COMPOSITION OF THE ATMOSPHERE 25 



are water vapor and CO, present, that we can speak of a triple at- 

 mosphere, one an intimate mixture of oxygen and nitrogen, and 

 the other two of water vapor and CO,, respectively, diffused through 

 the pure air atmosphere. 



Nitrogen is the inactive element, though animals and plants ap- 

 propriate it through the formation of nitrogenous compounds. 



Oxygen, the active element of the atmosphere, is consumed by 

 all animals and taken either directly from the air, or, in the case of 

 water-living animals, from the water in which it is dissolved. De- 

 caying organic matter consumes oxygen, and so do some minerals 

 during the process of oxidation. Oxygen is supplied to the air by 

 the growth of plant life, which breaks up CO,, using the carbon 

 and setting part of the oxygen free. Volcanic vents also supply 

 oxygen, while another source of supply is "found in the deoxidation 

 of minerals. Thus the quantity of oxygen withdrawn from the air 

 is balanced by that supplied, so that the relative amount remains 

 practically constant. 



Argon, first separated from the nitrogen of the air in 1895, is 

 like that element exceedingly inert, its power of combining with 

 other elements being even less than that of nitrogen. It forms about 

 0.94 per cent, by volume, or 1.44 per cent, by weight, of the at- 

 mosphere. Other previously unknown gases in the atmosphere are : 

 helium, i to 2 parts per million ; neon, i to 2 parts per hundred 

 thousand; krypton, i part in 20 millions; xenon, i part in 170 mil- 

 lions. These are as inert as argon. 



Carbon Dioxide (CO,). This is a relatively constant constitu- 

 ent of the air, making about 0.03 per cent, by volume, or three parts 

 in 10,000 of the entire atmosphere. It is supplied to the atmosphere 

 by the burning or decay of organic matter, by the respiration of ani- 

 mals, as well as by volcanic emanations and other agents. Artifi- 

 cial consumption of coal and other burnables furnishes a large sup- 

 ply of CO, to the atmosphere, a ton of bituminous coal (75 per cent, 

 of carbon) furnishing about 2^ tons of CO, (Salisbury-83 :575). 

 Since the amount of CO, in the air remains relatively constant, a 

 quantity equal to that supplied [estimated at several billion tons a 

 year (Salisbury)] must be removed from the air. The chief agents 

 active in abstracting CO, from the air are chlorophyll-forming 

 plants, which, as already remarked, find in it the source of carbon 

 for their tissues. Carbonization of mineral matter, or the com- 

 bination of the CO, with other elements, is another cause of the 

 reduction of the amount of CO, in the air. 



Water Vapor. The water vapor of the atmosphere varies with 

 temperature and other local conditions, and with the amount sup- 



