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vcflel intended to produce the heat or cold. Inthis inftance this latter was a matrafs full offnow: the mirrors were feparated to the dis-tance of I o-i- feet. At the inftant the matrafswas placed in one focus, the thermometer inthe oppofite focus began to fink, and defcendedfeveral degrees. When Stationary, nitrous acidwas poured on the fnow, which produced acold of much greater intenfity, and the ther-mometer in confequence immediately defcendedfeveral degrees lower. When taken out of thefocus, it again rofe to the common temperature.

Mr. Leflie alfo found, not only the fameeffect in his experiment, but that the action ofa cold-radiating furface upon the tin reflectorproduced exactly the Came proportional effectupon the differential thermometer as the hotradiating furface, only in the oppofite directionof the fcale. The differential thermometer,which is always at zero when both bulbs areequally heated, is beautifully calculated to Chewthis ftriking experiment. Thus if the differenceof temperature between the heat-radiating fub-ftance and the atmofphere be 60 degrees, andif this raifes the thermometer 45 degrees, thefame difference between the cold-radiating fub-ftance and the atmofphere will fmk the ther-mometer 45 degrees, and fo in proportion; fothat a cold of 16 degrees will fink the ther-mometer 12 degrees, for 60 : 45 :: 16 : 12.

The range of thefe experiments on cold isneceffarily more limited than that of heat, forit is not -eafy to keep up a cold lefs than o°(Fahr.) in any veffel, for a moderate length oftime, which is only about 6 o° below the averagetemperature of a room; whereas the heat ofboiling water is 150° above this average. Befides,a fource of error takes place in eftimating thepropulfive energy of a cold fubftance when at avery low temperature, which is, the conftantcondenfation of the moifture of the atmofphereon the furface, except the air is uncommonlydry, and this introduces an extraneous fubftanceinto the comparative experiments.

The theoretical explanation of thefe curiousfadts leads us immediately to the confideration of

The Equilibrium of Radiant Caloric.

The fimpleft explanation of what paffes whena heated body cools in a cold medium and raifesthe temperature of the furrounding fubftances,is to fuppofe a mere efflux of caloric from thehotter to the cooler bodies, radiating with moreor lefs force according to the greater or lefs dif-ference between the thermometrical heat of thetwo, and the nature of the heated furface, with* Prevolt, Ph . Tranf. 1803. • Recherches PJ>y;

regard to its heat-propelling quality. Thiswould explain without difficulty the reflexionof heat in the experiments already related, butwould not apply with equal eafe to the reflexionof cold. For fuppofing (as the fimpleft cafe) acanifter filled with iced water at 32 0 , Handingbefore a tin refledtor, and a thermometer bulbin the focus, with the common temperature of6o°; if no other radiation takes place than thatfrom the hotter bodies to the colder one, thecold canifter would merely make a kind of voidor gulph, into which the thermometer bulb, thereflector, and the ambient air would all pourtheir excefs of heat by direct radiation in thefhorteft path, and not in any other direction ;nor could any retrogradation or reflexion ofrays take place between the thermometer andthe mirror. n Hence the prefence of the mirrorcould make no difference in the temperature ofthe thermometer, and its focal point wouldnot be colder than any other contiguous fpot.

The very ingenious theory of M. Prevoftreconciles this difficulty without requiring thefuppofition of pofitive radiations of cold; thoughthis laft hypothefis will certainly apply withequal force to all the circumftances of thereflexion of cold as to thofe of heat.

According to M. Prevoft, 0 heat, when un-icombined, is a fubtile fluid, of extreme mobility,difperfing its particles in every direftion, whichexift in that ftate of mutual repulfion or fepa-ration, that contrary or different currents maycrofs each other, like thofe of light, withoutdifturbance or interference. This conftitutionof heat being admitted, a conftant interchangeof radiations of heat is kept up between allfubftances placed within the fphere of mutualinfluence, whether hot or (comparatively) cold.The interchanges arc equal when the fubftanceshave the fame thermometrical temperature, andthe mutual tenfion of heat is then in equilibrium.A hot body, in cooling, parts with more caloricthan it receives, and its excefs of heat difturbsthe tenfion of heat between it and the furround-ing bodies, till the inequality of the interchangeshas reftored the equilibrium. On the otherhand a comparatively cold body, in heating,receives more caloric than it parts with, as longas the equilibrium is reftoring.

To apply this hypothefis to the, two oppofiteexperiments of the reflexion of heat and of cold.

Where (in the apparatus of Mr. Leflie fooften referred to) the canifter is filled withboiling water, and therefore hotter than thefurrounding air and the mirror, the interchanges

ico-jvlecksnicjues fur la Chaleur, & Ph. Tranf, 1802,

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