526 HISTORY OF THERMOTICS. 



trate this process by some physical notions. Lam- 

 bert in 1755 published 5 an Essay on the Force 

 of Heat, in which he assimilates the communication 

 of heat to the flow of a fluid out of one vessel 

 into another by an excess of pressure ; and mathe- 

 matically deduces the laws of the process on this 

 ground. But some additional facts suggested a 

 different view of this subject. It was found that 

 heat is propagated by radiation according to straight 

 lines, like light ; and that it is, as light is, capable 

 of being reflected by mirrors, and thus brought 

 to a focus of intenser action. In this manner the 

 radiative effect of a body could be more precisely 

 traced. A fact, however, came under notice, which, 

 at first sight, appeared to offer some difficulty. It 

 appeared that cold was reflected no less than heat. 

 A mass of ice, when its effect was concentrated on 

 a thermometer by a system of mirrors, made the 

 thermometer fall, just as a vessel of hot water placed 

 in a similar situation made it rise. Was cold, then, 

 to be supposed a real substance, no less than heat ? 

 The solution of this and similar difficulties was 

 given by Pierre Prevost, professor at Geneva, whose 

 theory of radiant heat was proposed about 1790. 

 According to this theory, heat, or caloric, is con- 

 stantly radiating from every point of the surface 

 of all bodies in straight lines ; and it radiates the 

 more copiously, the greater is the quantity of heat 

 which the body contains. Hence a constant ex- 

 5 Act. Helvet. torn. ii. p. 172. 



