NITROGEN AND ITS OXIDES. 67 



acid upon the metals themselves. In the latter case the 

 acid in immediate contact with the metal gives some of its 

 oxygen to the metal forming an oxide, and the moment that 

 this is done another portion of the acid seizes this oxide, 

 forming with it the nitrate ; and this double process goes 

 on continually until the action stops. You see, then, that 

 a part of the acid is decomposed in order to provide an ox- 

 ide to unite with the other part. In this decomposition, the 

 acid losing a portion of its oxygen, fumes of nitric oxide 

 pass off. 



We will study the nitrates of the metals in connection 

 with the metals themselves. 



F6. Combustion by Nitric Acid. As this acid so read- 

 ily parts with some of its oxygen, it can set fire to certain 

 substances oh being applied to them. If you heat some 

 powdered charcoal, on pouring nitric acid upon it combus- 

 tion will at once take place. The cause is the rapid union 

 of the charcoal with the oxygen, which it takes from the 

 nitric acid. The reason that it is necessary to heat the 

 charcoal is that the union would not be sufficiently rapid 

 to produce a fire without the aid of heat. So, too, if you 

 pour some of the acid upon warmed oil of turpentine, the 

 oxygen which the turpentine takes from the acid sets it all 

 ablaze. Some caution is required in trying this experiment. 

 The test-tube containing the acid should be fastened to the 

 end of a stick a yard long, so that the experimenter may 

 be at some distance from the turpentine 

 as he pours the acid upon it. Phospho- 

 rus, if thrown upon some nitric acid in a 

 plate, will be set on fire, as seen in Fig. 

 13. The bits of phosphorus must be very 

 small, or some harm will be done by the 

 violence of the combustion. If the acid 

 be rather weak, as that which is bought at the shops often 



