96 EXPANSION OF BODIES . BY HEAT. 



is made to play upon a ball of lime, by which a most 

 intense light is produced,, the heat, which has not been 

 actually increased, acquires the power of being refracted 

 by a glass lens, and combustible bodies may be ignited 

 in its focus. 



It certainly appears from these results, that the 

 undulatory hypothesis holds true, so far as the motion of 

 the calorific power is concerned. At a certain rate the 

 vibrations are thrown back or stopped by the opposing 

 body, while in a state of higher excitation, moving with 

 increased rapidity, they permeate the screen.* This 

 does not, indeed, interfere with the refined theory of 

 Prevost,f which supposes a mutual and equal interchange 

 of caloric between all bodies. 



The most general effect of heat is the expansion of 

 matter ; solids, liquids, and airs, all expand under its 

 influence. If a bar of metal is exposed to calorific 

 action, it increases in size, owing to its particles being 



* For a careful examination of the several theories of heat 

 consult Dr. Young s Course of Lectures on Natural Philosophy, 

 &c., Lecture 52, On the Measures and the Nature of Heat; also 

 Powell's very excellent Reports on Radiant Heat Reports of the 

 British Association, 1832, 1840. The transcendental view which 

 the immaterial theory leads to, cannot be better exemplified than 

 by the following quotation from that inexplicable dream of a 

 talented man, Elements of Physiophilosophy, by Lorenz Oken, M.D. 

 (translated for the Ray Society, by Alfred Tulk) : 



" Heat is not matter itself any more than light is ; but it is 

 only the act of motion in the primary matter. In heat, as well 

 as in light, there certainly resides a material substratum ; yet, 

 this substratum does not give out heat and light ; but the motion 

 only of the substratum gives out heat, and the tension only of 

 the substratum light. There is no body of heat; nitrogen is 

 the body of heat, just as oxygen may be called the body of fire. 

 Heat is real space; into it all forms have been resolved, as all 

 materiality has been resolved into gravity, and all activity, all 

 polarity, into light. Heat is the universal form, consequently the 

 want of form." 



f Menioires de la Societe Physique, &c., de Geneve, torn. ii. 

 art. 2. 



