CAL

CAL

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the lower, and on changing the thermometers,that which was now at the top of the bar rofethe higheft in equal times as before. Alfo onobferving the comparative rates of cooling, thedifference was ftill more ftriking; for whilft thelower thermometer rapidly cooled, the upperone for fome time did not fink at all, its lofs ofheat being exaCtly balanced by the influx fromthe lower part of the bar.

The above experiment cannot be confideredas quite accurate, as no air-pump vacuum is aperfect one, and even if free from air it ftillcontains fome vaporous moifture, which isitfelf a conductor of caloric; and thereforethe fource of error intended to be avoided bythe air-pump is not entirely removed. Anotherdifficulty arifes from the extreme difficulty ofenfuring exactly the fame extent and accuracyof contaCt of the upper and lower thermometerbulbs with the folid copper bar ; but with allthefe deductions we may ftill confider it asnearly certain that caloric is more readily con-veyed through folids in an upward than in anyother direction. In the fame apparatus ano-ther upper and lower thermometer were fuf-pended, fo as nearly, but not quite, to touchthe heated metal, and therefore were infulatedin a vacuum. The great fluggifhnefs of theirrife and fall, compared with the two that actu-ally touched the heated bar, fhewed (what hasalready been mentioned) the difficulty with whichcaloric radiates through a vacuum.

Metals are by far the belt folid conductorsthat we are acquainted with. Every one knowsthat if one end of a fhort {lender iron rod bethruft in the fire, the other end foon becomesintolerably hot to the fingers ; whereas the famelength of a fimilar wooden rod may be heldwithout any inconvenience, and may even beconfumed to within nearly a quarter of an inchof the fingers that hold the other- extremitywithout burning them. The comparative con-ducting power of different fubftances is irr-geniouily {hewn in the following way. Takefhort thin rods, of equal fize and length, of anyof the different metals, of glafs, and any otherfubftance required -, coat one of the ends bydipping it into melted wax or tallow, and im-merfe the other into boiling water, or anyother hot medium to which they will all beequally expoied. After a while the wax will

melt, firft from the metallic rods in the orderof their conducting power, then from the glafsor other fubftance ufed, and the comparativetimes required, to melt the wax from the refpec--tive rods will give their comparative conductingpower.

In this manner Dr. Ingenhouz * found thecomparative conducting power between feveralmetals to be in the following order : filver, gold;copper and tin nearly equal; platina, iron, fteeland lead, all much inferior to the others. Hencethe conducting power by no means correfpondswith the denfity, tenacity, oxidability, fufibility,.or any known quality common to all metals.

Next to metals, ftones, glafs, and hard earthyfubftances or compounds appear to have thegreateft conducting power.

The conducting power of feveral woods hasbeen afeertained with apparent fkill and accu-racy by Mayer, profeffor at Erlangen .'

Balls of the different-woods were turned ofthe fame diameter, and hollowed out fo as juftto admit the bulb of a fine thermometer. Thefeballs were firft dried in a gentle ftove-heat forfeveral days, and then heated uniformly to acertain point, and brought into a cold roomwithout a fire. The refpeCtive times taken byeach ball to cool to certain fixed points indicatedinverfely their comparative conducting power.A correction became neceffary however onaccount of the unequal fhrinkage of the ballsduring the drying. This was eafily calculatedfrom the law that in veffels of the fame fub-ftance the comparative time of heating or coo-ling is at tire diameter of the veflel. The ballsbeing thus reduced by calculation to the famedimenfions, the times of cooling from one givenpoint to another were determined by the num-ber of vibrations of a pendulum, which ofcourfe would be inverfely as the conductingpower. Elm wood was taken as a ftandard, towhich all the others were firft referred, andafterwards referred to that of water, wholeconducting power was found by confining it ina hollow glafs- ball of. the fame dimenfions asthe wooden one, and deducting the feparateeffeCt of the glafs. The fpecific gravity of thedifferent woods was alfo afeertained, which isworth notice, as it {hews fome kind of accordbetween it and the conducting power, inverfely,though not with much proportional accuracy,

* J\ Phyf, tom. 34. ' An. Chim, tom. 30. from Crcll’s Journal, 3358.