138 THE FITNESS OF THE ENVIRONMENT 



that for the ordinary temperature of the earth's 

 waters, where they are in contact with car- 

 bonic acid gas, it is very close indeed to 1.0. 

 Hence, when water is in contact with air, and 

 equilibrium has been established, the amount 

 of free carbonic acid in the water is almost 

 exactly equal to the amount in the air. Unlike 

 oxygen, hydrogen, and nitrogen, carbonic acid 

 enters water freely; unlike sulphurous oxide 

 and ammonia, it escapes freely from water. 

 Thus the waters can never wash carbonic acid 

 out of the air, nor the air keep it from the 

 waters. It is the one substance which thus, 

 in considerable quantities relative to its total 

 amount, everywhere accompanies water. 1 In 

 earth, air, fire, and water alike these two sub- 

 „ stances are always associated. 



Accordingly, if water be the first primary 

 constituent of the environment, carbonic acid 

 is inevitably the second, — because of its 

 solubility possessing an equal mobility with 

 water, because of the reservoir of the at- 

 mosphere never to be depleted by chemical 



1 "Carbonic acid being more soluble than the other gases, 

 is contained in rain water in proportions between 30 and 40 

 times greater than in the atmosphere." — Geikie, "Text- 

 book of Geology." 4th ed., Vol. I, p. 449, 1903. 



It must not be forgotten that carbonic acid in subterranean 

 water, by which so much geological change is accomplished, 

 originates, not in the air, but from organic matter in the soil. 



