32 CROONIAN LECTURES 



peculiar motions, excited only in the molecules 

 of ponderable matter. 



In other words, the third stage of ideas re- 

 garding the union of matter and force may to 

 some extent be received by every one ; but, in 

 its full extent, it is only gradually becoming 

 recognised as undoubted truth. 



As soon as it is admitted that force is absolutely 

 inseparable from matter, whether gas, liquid, or 

 solid, it will become as impossible to think that 

 matter can consist only of centres of force, as to 

 think that matter can be inert or void of all force. 



Matter, if it be inseparable from force, must 

 always be in a state of motion, or of tendency 

 to, or of resistance to, motion ; it can never be 

 in a state of perfect rest. 



Immediately connected also with the idea of 

 the inseparability of matter and force is another 

 modern idea regarding force, the history of 

 which in England must be given here. 



This idea was first represented by the term 

 conversion of force, then by correlation of 

 forces, then by conservation of force, and now 

 by conservation of energy. 



In 1839, Mr. Faraday, in a paper on the 

 Improbability of Contact exciting Voltaic 



