14 CEOONIAN LECTURES 



substance as the cause of light lasted until the 

 time of Dr. Young and Fresnel. 



The materiality of light led directly to the 

 assumption of the materiality of heat. 



Lambert, in 1755, published an essay on the 

 force of heat, in which he likens the communi- 

 cation of heat to the flow of a fluid. To account 

 for the phenomena of heat, a combustible 

 element, an igneous matter, capable of com- 

 bining or separating itself from other substances, 

 was assumed ; this was phlogiston. It was 

 considered, until the time of Black and 

 Lavoisier, as just as much a chemical element 

 as any ponderable element. It was assumed 

 even to have a principle of lightness. 



The imponderable materialism of heat, how- 

 ever, survived in the idea of caloric ; material 

 heat was an actual flow and emission of material 

 particles. Leslie, in 1804, says, " What is this 

 calorific and frigorific fluid ?" It is merely the 

 ambient air. But afterwards he says, "It is 

 the same subtle matter that, according to its 

 different modes of existence, constitutes either 

 heat or light." Even as late as 1832, in the 

 Bibliotheque Universelle de Geneve, vol. 49, and 

 again in 1834, in \\\QAnnalesde Chimie, vol. 58, 



