THE ENVIRONMENT 69 



quired to tear apart molecules of water, and 

 to liberate hydrogen and oxygen, is very great 

 indeed, and when hydrogen and oxygen re- 

 combine to form water, this energy must 

 reappear, — under ordinary circumstances as 

 heat. This fact, too, is very favorable for the 

 organism, because almost all compounds which 

 contain hydrogen yield a great deal of energy 

 when they are burned ; they are, in short, great 

 reservoirs of energy which can be tapped in 

 the process of metabolism. If, therefore, the 

 heat of combustion of hydrogen be nearly or 

 quite a maximum, as it is, among all sub- 

 stances, it is clear that water is again, in an- 

 other respect, most wonderfully fitted for life. 



Finally, if it be true, and such is the case, J 

 that very few of the substances which share 

 the fitness of water in one of these character- 

 istics also share or approach its fitness in 

 either of the others, and that none possesses 

 all these qualifications in a degree that merits 

 consideration, it must, I conceive, be admitted 

 that so far as the investigation has proceeded 

 water is the only possible fit substance. 



A criticism may here be made; are there 

 not other substances which possess other 

 groups of qualifications which water lacks ? 

 And that is a difficulty which is even harder 

 to meet. But in the first place it is evident 





