140 THE FITNESS OF THE ENVIRONMENT 



nomenon, 1 can escape into air which is charged 

 with but little of the gas. 2 Were carbon 

 dioxide not gaseous, its excretion would be 

 the greatest of physiological tasks ; were it 

 not freely soluble, a host of the most universal 

 existing physiological processes would be im- 

 possible. 



II 



ACIDITY 



The only other property of this substance 

 which calls for consideration at this point is 

 its acid nature and strength. Very few min- 

 erals are freely soluble in pure water, and 

 nearly all yield to the process of weathering 

 far more readily because of the carbonic acid 

 which all natural waters contain. 3 The latter 



1 The determination of the exact nature of the process 

 by which carbonic acid is excreted across the lung membrane 

 is one of the standing difficulties of physiology, but we need 

 not here enter upon its consideration. 



2 Expired air contains on the average more than 4 per cent 

 by volume of carbonic acid. 



3 "A few minerals (halite, for example) are readily soluble 

 in water without chemical change, and without the aid of any 

 intermediate element; hence the copious brine-springs of 

 salt regions. In the great majority of cases, however, solu- 

 tion is effected through the medium of carbonic acid or other 

 reagent. Limestone is soluble to the extent of about 1 part 

 in 1000 of water saturated with carbonic acid. The solution 

 and removal of lime from the mortar of a bridge or vault, 



