238 THE FITNESS OF THE ENVIRONMENT 



not so those of hydrogen and oxygen. In 

 almost equal frequency the latter elements 

 take part in the reactions of inorganic chem- 

 istry, and help to form its molecular struc- 

 tures. As an illustration of their importance 

 in this department of the science I have 

 counted the compounds and the classes of 

 compounds mentioned in the table of con- 

 tents of the second edition of Erdmann's 

 " Lehrbuch der Anorganischen Chemie." In all 

 435 substances are referred to ; of these 259, 

 approximately 60 per cent, contain oxygen; 

 130, or 30 per cent, contain hydrogen. There 

 seems to be little doubt that this is a fair 

 test, for the work is compendious, and all im- 

 portant substances and classes of substances 

 are mentioned. Even if the acids, and the 

 small number of bodies which are referred to 

 in connection with their water of crystalli- 

 zation, be eliminated from the above count 

 the great importance of the two elements 

 remains clearly evident. 



Only about one fourth of all the compounds 

 mentioned contain neither hydrogen nor oxy- 

 gen. A very large proportion of these con- 

 sist of the chlorides, bromides, iodides, sul- 

 phides, fluorides, and other similar binary 

 compounds, whose importance certainly does 

 not depend upon the variety of chemical 



