CAL

CAL

( 199 )

Experiments were alfo made by Count Rum-ford to determine whether heat could be propa-gated downwards through oil and mercury.Thefe fluids being uniformly dilated with anyencreafe of heat from any temperature at whichthey remain uncongealed, no fuch precautionswere necefl'ary as when water near the freezingpoint was ufed. The apparatus ui'ed was a cakeof ice frozen to the bottom of a jar, with aprojedting point or nipple at the furface: thiswas entirely covered with the oil or the mer-cury, and a folid cylinder of iron heated inboiling water was let down through the oil ormercury to within T V of an inch of the nippleof ice. Though the heated iron was fo near it,no fenflble quantity of ice was melted; and asall the motions produced in the fluid by theunequal heating took place a little above thelevel of the ice, it Ihewed that the communi-cation of heat through the fluids could only bemade by means of heated currents, and there-fore could not properly be conducted throughthem.

Though thefe very curious experiments fhewinconteftibly that the heating and cooling offluids is chiejly carried on by means of the cur-rents into which they are thrown when heat ispartially communicated, there is very ftrongevidence from fubfequent experiments that thisis not the only way in which caloric is propa-gated through them; or in other words, thatfluids are not abfolute non-condu£tors of heat,though they are certainly very bad conductors.

The aftual motion of the currents of dif-ferently heated particles in any mafs of fluid,has been attempted to be fhewn in differentmethods. One of them, which was adopted byCount Rumford , is to fufpend in the fluidminute particles of any opake folid fubftancereduced to powder, which is not chemicallyafted on by the fluid, and whofe fpecific gravityis fo nearly the fame as that of the fluid in whichit is immerfed that it will rife or fall merelywith the currents. Amber is a little heavierthan pure water, but if a fmall quantity of car-bonat of potalh or any other fait be added, itmay ealily be brought to the requifite denlity,and no chemical aCtion takes place between thetwo.

When a glafs jar thus filled, and the fluidcold, is immerfed in another large jar containingvery hot water, the particles of amber whichbefore were at reft in every part of the water,immediately begin to fall into regular currents,and all that are near the fides of the jar, being

* Ph: Jour. ■

the neareft to the hot water on the outfide,begin inftantly to afcend with great rapidity andregularity, whilft of courfe a defending currentis formed in the centre of the jar. This con-tinues, but with diminifhing velocity, till thewhole of the water in the inner jar is broughtnearly to the fame temperature, when the par-ticles again remain at reft. If the procefs isreverfed, and the jar now hot is immerfed incold water, or even Amply expofed to cold air,the currents now run exactly in a contrarydirection, the outer circumference cooling firft,and forming a defcendmg ftream, whilft the innercylinder of water of courfe afcends; and thefecurrents again continue till the temperature isequalized through the whole jar.

This moft entertaining experiment, though itcorrefponds very well in form with what actuallytakes place in the water itfelf, differs eflentiallyin degree ; for the rapidity of the motion of theamber particles is much beyond what would berequired to account for the actual rate of heat-ing or cooling in the water itfelf. Therefore,much of the effect is owing to the expanflon orcondenfation of the amber itfelf, rendering itlighter or heavier than the liquor in which it isimmerfed ; and thefe refpective currents of thefloating particles muft be confldered only as animitation, or rather an exaggerated reprelenta-tion of what actually takes place in the water.This was very fatisfadtorily proved by Dr.Thompfon, b who repeated the experiment nearlyin the fame way, but with the ingenious con-trivance of colouring blue the lower half of theliquor, and leaving the upper part uncoloured.On applying heat at the bottom, the amber par-ticles were readily fet in motion, and they r-ofynot only to the top of the blue part, but throughthis into the colourlefs portion, without beingaccompanied by any of the coloured folution, whichof courfe would have been readily viflble. Bycontinuing the heat for ten minutes, the twoliquors did indeed become uniformly mixed, butthe motion of the amber particles began almoftimmediately on applying heat, and were fullyeftablilhed through the whole of both liquorsbefore the colour had fpread upwards in anyfenflble degree.

Hence it follows, that the only fatisfadtoryway of rendering the currents of the water itfelfviflble, is by tinging a part, and then the gradualadmixture of colour can only be brought aboutby the actual admixture of the different particlesof the fluid itfelf.

To prove the real conducting power of water,ol» i. 8vo»