CHAPTER IX 

 Air 



67. Air a flechanical flixture. Air is a mechanical 

 mixture composed of a number of gases and compounds 

 in about the following proportions : ( i ) nitrogen, 79 per 

 cent.; (2) oxygen, 20 per cent.; (3) carbon dioxid, 0.04 

 per cent.; (4) ammonium compounds in small amounts; 

 (5) moisture; (6) ozone; (7) hydrogen peroxid ; (8) 

 argon; (9) dust: organic matter, and micro-organisms. 

 That air is a mechanical mixture is shown by its not hav- 

 ing a constant chemical composition, which is necessary 

 for all compounds, and when nitrogen and oxygen are 

 mixed, in the same proportion as in air, there is no evi- 

 dence of a chemical reaction, as change of volume or 

 temperature. The air that is dissolved in water is of 

 different composition from atmospheric air, due to the fact 

 that oxygen is more soluble in water than is nitrogen. 

 The occurrence of nitrogen and oxygen in the air, and 

 the chemical and physical properties of these gases have 

 been discussed in Chapters IV and V. 



68. Carbon Dioxid. The amount of carbon dioxid in 

 the air is small, about 0.04 per cent., and it is supposed 

 to remain fairly constant in amount. It is produced 

 from : (i) combustion of carbon-containing materials, as 

 fuels ; (2) decaying of organic matter ; and (3) respira- 

 tion of animals. The carbon dioxid of the air serves as 

 food for plants and is used for the construction of plant 

 tissue. The amount produced and that used by vegeta- 



