84 THE FITNESS OF THE ENVIRONMENT 



It must be confessed that such data are not 

 a brilliant confirmation of the law. A series 

 of numbers which vary from 2.0 to 6.0 is 

 something quite different from constancy, 

 and every one of these numbers is less than 

 6.4. It is, however, certain that these quanti- 

 ties are uniformly of the same order of magni- 

 tude, and this is all that is of importance for 

 our present purpose. For accordingly they 

 prove that unless the average atomic weight 

 of a substance be very low its specific heat 

 cannot be very high. Of course only com- 

 pounds which are largely made up of hydrogen 

 can possess very low average atomic weights, 

 and among such those will be lowest in this 

 respect which contain a relatively small number 

 of atoms of another element of low atomic 

 weight, like carbon, nitrogen, oxygen, etc. 

 Of such substances the hydrocarbons make up 

 the only numerous group, and for the most 

 part their specific heats appear to be, like 

 that of elementary carbon itself, considerably 

 lower even than would be predicted by the 

 rule. So it is that the conclusion is warranted 

 that water shares the characteristic of very 

 high specific heat with a very small number of 

 substances, among which hydrogen and am- 

 monia are probably the only important chemi- 

 cal individuals. From this conclusion another 



