102 LIFE AND DEATH. 



which have already been studied and recognized m 

 general physics. They are the same energies : 

 chemical, thermal, mechanical, with their char- 

 acteristics of mutability, their lists of equivalents, 

 and their actual and potential states. 



In the second place, it may happen, and it prob- 

 ably will happen, as it happened in the last century 

 in the case of electricity, that some new form of 

 energy will be discovered belonging to the universal 

 order as to the living order. This will be a conquest 

 of general physics as well as of biology. 



And finally we may rigorously and provisionally 

 admit a last category of vital energies properly so 

 called. 



It is difficult to give much precision to the idea of 

 vital energies properly so called. 



It will be easier to measure them by means of 

 equivalents than to indicate their nature. Besides, 

 this is the ordinary rule in the case of physical agents. 

 We can measure them, although we know not what 

 they are. 



Characteristics of Vital Energies. — We see why we 

 cannot exhibit with precision, a priori, the nature of 

 vital energies. In the first place, they are expreseed 

 by what takes place in the tissues in activity, and 

 this cannot at present be identified with the known 

 types of physical, chemical, and mechanical pheno- 

 mena. This is a first, intrinsic reason for not being 

 able to distinguish them readily, since what takes 

 place is not distinguished by the phenomenal appear- 

 ances to which we are accustomed. 



There is a second, intrinsic reason. These vital 

 phenomena are intermediary, as we shall see, between 

 manifestations of known energies. They lie between 



