THE AIR AND LIFE. 527 



the j>roportions of this gas iu the atiiiosplieie as a whok^ can vary 

 but slightly, owing- to the power of the sea to absorb the gasandmaiu- 

 tain the equilibrium. 



There is no doubt that oxygen, nitrogen, and carbonic acid are the 

 most im[)ortant elements of the air from a chemical standpoint. Other 

 bodies, however, are normal iu the atmosphere. Ammonia is one of 

 them. It has been found by G. Ville in the extremely small proportion of 

 24 grams per 1,000,()()0 kilograms of air. Nitric acid is another which 

 has been discovered in rain w^ater iu the proportion of 1 to 10 milligrams 

 in a liter of water. Ozone also is found, oxygen that has become con- 

 densed or in some way more potent under the induence of atmospheric 

 electricity. But as a matter of fact these bodies exist in the- air in 

 very trifling proportions and there is no need to consider them further 

 at present. Their functions with regard to life are, however, cleaj, 

 and we will mention them again further on. 



II. — CHEMICAL EFFECTS OF ATMOSPHERIC GASES. 



Having iu)w become acquainted with the elements of the atmos- 

 phere, their relative proportions in the aerial mixture, the manner in 

 which they are produced and destroyed, and assuming that it is a 

 fairly well establislied principle that the composition of air varies l)ut 

 little, remaining the same within the above indicated limits, and having 

 shown the part i)erformed by aninuite life in maintaining the composi- 

 tion of the atmosphere, we may now discuss the functions of the atmos- 

 l^here with reference to animate life. 



For simpliflcation Ave shall make a separate examination of the func- 

 tion performed by the several constituent elements of air. Oxygen is, 

 to all appearances, the vital gas par excellence, and with it we shall 

 begin. It is universally known to be necessary for the respiration of 

 animals and men, and its great usefulness lias been clearly <lem<m- 

 strated by physiology. Man is a great consumer of it, inhaled aircou- 

 taining on an average from 20 to 21 volumes of oxygen while there are 

 but 16 volumes in exhaled air, the organism therefore has absorbed 4 

 volumes. In twenty-four hours the human body retains over 740 grams, 

 or 510,500 cubic centimeters, representing for all mankind an aggregate 

 of 500,000,000 cubic meters per day. Children and aged people con- 

 sume less oxygen than the middle aged, the latter absorbing 014 grams 

 in twenty-four hours, while 375 grams suffice for a child of 8 years. 

 Various other conditions also exert a perceptible influence, a person's 

 vigor, sex, external temperature, inaction or activity either increasing 

 or diminishing the consumption of oxygen. The gas is absorbed by the 

 tissues which it reaches chiefly through the lungs and blood, although 

 one eightieth as much as is taken up by the lungs is absorbed by the skin. 

 All living tissues require oxygen, for all breathe, and the lungs are noth- 

 ing but the instrunumt of respiration. The chemical operation by which 

 respiration is essentially constituted is jjerformed elsewhere, in the 



