SOURCE OF ORGANIC ENERGY. 35 



"We know that the molecules of all bodies are 

 in motion. In gases and liquids the motion is 

 such that there is nothing to prevent any molecule 

 from passing from any part of the mass to any 

 other part ; but in solids we must suppose that 

 some, at least, of the molecules merely oscillate 

 about a certain mean position, so that, if we con- 

 sider a certain group of molecules, its configuration 

 is never very different from a certain stable con- 

 figuration about which it oscillates. 



"This will be the case even when the solid is 

 in a state of strain, provided the amplitude of the 

 oscillations does not exceed a certain limit ; but if 

 it exceeds this limit the group does not tend to 

 return to its former configuration, but begins to 

 oscillate about a new configuration of stability, the 

 strain in which is either zero, or at least less than 

 in the original configuration. 



"The condition of this breaking up of a configu- 

 ration must depend partly on the amplitude of the 

 oscillations, and partly on the amount of strain in 

 the original configuration ; and we may suppose 

 that different groups of molecules, even in a homo- 

 geneous solid, are not in similar circumstances in 

 this respect. 



"Thus we may suppose that in a certain number 

 of groups the ordinary agitation of the molecules 



