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, 

 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 



