Air 217 



When the phosphorus has ceased burning, allow 

 the white fumes to settle, and then raise the bell- jar 

 and examine the substance left on the tile. This is the 

 oxide of phosphorus. Should the phosphorus begin to 

 burn again when the jar has been removed, a further 

 quantity of the oxide may be obtained by replacing the 

 jar, and waiting till the phosphorus again stops burning. 



(Note what happens to the oxide if it is allowed to 

 remain in the open air for a few minutes.) 



147. Such changes as those considered in this 

 chapter are termed " chemical " changes. The union 

 of two distinct substances to form a third, entirely 

 different in properties from either, is often briefly 

 expressed by such statements termed " chemical 

 equations " as the following : 



(1) Phosphorus + oxygen = phosphorus pentoxide. 



(2) Iron + oxygen = iron oxide. 



Here the sign " -f " means " combines with " and 

 "=: " means "to form". 



Thus statement (1) should be read " Phosphorus 

 combines with oxygen to form phosphorus pentoxide ". 

 Phosphorus pentoxide is the exact chemical name of 

 the substance we have hitherto called " oxide of 

 phosphorus", and is required because we know of 

 other distinct compounds of phosphorus and oxygen. 

 Similarly iron rust, which we have called simply " iron 

 oxide ", chiefly consists of one of the three oxides of 

 iron, called " ferric oxide " (Latin, ferrum, iron), com- 

 bined with a certain amount of water. 



148. We will next consider the action of air on the 

 metals magnesium and copper. If magnesium, a metal 

 with a bright silvery lustre, be allowed to remain in 

 contact with moist air, the surface of the metal becomes 

 somewhat dull, but no other change of importance can 



