THE ATMOSPHERE 87 



much used for this purpose. The greatest supply of 

 available nitrogen for use in commercial fertilizers and 

 explosives is found in Chile in the form of Chile salt- 

 peter, which is mined in great quantities and exported 

 to all parts of the world. 



Protein is one of the necessary foods of animals, and 

 since they cannot make their own protein, the animals 

 must get it from the plants. Plants make protein 

 from nitrogen and other elements such as carbon and 

 oxygen, and this in turn becomes natural food for 

 animals. 



Air, a Mixture. We are now able to answer the 

 question as to whether air is a chemical compound of 

 nitrogen and oxygen or simply a mixture of the two gases. 

 We can do this by comparing the properties of nitrogen 

 and oxygen with those of air. The properties of air 

 are simply those of oxygen modified by those of nitrogen. 

 For example, air supports the combustion of other sub- 

 stances in the same way that oxygen does but much less 

 energetically, because the nitrogen interferes by dilut- 

 ing the oxygen. We have already compared the burning 

 of sulphur in air with its burning in pure oxygen. Since 

 only about one fifth of air is oxygen, a substance burned 

 in air is supplied with oxygen only about one fifth as 

 fast as when burned in pure oxygen. If air were a chemi- 

 cal compound, its properties would in all probability 

 be very different from those of the two gases which com- 

 pose it. Another proof is that the weight of the air 

 may be calculated by calculating the weight of each gas 

 separately in the proportion of four of nitrogen to one of 

 oxygen. In 100 liters of air there are 79 liters of nitrogen 

 which will weigh : 



1.257 X 79 = 99.3 grams 



