vi THE COMPOSITION OF THE AIR 55 



with the burning phosphorus, another which 

 does not so combine, and is left behind. 

 The gas which supports combustion is called 



Oxygen 



The gas which does not support combustion is 

 called 



Nitrogen 



We can now determine the volumes in which 

 these two gases are present in the air, as the water 

 which has rushed into the flask represents the 

 volume of oxygen which has disappeared. Pour 

 this water into a measuring glass and note the 

 amount. An ordinary ounce-measuring glass will do. 

 Next fill the flask with water, allowing a little for 

 the cork, and empty it into the measuring glass as 

 well, so as to determine the volume of the flask. 



On doing this we shall find that the oxygen occu- 

 pied about one-fifth part of the volume of the flask. 



The air therefore contains four- fifths by volume of 

 nitrogen mixed with one-fifth by volume of oxygen. 



This experiment gives us an example of 

 what we mean by chemical combination, the 

 oxygen of the air combining with the phos- 

 phorus to form a third new body, which we 

 call oxide of phosphorus. 



During this operation no substance has been lost 

 or gained, the flask and its contents weighing the 

 same at the beginning and the end of the burning 



