fuch practices, may enable us to acquire the fame advantages by employing more elegant means. A knowledge of the manner in which heat and cold are excited, would enable us to take measures for these important purposes with perfect certainty: In the mean time, we may derive much ufeful information by a careful examination of the phenomena which occafionally fall under our obfervation.

If it be true, that the black colour of a negro, by rendering him more fenfible to the few frigorific rays which are to be found in a very hot country, enables him to fupport the great heats of the tropical climates without inconvenience, it might be afked, how it happens that he is able to fupport, naked, the direct rays of a burning fun?

Those who have feen negroes expofed naked to the fun's rays in hot countries, must have obferved that their fkins in that fituation are always very fhining. An oil exudes from their skin, which gives it that fhining appearance; and the polifhed furface of that oil reflects the fun's calorific rays.

If the heat be very intense, sweat makes its appearance at the furface of the skin. This watery fluid not only reflects very powerfully the calorific rays from the fun, which fall on its polifhed furface, but alfo by its evaporation generates cold.

When the fun is gone down, the fweat difappears; the oil at the furface of the skin retires inwards; and the skin is left in a ftate very favourable to the admiffion of thofe feeble frigorific rays which arrive from the neighbouring objects. p. 132.

It is fcarcely neceffary to remark how completely all this explanation is at variance with the fpeculative obfervations last

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quoted.

quoted. There we were taught to confider cooling as the effect only of abforbing frigorific particles; now, the exclufion of heat is the cause of comfort to the negro. Formerly, we were told, that he only benefited from his colour by abforbing an extraor dinary portion of cold; now, he is provided with a coat of greafe, which must exclude both heat and cold. Thefe fanciful remarks are followed by fone experiments to prove that the cooling of hot bodies depends on the abforption of frigorific rays. They are inconclufive; because they either prove that the cooling body abforbs heat, or that the cooled body abforbs cold; and in every inftance the effects are exactly proportional, and the terms of the explanation convertible. The author is req duced, at every step, to this dilemma; either cold exists without heat, which must follow if you maintain that bodies are cooled only by receiving cold, and that they are heated only by giving out cold; or heat can never be emitted without an equal abforption of cold, nor cold emitted without an equal abforption of heat. If you chufe the latter pofition, what fort of feparately exifting bodies must those be which are fo mutually dependent on each other? If you take the former, are there not at least as many proofs adducible of the existence of heat, as of the exiftence of cold?

Before leaving the experimental branch of this fubject, we fhall notice the most original of all Count Rumford's experiments -thofe which he made to prove the great effect of polished furfaces in reflecting heat. They do not indeed demonftrate any new propofition, but there is fomewhat in the refults of them, and in his way of ftating others, which cannot fail to furprife us. A drop of water rolls about on a red-hot iron without evaporation, because its furface becomes fo highly polifhed as to reflect all the heat. If the heat be lefs, the water penetrates the pores of the oxidated iron, and, lofing its polifh, is evaporated. If the metal be lefs oxidable, the water remains unevaporated even at a low temperature. If the infide of a filver spoon be covered with candle fmoke, and a drop of water be put into it, you may hold the spoon over a lamp until it becomes violently heated, without affecting the water, which is fcarcely warmed by the heat at laft, the foot adhering to the drop, facilitates the tranfmission of heat, and the water gradually evaporates. A drop may, in like manner, be introduced into the centre of a lamp flame, without being affected, until it receives heat by the conducting power of the body which holds it. This last experiment, by the way, appears to be explicable on the commonly received doctrine, that the centre of a flame, having no accefs to oxyge nous gas, is not in a state of combuftion at all, while the furface

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