74 CHEMISTEY. 



qualities! one being one of the blandest of all substan- 

 ces, flowing into the lungs without irritating in the least 

 the delicate air-cells, while the other is powerfully acid, 

 and dangerous to breathe. The chief reason of this differ- 

 ence is that the air is a mere mixture of oxygen and nitro- 

 gen, and therefore partaking of the properties of both of 

 these gases, while nitric oxide is a compound, a new sub- 

 stance formed by the chemical union of the two gases. 

 You have here illustrated in a striking manner the grand 

 difference between mixtures and compounds, the mixture 

 having properties intermediate between those of its ingre- 

 dients, while the compound generally has properties differ- 

 ing widely from those of either of the substances of which 

 it is composed. 



86. Nitrous Anhydride. This is a thin, mobile, blue liquid 

 at a very low temperature, otherwise it is an orange-red gas. 

 Dissolved in water, it combines with it and forms nitrous 

 acid, which is of no great importance, though some of its 

 compounds are useful in the arts ; they are called nitrites. 



87. Nitrous Anhydride in Nitric Acid. It is the nitrous 

 anhydride that gives the yellow color which nitric acid so 

 commonly has. But how is this gas generated in the nitric 

 acid ? The explanation is easy. Nitric acid, we have told 

 you, is very ready to part with a portion of its oxygen. 

 Even exposure to light will make it do this ; so that if we 

 wish to preserve the acid pure, we must keep it in a dark 

 place or in dark-colored bottles. As we commonly see it, 

 a portion of it has become, by a loss of one fifth of its oxy- 

 gen, nitrous anhydride, which, readily dissolving in the nitric 

 acid, gives it a yellow color. Sometimes the oxygen which 

 is disengaged in this decomposition of the nitric acid, to- 

 gether with some of the nitrous anhydride, forces out the 

 stopper of the bottle. 



88. Nitric Peroxide. To obtain this substance in a liquid 



