CHAPTER VII. 



THEORETICAL DISCOVERIES IN PHYSICS. 



Has matter motion ? Then each atom, 

 Asserting its perpetual right to dance, 

 Would make a universe of dust ! 



For the world was built in order, 

 And the atoms march in tune. 



Emerson. 



THE theoretical discoveries in the domain of physics 

 (besides those already referred to) have been very nu- 

 merous, but only a few of them have enough generality 

 or have become sufficiently popular to require notice in 

 the present sketch. Two of these discoveries, however, 

 stand above the general level as important contributions 

 to our knowledge of the material universe. These are 

 (1) the determination of the mechanical equivalent of 

 heat, leading to the general theory of the conservation of 

 energy, and (2) the molecular theory of gases. 



Down to the beginning of this century heat was gen- 

 erally considered to be a form of matter, termed caloric 

 or phlogiston. The presence of phlogiston was sup- 

 posed to render substances combustible, but when the 

 chemical theory of combustion was discovered by Lavoi- 

 sier, phlogiston, as the cause of combustion, disappeared, 

 although caloric, as the material basis of heat, still held 

 its ground. Close to the end of the last century Count 

 Rumford showed that in boring a brass cannon the heat 

 developed in 2-| hours was sufficient to raise 26-J Ibs. of 



