CARBONIC ACID 139 



action in the oceans, lakes, and streams. In 

 truth, so close is the association between these 

 two substances that it is scarcely correct logi- 

 cally to separate them at all ; together they 

 make up the real environment and they never 

 part company. Carbonic acid thus possesses 

 the first great qualification of a food: its 

 occurrence is universal and its mobility a 

 maximum. This is due to the fact that its 

 absorption coefficient is on the average ap- 

 proximately one, the most favorable value. 



Needless to say the absorption coefficient 

 of carbonic acid is also of great importance 

 in many physiological processes, chiefly per- 

 haps in excretion. In the course of a day a 

 man of average size produces, as a result of 

 his active metabolism, nearly two pounds of 

 carbon dioxide. All this must be rapidly re- 

 moved from the body. It is difficult to im- 

 agine by what elaborate chemical and physical 

 devices the bodv could rid itself of such enor- 

 mous quantities of material were it not for 

 the fact that, in the blood, the acid can cir- 

 culate partly free * and, in the lungs, by a pro- 

 cess which under ordinary circumstances has 

 all the appearances of a simple physical phe- 



1 Of the total carbonic acid of the blood 5-10 per cent 

 exists as the free acid, partly in the plasma, partly in the cor- 

 puscles. 



