DECOMPOSITIONS OF N1TKIC OXIDE. 195 



Sometimes half the nitrogen only is set free, a slow reaction 

 observable with essence of turpentine or benzene, which leave 

 a residuum of nitrogen equal to the fourth of the volume of 

 the nitric oxide. Sometimes nitrogen monoxide is set free, 

 another slow reaction observable with sodium sulphide or 

 stannous chloride, which leave nitrogen monoxide and nitrogen 

 in equal volumes. 



Sometimes even ammonia is set free, with the aid of the 

 hydrogen of water, or various organic compounds. 



Nitrogen monoxide, nitrogen, and ammonia are formed from 

 the same causes in the greater number of reactions where an 

 oxidisable body tends to bring nitric acid to the state of 

 nitric oxide. Hence the latter gas, prepared by the reaction of 

 the metals on dilute nitric acid, is seldom pure. 



A similar tendency to slow and multiple decompositions 

 is the distinctive character of unstable compounds formed 

 with absorption of heat. Nitric oxide is comparable, under 

 this head, with cyanogen and acetylene. Now, all these 

 endothermal compounds have a capacity for entering into 

 reaction, a sort of chemical plasticity very superior to that of 

 their elements, and comparable to that of the most active 

 radicals, a circumstance which may be explained by the excess 

 of energy stored up in the act of their synthesis. 



The potential energy of the elements generally diminishes in 

 the act of combination ; acetylene, cyanogen, and nitric oxide, 

 however, form exceptions. There is no doubt some relation 

 between this increase of energy and the capacity possessed by 

 these compound radicals for entering directly into new com- 

 binations with the elements. 



Under the influence of electricity we obtain the direct, 

 though always endothermal reunion of the elements which 

 form either acetylene itself or the hydrogenated combination 

 of cyanogen, or the super-oxidised combination of nitric oxide. 



4. Nitrogen trioxide. Let us first note the following thermal 

 relations concerning anhydrous nitrous acid : 



N 2 3 = 2NO + would absorb - 10-5 Cal. 

 N 2 3 = 2N0 2 liberates + 8-5. 



Hence it follows that the breaking up of nitrous acid into 

 nitric oxide and peroxide, 



N 2 3 = NO + NO* would absorb - 2'0 Cal. 



In fact, the three bodies contained in the last equation con- 

 stitute a system in the state of dissociation, a system of which 

 the equilibrium varies with the relative proportions, temperature, 

 condensation, etc. 



Gay-Lussac observed that oxygen and nitrogen, mixed in 

 volumes in the ratio of 1 : 4 in the presence of a concentrated 

 solution of potash, yield only nitrite. 



o 2 



