INTRODUCTORY 11 



more exactly, 21 per cent, of the former and 79 

 per cent, of the latter by volume. The gases are not 

 chemically united, merely mixed together, but the pro- 

 portion is remarkably constant. In addition to these two 

 main constituents, the atmosphere always contains a cer- 

 tain quantity of carbon dioxide, some water vapour, and 

 generally traces of nitric acid, ammonia, particles of dust, 

 micro-organisms and other impurities. 



Oxygen. Oxygen, which forms about one fifth part of 

 the total bulk of dry air, is a chemical element, i.e., it 

 cannot be split up or resolved into anything simpler. It 

 is a very active substance. It combines readily with other 

 elements forming compounds called oxides. The process 

 is called oxidation. When it takes place heat is given 

 out, and, if it is very rapid, flame is produced. Ordinary 

 combustion is simply a process of rapid oxidation. When 

 oxidation takes place more slowly the temperature does 

 not .rise so high, but the material results are the same 

 the elements unite with oxygen to form oxides identical 1 

 with those produced by burning. The respiration of 

 animals and decay of organic matter in the soil are 

 examples of slow oxidation. The bodies of animals 

 and all kinds of organic matter consist largely of the ele- 

 ments carbon and hydrogen. When they are oxidised, 

 oxide of carbon and oxide of hydrogen (water) are formed, 

 whether the process takes place rapidly by burning or, 

 more slowly, by breathing or decay. Plants also are sub- 

 ject to a process analogous to the breathing of animals. 

 They take up oxygen from the air and give off oxide of 

 carbon. This phenomenon is not noticeable in daylight 

 as it is then obscured by a reverse action. The reverse 

 action is one of de-oxidation ; the plants take up oxide 

 of carbon and give off oxygen. 



1 In some cases other oxides are formed. 



