182 Mr. Powell on Terrestrial Light and Heat. [Sept. 



clearly pointed out by Prof. Leslie himself. (See his " Inquiry," 

 p. 54, &c.) 



The same distinction is also remarked by Biot as having been 

 originally pointed out by Marriotte, the first observer of the 

 reflection of heat by the same laws as those nf light: " Qui a 

 indique aussi la necessite de distinguer le calorique obscw et le 

 calorique iumineux." (Biot. Traite de Phys. iv. 606.) 



(3.) The subject having been taken up by M. De la Roche, 

 results of the most important nature were obtained in several 

 series of experiments, which, from their accuracy and elegance, 

 have justly excited the admiration of the philosophical world. 

 The account of them was communicated to the French Institute, 

 and, according to the excellent plan adopted by that body, a 

 report upon the examination of them was afterwards drawn up 

 and published by the commissioners appointed for that purpose, 

 MM. Berthollet, Chaptal, and Biot. A translation of their 

 report may be seen in the Annals of Philosophy, O. S. ii. 161. 



This distinguished philosopher extended the examination to a 

 series of radiating bodies, extending from boiling mercury to 

 incandescent metal, and flame in different stages of combustion. 

 He tried the interceptive effects both of transparent and opaque 

 screens on the radiant matter or influence proceeding from the 

 substances in question, and acting on a thermometer, with its 

 bulb blackened, and placed in the focus of a reflector, his results 

 are given by Biot, Traite de Phys. iv. 640. The following 

 extracts will best display the nature of the conclusions. 



A greater effect was observed on the blackened thermometer 

 beyond the glass screen in proportion as the temperature of the 

 source of heat was elevated, and as it approached the state of 

 luminosity, or became more perfectly luminous. 



" Mais dans cet etat meme il y a encore des differences ; car 

 selon l'observation de Mariotte si Ton fait reflechir la lumihe 

 du soleil au foyer d'un miroir concave de metal, etqu'ensuite on 

 mette au-devant de ce miroir un ecran de verve, il n'en resultera 

 dans la temperature du foyer qu'unefaible diminution, telle que 

 J- ou 4-; mais si Ton fait la meme experience sur lefeu d'un foyer 

 ou d'un fourneau on trouvera que la reflexion directe sur le 

 miroir sans ecran, pi'oduira unechaleurtres vive, et n'en produira 

 qu'une tresfaible, ou insensible, si Ton interpose la lame de vene 

 meme en se rapprochant assez du feu pour que l'image lumi- 

 neuse formee au foyer soitplus vive qu'auparavant." 



Upon which M. Biot makes the following remark : " Or ce 

 resultat que je ne fais qu'enoncer est de la plus haute import- 

 ance ; car d'abord l'inegalite de la transmission a diverses temper- 

 atures de la source rayonnante montre que les emanations 

 calorifiques qui en partent dans les diverses circonstances sont 

 differement modifees : et en second lieu, la transmission plus 

 abondante a mesure que le rayonnement calorifique s'approche 



