GAL
CAL
( 221 )
any fimplc condenfation of the fides of theremaining hollow cylinder of brafs would domore. Frielion therefore is a totally diftindlfource of caloric, and apparently unlimited inits a&ion; for as long as it is kept up, theevolution of heat continues uninterruptedly, norhas any approach been made to an exhauftionof the fupply. In this refpedt it differs frompercuffion. It may be added that the mollpowerful arguments againft the materiality ofcaloric are drawn from this fource.
The direft paffage of caloric from the fun,and Dr. Herfchel’s important difcovery of calo-rific rays feparate from thofe that occafion lightand colour, have already been noticed underthe head of radiant calorie. Some other of thechemical fadts connected with the folar rayswill be defcribed under the article Light .
The intenfity of the heat produced by thediredt rays of the fun, collefded by a lens orrefledted by a concave fpeculum, is equal, withpowerful inftruments, to any other method ofrailing heat; but of courfe it muft be confinedto a very fmall fpot. Solar heat, when it canbe procured fteady and for a fufficient lengthof time is molt admirably adapted for very deli-cate and complicated experiments, as no errorcan arife from the introduction of burning fuel:and with caution, the pencil of rays from a lensor mirror may be tranfmitted through a glafsveffel to any objedt contained within it.
The heat of the diredt rays of the fun (limplythrown on any objedt without being collectedby any glafs or mirror) is much greater thangenerally appears by the thermometer, much ofit being difperfed by the air through which it istranfmitted, efpecially when in brilk motion.When the fun’s rays are confined to any fpot,as much as may be, by being furrounded withbad condudtors and protedted from wind, theaccumulated heat eafilv rifes to a much greaterintenfity than the higheft temperature of a tro-pical fun. M. de Sauffure made an experimentto this point* a He made an oblong box onefoot long and nine inches in width and depth,of half-inch deal, and lined it on the infide withcork an inch thick, and charred on the furface.A cover was fitted on, made of three panes ofglafs clofe fitting and parallel to each other,with i y inch of fpace between each. Whenthis was fet in a hot fun the heat had to pene-trate the three glafles, and was conftantly accu-mulating in the box to a given degree, beingconfined on the three fides by the burnt cork
* Voyages dans lcs Alpcq tom. S. p-
and thick walls of the box, and on the fourthby the two ftrata of air at reft contained between:the glafles; all bad conduftors of heat. Onthe top of a high peak of the Alps (on whichfor a particular purpofe the experiment was firftmade) the box was expofed to the fun’s rays fora confiderable time, and a thermometer, whichin the free air (hung 4 feet above the ground,and in the full fun-{hine) flood only at 454°, rofegradually within the box to 1 yol-°. The rife ofthe laft 46 degrees was effedted in an hour*The thermometer laid on a piece of charred corkin the open air rofe to 79i°. At the bottom ofthe mountain, nearly 800 fathom lower, the-refpetlive temperatures under fimilar circum-ftances of expofure were, fulpended in the freeair 74|-°, within the box 187^°, and laid oncork 92!°. Similar experiments by ProfefforRobifon 6 gave a much greater effedl, the ther-mometer rifing within the box to 23 o° and2 37 °*
Of Cmnbufion.
The laft fource of caloric to be mentioned isthat which appears, in union with light (to-gether conftituting Fire) in the procefs of com-buftion: and the heat employed for all culinary,domeftic, and manufacturing purpofes, is almoftentirely derived from this fource.
The neceflity for a eonftant fupply of air to'maintain combuftion in all common cafes, firftled chemifts to the important analyfis of atmof-pherical air, and the diftinft properties of thatconftituent of air which alone is active in main-taining combuftion and refpiration, and to whichthe modern fyftem of nomenclature has giventhe name of Oxygen. Under this article, andthat of Phlogiston, the different theories ofcombuftion will be defcribed, but it may- be 1here added that the caloric contained in oxygenand given out during combuftion is not- folelythat which is requifite to the gafeous ftate, butin all probability is an effential conftituent ofthis fubftance.
When a folid fubftance is- burnt in oxygen:gas, and the refult is a folid or liquid, it might'indeed be fuppofed that the evolution of caloricwas owing to the change from the gafeous ftate.to one of much lefs.capacity for heat; but in avariety of inftances the caloric produced by-combuftion is by no means proportioned to thequantity of oxygen- gas confirmed, and whatentirely overthrows this liypothefis is the intenfcrheat given out by gunpowder and various othercombuftible mixtures with the nitric or oxy-muriatic falts, in which the oxygen is not in.
* Notes to Black’s Le&ures, p. 547.