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of caloric are unequal, and (comparatively) hotrays proceed from the canifter. As the raysfrom this quarter are the only ones which canproduce any effect in the focus of the mirror,the temperature of the focus, like and fuperior indegree to the rays from the canifter, is hot; andthe thermometer proportionally raifed. On theother hand, when the canifter is filled with icemutual interchanges of caloric take place, asbefore, between all the fubftances concerned inthe experiment; but the rays that proceed fromthe canifter are now (comparatively) cold, andhence a fimilar and more intenfe cold is pro-duced in the focus, which is {hewn by the fink-ing of the thermometer.

It does not appear that any experimental proofcan be devifed to confirm or difprove thetruth of the above hypothefis of the conftantand mutual radiations of caloric between fub-ftances whole thermometrical temperature is thefame; fince the thermometer is the chief, ifnot the only inftrument, which we can applyto detedt the paiTage of heat, and it is obviousthat it only indicates the difturbance of theequilibrium, and not the mutual interchangesthat maintain this equilibrium. The relationand correfpondence between the powers of pro-pelling and of abforbing caloric poflefled by dif-ferent furfaces, and fo beautifully illuftrated byMr. Leflie’s experiments, may perhaps be afupport to the above hypothefis, fince it recon-ciles the maintenance of the equilibrium of theconducted heat in liquids, however enclofed byveflels of any folid materials, and in folids them-felves, with that of the radiant heat of the fur-rounding atmofphere.

The cooling of hot bodies by fimple radiationinto the furrounding cold medium, and theheating of cold bodies by abforption from thefurrounding hot medium, are regulated by cer-tain laws, which have been well explained bythe experiment of M. Kraft and Richman. «>The moft important of thefe is the following.When a hot body is immerfed in a cold mediumof uniform temperature (in the common atmof-phere for example) the lofs of heat proceeds inan inverfe geometrical progreflion, whilft thetime proceeds in a direft arithmetical progreflion.The fame rule alfo applies to the heating of acold body in an uniformly hot medium. Forinftance, fuppofing the propulfion of caloricfrom the heated body is of fuch an intenfitythat one-tenth of the whole excefs of heat,above that of the furrounding medium, is loftin one minute; then, at the end of that time

there will remain of the whole. After thefecond minute, the tenth of the heat that wasleft is diffipated, and hence there now remainsin the heated body r \ of or T \)\ Afterthe third minute there will remain 0 f _a_.) *or -j%-) 3 . After the fourth minute -fo-) 4 , and fo-on, the quantity of heat decreafnig in geometri-cal progreflion, whilft the time encreafes inarithmetical progreflion.

Hence too it appears that whilft the equili-brium of heat is reftoring, the actual quantityof caloric palling from the hotter to the colderbody is conftantly decreafing in equal times:thus if the firft excefs of heat be equivalent toioo, the firft minute will fubftract 10, thefecond minute 9, the third 8.1, the fourth 7.29,and fo on in proportion, always leaving T ^- ofthe laft refidue. Hence, after 10 minutes haveelapfed' (at which time the whole excefs ofheat would have been fpent, had the actual lofsbeen uniformly the fame as during the firftminute) more than one-third of the original100 parts, or 34.86 parts, ftill remain in theheated body. This encreafing flownefs of heat-ing or cooling, as it approaches the equilibrium,is a matter of common obfervation in number-lefs experiments. Thus if a thermometer beplunged in boiling water and then expofed tothe air, it will fink at firft vifibly and rapidly,but this finking will foon be found more andmore retarded, and at laft many minutes will-elapfe before it has arrived at the exa£t tem-perature of the air. The fame of cold bodies,in acquiring the heat of the furrounding me-dium.

The fame reafoning will apply to the hypo-thefis of the mutual interchange of caloric be-tween bodies, whatever be their thermometricaltemperature. For if a cold body is expofed toa hot medium, the heat of this laft may be con-fidered as divided into two parts, that which isequal to the cold body, and that which formsthe excefs. With regard to the firft, as it onlyferves to equalize what it receives from the coldbody, and to produce an equilibrium, it is putout of the queftion, and it is only the excefs towhich the laws of progreflive diminution canbe applied, a

The inveftigation of the above law of Richmanbecomes fomewhat more complex when boththe hot and the cold body are changing their-temperature, the former by lofing, the latter bygaining caloric; for in the heating or cooling ofany fubftance in the atmofphere, the proportionbetween the bulk of the body and the medium'

* N. Conun, Acad. Petrop. ’ Prevoft,