8o "IDEAS VERY UNSETTLED. 



" The ideas of the best-informed philosophers are as 

 yet very unsettled as regards the exact nature of heat. 

 The great starting-point is to regard heat as motion of 

 some kind; leaving its more precise character to be 

 dealt with by other investigators." 



The sensation of heat, produced by its action 

 on the nerves of feeling, he explains in the follow- 

 ing words : 



" The impression of heat which one receives on enter- 

 ing the hot room of a Turkish bath, is caused by the 

 atomic cannonade which is there maintained Against 

 the surface of the body." 1 



He further specifies : 



" We are to figure a gaseous body as one whose par- 

 ticles are flying in straight lines through space ; imping- 

 ing, like little projectiles, upon each other, and striking 

 against the boundaries of the space which they occupy. 

 ... So likewise in regard to forming steam : the heat is 

 consumed in pulling asunder the liquid particles of water, 

 and in conferring upon them a still greater amount of 

 potential energy. . . . When the heat is withdrawn, the 

 vapor condenses, and the particles again clash together 

 with a dynamic energy equal to that which was employed 

 to separate them. The heat then reappears. . . . 



" The disappearance of heat, which enters bodies 

 while changing from solids to liquids, and from liquids 

 to aeriform states, is ascribed to internal work done 

 among the molecules, which is latent heat ; the exter- 

 nal work being denoted sensible heat. . . . 



" The percussion of the earth against the central orb 

 of the solar system would produce an extent of heat 

 equal to that producible by the combustion of fourteen 



1 Heat considered as a Mode of Motion, lecture iii. 



