20 Life and Matter [chap.h. 



merely to transference and transformation, 

 and can only be increased or diminished by 

 passing fresh energy in or out through the 

 walls of the system. So far from this being 

 self-evident, it required very careful measure- 

 ment and experimental proof to demonstrate 

 the fact, for in common experience the 

 energy of a system left to itself continually to 

 all appearance diminishes ; yet it has been 

 skilfully proved that when the heat and 

 every other kind of product is collected and 

 measured, the result can be so expressed as 

 to show a total constancy, appertaining to 

 a certain specially devised function called 

 " energy," provided we know and are able 

 to account for every form into which the 

 said energy can be transformed by the activity 

 going on. A very important generalisation 

 truly, and one which has so seized hold of 

 the mind of the physicist that if in any 

 actual example a disappearance or a genera- 

 tion of energy were found, he would at once 

 conclude either that he had overlooked some 

 known form and thereby committed an error, 



