CAL

CAL

( 200 )

or the paflage of heat through it, unaccompaniedby correfponding currents, very fatisfa&ory ex-periments have been made by Dr. Thompfon, cMr. Dalton, d and Mr. Murray. e All of themproceed on the general principle of endeavouringto communicate caloric in a direction contraryto that of the currents which are excited inpartially heated fluids. In many of them a veffelwas filled with water unequally heated, in which,during the procefs of cooling, no alteration offpecific gravity could make any current; and asnotwithftanding, the temperature of the waterin the different parts of the vefiel graduallybecame equal, it was inferred that water (orwhatever was the fluid ufed) poffefied a realconducting power.

To fulfil thefe conditions, either a hot liquidmuft be put upon a ftratum of the fame liquidcold, or a hot liquid of greater fpecific gravitymuft be put at the bottom, and a cold liquid oflefs gravity at the top. In either cafe there canbe no current from one to the other. The par-ticulars of thefe and other fimilar experimentsneed not be related; in all there was a gradualequalization of the temperature, but not .till aconfiderable time had elapfed.

Some nicety of management is required inpouring one liquor down a tall jar on the fur-face of another liquor without mixing the two•in any material degree. To do it in the moltexpeditious and belt way, a thin wooden floatfhould firft be let down on the lower liquid andthe ftream of the upper liquid directed upon it,which will break the force of the current, andkeep the two fo much feparated, that if only'one be coloured, the line of diftinCtion will beobvious, and pretty well defined.

As caloric cannot radiate through denfe fluids,if it paffes through them without the afliftanceof currents (that is, without being conveyed), itmuft be conducted through, unlefs it is com-municated through the fides of the veflel thatmuft be ufed to make the experiment. Thishas generally been of glafs, which is itfelf a badconduftor of heat; but ftill it cannot be deniedthat where one part of the fame veflel confinesa hot and the other part a cold fluid, the varyingquantities of heat abforbed by the veflel itfelfmuft after a time be equalized by the conductingpower which folids indifputably poffefs, and aconfequent equalization of temperature muftfooner or later take place in their contents bythis caufe alone, even were no other to operate.In thofe of the above experiments that lafted•many minutes., even as much as one or twoc Ph.Jour. vol, 4. d Manch, Mem. vol. 4

hours, a very large deduction muft be made forthe conducting power of the veflel, but a beau-tiful experiment of Mr. Murray’s, made in aveffel of ice (and therefore a perfect non-con-duCtor for all temperatures above 32 0 , whilft itcontinued to be ice), {hews a conducting powerin fluids independent of the veflel containingthem. f Some of the particulars may be men-tioned. A quantity of water was frozen in atin mould, fo as to form a hollow cylinder 3inches in diameter, 3 t in depth, and of i-j thick-nefs of ice. A thermometer was introducedinto it horizontally, at the depth of one inch,the bulb being in the axis of the cylinder, andwas frozen in. A quantity of almond oil wasthen poured into the ice veflel, fo as to coverthe bulb of the thermometer one quarter of aninch. A fmall flat-bottomed iron cup, holdingtwo ounces by meafure, was then fufpendedover the oil, fo as merely to touch the furface,without dipping to any fenfible depth. Thiscup was then filled with boiling water; fo thatthe heat palling through its bottom woulddireCHy touch only the furface of the oil, whichwas a quarter of an inch above the thermometerbulb. Thus Gtuated, if the thermometer rofe,it Ihewed that the heat muft have been fairlyconducted downwards through this depth of theice-cold oil, for the lides of the veflel being icecould not have conveyed it, nor would anyheated currents reach fenfibly lower than thebottom of the heated cup. The mercury rofefrom 32 0 to 324- in a minute and a half, and to374- in five minutes, after which it remainedftationary for a while and then gradually funkagain to 32° The effeCt of the heat had exca-vated a portion of the top of the ice veflel, andeven as low as the; bottom of the thermometerbulb. The fame experiment was repeated, fub-ftituting mercury to the almond oil, and theiron cup was now allowed to float on the fur-face of the mercury, but ftill at a fenfible dif-tance from the thermometer. This rofe inftantlyon the pouring the boiling water into the cup,from 3 2° to 36°, and again flowly funk to thefreezing point.

Each of thefe experiments were repeated withdifferent heights of fluid over the thermometer,and in all, fome rife of the thermometer wasobfervable, though when covered with one inchof mercury it amounted to no more than three-fourths of a degree.

Having thus related the principal faCts con-cerning the paflage of heat through folids, liquids,and gaffes, in its radiated and conduced ftates,

* I'h. Jour. 8vo. vol. I, 1 Ph. Jour.