304 Domestic Science 



The nitrogen and oxygen of air may be separated 

 to some extent by such simple physical operations as 

 diffusion and solution in water (see Chapter xiv, Expt. 

 105), each gas exercising its own powers quite inde- 

 pendently of the other. No such simple means suffice 

 for the separation of the two gases contained in water. 

 A striking example of the independence of the oxygen 

 and nitrogen in this respect is afforded by a manu- 

 facturing process for obtaining oxygen from air. 

 Liquid air, produced by exposing air to a low tempera- 

 ture and allowing it to cool itself by rushing through 

 a very small hole under very great pressure, is caused 

 to evaporate. The boiling-point of liquid oxygen is 

 182-5 C., while that of the liquid nitrogen is 13 lower. 

 As a consequence, the latter boils away first, and, by 

 careful manipulation of the conditions of the experi- 

 ment, the nitrogen may be separated from the oxygen 

 so completely that practically pure oxygen is left. 

 Compare this result with the behaviour of water on 

 boiling. 



The small variations noted in the composition of 

 different samples of air are larger than the limits of 

 experiment, hence it does not obey the Law of Constant 

 Composition formulated for chemical compounds. 



No less than six distinct compounds of nitrogen 

 and oxygen are known to chemists, and these all obey 

 the same laws as other chemical compounds. This 

 may be taken as indirect evidence in favour of the idea 

 that air is a mixture, and not a compound, of the two 

 gases which are its chief constituents. 



Other points of difference exist, but those already 

 mentioned will be sufficient to demonstrate the great 

 differences which exist between a mixture and a chemical 

 compound. 



