CAL

C AL

( 179 )

■with more facility than glafs, glafs eafier thanwood, wood eafier than charcoal, and the like.So that in treating of the fubjedt of conductedcaloric, the conducting power of the medium multbe taken into confideration. Late experimentshave alfo fhewn a peculiarity in the mode inwhich heat paffes through liquids, which mightjudify in them the term conveyed a rather thanconducted, but dill totally didindt from radiated.

Ladly, there are feveral cafes in which caloricconveyed into a fubftance, either by being radi-ated or conduced, totally combines with it,lofing thereby the fenfible properties which itbefore poffeffed when in a free Hate, not raifingthe temperature of the body with which it isunited, and contributing to form the dates ofliquidity and vapour. The caloric which icereceives from the furrounding bodies whenthawing, enters into this chemical union andConverts it into the date of water. The caloricis then termed combined, fixed, or latent, andcomparatively in different bodies, fpccific-caloric ,and many mod important enquiries are con-nedted with this part of the fubjedt.

Therefore in invedigating the laws and pro-perties of heat it will be convenient fird to fallin with the natural fubdivifions of the fubjedt asrelating to radiant, conduced, and combinedcaloric.

Of Radiant or Transmitted Caloric.

The fird and mod magnificent fource ofradiant heat is the fun, whofe rays are per-petually tranfmitting a vad fupply of light andheat with immenfe velocity. The connexionbetween light and heat was long a fubjedt ofmuch doubt and difficulty, but a late difcoveryof Dr. Herfchel has edablifhed the very im-portant fadt of the feparate identity of thefetwo bodies in the folar ray. The well-knowncircumdance that the moon’s rays, though con-denfed by the mod powerful lens or mirror,poffefs no afcertainable heating power, is a proofof the exidence of light, feparate from heat,but dill capable of refledtion and refradtion bythe fame laws that regulate the motion of diredtfolar rays. Dr. Herfchel b in his optical experi-ments on coloured glaffes was led to examinethe difference between the coloured rays of thefun, with regard to their heating power. Hethereby difcovered that the mod refradted raysof light, the violet, poffefs the lowed heatingpower; and the lead refradted, the red, thegreated power, and the mean rays of the prif-matic fpedtrum fhewed an intermediate power.Thus, in the red rays the thermometer (by the

* Thompfon’j Chcmiftry. k Phil. Tranf.

average of feveral experiments) rofe 6f degrees;in the green rays, 3i degrees ; and in the violet,2 degrees; or in round numbers, the effedt ofthe red rays was to that of the green as 2.25 to1, and to that of the violet as 3.5 to 1.

Purfuing thefe experiments, the fame phi-lofopher found the range of difperfion of therays of heat by the prifm to differ mod effen-tially from that of light; for on applying ther-mometers of great fenfibility fucceflively in aline, beginning at the violet rays and proceedingalong the prifmatic fpedtrum, he found not onlythat the heat encreafed by advancing towardsthe red or lead refradted rays, but that the heatwas greated at a fmall didance beyond theextreme limits of the fpedtrum, that is, whereno rays of light at all fell; and dill continuingto advance the thermometer in the fame line,the heat then gradually diminilhed till it becametoo fmall to be noticed. This mod curious andimportant difcovery diews therefore both anentire feparation of heat from light in the folarray, and a very different degree of refrangibilityof one from the other, which together go nearto edablifh the feparate identity of caloric andlight, and caufe precifely the fame argumentsufed to demondrate the materiality of light toapply to the materiality of heat.

Thefe experiments of Dr. Herfchel have beenfully confirmed by Sir H. Englefield, c whofeapparatus was fomewhat different and moreaccurate. The particulars are as follows: thecoloured rays of the fpedtrum were fucceflivelyand fingly thrown on a lens (all the others beingexcluded by a fcreen) and a thermometer witha blackened ball was placed in its focus, andallowed to remain there fome time after it hadceafed to rife, that the full effedt might be fe-cured. Thus circumdanced it rofe

In the blue ray from 55° to 56°

-green ■

— yellow-

— red

Quite out of vifible light 61 — 79Thefe experiments were repeated feveral time#,and in all with very clofely correfponding refults,and the mod driking and novel phenomenonwas manifed in all, namely, the rife of thethermometer when palled beyond the extremepoint of the luminous fpedtrum on the red fide,and its fall when again carried back into thered light. It is to be obferved that as theieexperiments were made in a London atmoi-phere, in weather not uniformly bright, the

numbers expreffmg the rife of the thermometer

for iEOa. • Jour, Royal Inftit. No. 13.

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