CAL

CAL

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examined by Count Rumford , whofe beautifulexperiments on the propagation of heat in fluids*rank with the moil ingenious refcarches in natu-ral philofophy. .

It may be laid down as a general law, thatwhen any portion of a fluid is heated above thecommon temperature of the contiguous mafs,the hot particles immediately expand, and in fodoing become fpecifically lighter than the reft jand therefore, if the heat is applied at bottom,the warmer fluid rifes through the whole mafsto the furface, giving off however on every fidea portion of its excefs of heat to the coolerliquid during its paffage. But if the heat isapplied at the furface, the upper part alonebecomes heated, and being fpecifically the lighteftit can only remain at the furface, and nevercomes in contaft with the lower part of themafs, and confequently it cannot convey or carrycaloric to it as in the former cafe. Many ftrikingexperiments, eafily performed and perfectlyconvincing, prove that this is by much the prin-cipal method in which fluids become heated orcooled, fo that for all general purpofes it maybe affirmed to be impoflible to propagate heatdownwards or cold upwards in fluids withoutthe affiftance of mechanical agitation. Werethis law abfolute, it would follow that fluids areperfe£t non-conductors of caloric, fince the con-ducting power implies the paffage of caloricthrough fucceflive particles or ftrata of bodies,and not the conveyance of caloric by the aflift-ance of the movements of the heated bodiesthemfelves.

Many are the experiments that fhew the car-rying power of heated fluids and the afcent ofthe heated particles. Thus if a quantity of hotwater is poured into a jar of cold water througha long funnel touching the bottom of the vefl'el,the hot water will rife almoft immediately tothe top, encreafing the temperature of the wholemafs in its paffage. But if on the other handcold water is poured down in the fame mannerinto a veffel of hot water, it remains at thebottom, fcarcely mixing with the hot,.the furfaceof which will not have its temperature fenfiblylowered for a confiderable time.

The melting of ice under different circum-ftances of polition is adduced by Count Rumford as a ftriking illuftration of this principle. Apiece of ice was fafhioned fo as to fill prettyclofely about 3-j inches in diameter of a tallcylindrical glafs jar. Boiling water was firftpoured into the jar, and when nearly full theice was dropped in, and melted on the furface

* Effays. r

of the hot water in 2 minutes 58 feconds. Theexperiment was then repeated with this differ-ence, that the ice was firft fixed to the bottomof the jar by two thin flips of deal, a very fmallquantity of cold water poured on it to fill theinterftices, and boiling water poured upon thewhole. In this laft experiment the circumftanceswere fo different that in more than double thetime in which the former ice had entirely dif-appeared, the latter fhewed not the leaft dilpo-fition to melt. The temperature of the waterbeing taken at different heights, during fcveralintervals for two hours, it was found that in allthe trials the bottom of the water (or the partimmediately refting on the ice) was uniformly40° •, but near the furface of the water, or feveninches above the ice, it was found at intervalsof from 15 to 30 minutes, to have fallen to160 0 , 149 0 , 131 0 , 117°, and at the end of twohours to io8i°- At three inches above die icethe temperatures at the fame periods were re-fpedlively, 159 0 , 148°, 128°, 116 0 , and finally106°. During this long period, at the end ofwhich the furface of the hot water was ftili108 0 , it was found that full half the ice remainedunmelted, fo that the thawing had been fulleighty times flower than when the ice was atthe furface ; and as it would have continued togo on ftili flower, many hours would doubtlefshave elapfed before the whole was melted.

This leading experiment, without enteringinto the particulars, lhews in a moft ftrikingmanner the effeft of fituation in heating andcooling fluids, and fully proves that the equaldiffufion of heat through fluids is chiefly owingto the carrying power, or the motions occafionedby the inferior fpecific gravity of the hottercompared with the colder part of the fame fluid.

A fimilar experiment y , but ftili more ftriking,is the actual boiling, of water over the furfaceof ice for a. length of time, without meltingany confiderable portion of it; and in a generalway, no experiment that could be devifed illus-trates more remarkably the extreme difficultyof the paffage of heat downwards through denfefluids. It is thus performed. Take a thin longglafs jar or tube clofed at one end, fill aboutthree inches of its height with water, and freezeit into a very folid mafs, faft, adhering to thebottom of- the jar.. This is done by immerfingthe jar in a mixture of fait and. fnow, till theice within is quite firm and hard. Then takeit out, pour on the ice about two or three inchesof ice-cold water, and wrap the outfide of thejar with feveral folds of flannel to the level ofRumford ’s Effays,