198 OXYGENATED COMPOUNDS OF NITROGEN. 



But the progressive dilution of the portion of hydrated 

 nitrogen trioxide which remains undecomposed (a dilution 

 resulting from the reaction itself) limits more and more the 

 relative proportion of anhydrous acid up to the point at which 

 the small quantity of nitric oxide remaining dissolved suffices 

 to ensure the stability of the system. Perhaps dilution, carried 

 out to a certain degree, completely arrests the decomposition 

 of the hydrated nitrogen trioxide, no longer permitting any 

 portion of the anhydrous acid to subsist. 



In practice it is certain that a final system is realised contain- 

 ing at one and the same time water, dilute nitric acid, and 

 hydrated and diluted nitrogen trioxide. By diminishing the 

 relative proportion of water, the equilibrium would be destroyed ; 

 it would also be destroyed by raising the temperature, which 

 gives rise to a liberation of nitric oxide. Conversely, the 

 diminution of water may be compensated for by the lowering of 

 the temperature. 



5. Nitric peroxide. We shall now examine the degree of 

 stability of nitric peroxide. This body is rightly regarded as 

 the most stable of the oxides of nitrogen ; in fact, it may be 

 heated in a sealed glass tube to about 500 for an hour, without 

 showing the least sign of decomposition. It moreover exerts no 

 reaction, either on oxygen in a cold state, or on free nitrogen at 

 a dull red heat under the same conditions. However, under 

 the influence of the electric current the mixture of oxygen 

 and nitric peroxide becomes discoloured, and gives rise to 

 a new compound, pemitric acid, 1 about which very little is 

 known. 



Nitric peroxide is decomposed into its elements by the electric 

 spark 



2N0 2 = T 2 + 4 . 



^ After an hour, as much as a quarter was decomposed. After 

 eighteen hours, a mixture was obtained containing in volume 



1ST = 28 ; = 56 ; N0 2 = 14 



We should note that the decomposition stops at a certain 

 point, as in all cases where the electric spark develops an 

 inverse action. It has, indeed, been known since the time of 

 Cavendish that the spark effects the combination of nitrogen 

 with oxygen. But this combination, effected with dry gases, 

 cannot yield anything but nitric peroxide, seeing that free 

 oxygen always remains, as will now be shown. Operating upon 

 atmospheric air it was found that after an hour 7'5 per cent., 

 that is, a third by volume, had yielded nitric peroxide. Eighteen 

 hours of electric action did not sensibly alter this ratio. 



This numerical value is not absolute. An exact measurement 

 would call for more numerous experiments, made under more 

 1 " Annales de Chimie et de Physique," 5 e s^rie, torn. xxii. p. 439. 



