1 1 2 THE AMERICAN NATURALIST [Vol. XL VII 



mitted that so far as the investigation has proceeded, 

 water is the only possible fit snbstance. 



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 

 that there are not an infinity of important physical prop- 

 erties; in fact there are very few. And in the second 

 place it is evident, both from centuries of experience in 

 physical science and from the postulates above mentioned 

 regarding life, which undoubtedly do in the main describe 

 its physico-chemical characteristics, that very few prop- 

 erties indeed are of importance in the least comparable 

 with those which I have mentioned. Finally it is in the 

 highest degree probable that we are acquainted with most 

 of the truly essential physical properties, and know them 

 as biologically important, when they are so ; and I believe 

 it has been possible to consider them all, and thus make 

 the argument complete. 



Such is the nature of the argument ; the facts, though 

 no less important than those above indicated, are far too 

 numerous to mention. They include the unique surface 

 tension of water and its very great ionizing power, the ab- 

 sorption coefficient and ionization constant of carbonic 

 acid, the extreme chemical activity of oxygen and hydro- 

 gen, the unique chemical combining power of carbon, the 

 number, complexity, variety and chemical activity of the 

 compounds and processes of organic chemistry, and the 

 vast complexity of the chemical system which inevitably 

 results from the reduction of a mixture of carbonic acid 

 and water. These properties result directly in a be- 

 wildering variety of conditions which in the most varied 

 ways promote complexity, durability and metabolism. 



Analysis of all the facts justifies the following con- 

 clusions. 



The physical and chemical properties which have been 

 taken into consideration include nearly all those which 

 are known to be of biological importance or which ap- 



