134 THE AIR 



suggestion is interesting. If the air is imagined to be sepa- 

 rated into its several parts and these to be arranged around 

 the earth in the order of their specific gravities, water vapor 

 being condensed, there would be first a layer of water five 

 inches thick, then thirteen feet of carbon dioxide, next ninety 

 yards of argon, then a mile of oxygen, and finally four miles of 

 nitrogen, with possibly three or four feet of hydrogen on top. 



(a) Nitrogen. — The amount of nitrogen in the air varies 

 but little. It is the most constant of all the constituents. 

 It is a very inert gas, uniting with other elements only at 

 high temperatures. It acts in the air in part as a diluent, 

 rendering the activity of oxygen less energetic. It is the 

 ultimate source of all nitrogenous compounds. The means 

 by which it has been made to combine with other elements 

 is bacterial in nature (see Section 130). By these means 

 it is removed from the air, but is returned in small measure 

 in the free state by the decomposition of nitrogenous organic 

 matter, and by the burning of all kinds of fuel or other or- 

 ganic material containing nitrogen. The ordinary com- 

 bustion of one ton of coal releases from one to five pounds 

 of nitrogen. 



(6) Oxygen. — This constituent, although fairly constant 

 in amount, has been known to vary from 20.53 per cent, by 

 volume to 21.03 per cent. Since a man consumes about 

 600 liters of oxygen in a day, and a ton of coal in burning 

 consumes about 1,500,000 liters, it can easily be seen that 

 the air of cities, which are densely populated and where 

 much manufacturing is carried on, has a lower percentage 

 of oxygen than the open country. And this is further empha- 

 sized by the fact that the country is where large numbers of 

 growing plants are to be found, and in photosynthesis oxygen 

 is given off by plants. Oxygen, as has been noted in Chapters 

 I and II, is necessary for the germination of the seed and the 

 growth of the plant. It is, moreover, absolutely necessary 

 for the life of man and other animals. 



(c) Carbon Dioxide. — Of the important constituents of 

 the air, carbon dioxide is the smallest in amount and the most 

 variable, with the exception of water vapor. Although 

 normal, pure air contains about 0.03 per cent, by volume, 



