COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 261 



So N 2 O 4 gives N 2 O 3 and N 2 O 3 , or, in the presence of water, their 

 hydrates. 



We have already seen that, under certain conditions, nitrogen 

 combines with oxygen, and we know that ammonia may be oxidised. 

 Tn these cases various oxidation products of nitrogen are formed, but 

 in the presence of water arid an excess of oxygen they always give 

 nitric acid. Nitric acid, as corresponding with the highest oxide, is 

 able, in deoxidising, to give the lower oxides, and for this reason we 

 will begin with it. 



Nitric acid, NHO 3 , is likewise known as aqua fortis. In a free 

 state it is only met with in nature in small quantities, in the air and 

 rain-water after storms ; but even in the atmosphere nitric acid does 

 not long remain free, but combines with ammonia, traces of which are 

 always found in air. On falling on the soil and into running water, 

 Ac., the nitric acid everywhere comes into contact with bases (or their 

 carbonates), which easily act on it, and therefore it is converted into the 

 nitrates of these bases. Ammonia and other compounds of nitrogen, 

 if oxidised in the soil, are always in the presence of bases, and there- 

 fore also give salts of nitric acid, and not the free acid itself. Hence 

 nitric acid is always met with in the form of salts in nature. These 

 salts are called nitres. This name is derived from the Latin sal nitri. 

 The potassium salt KNO 3 is common nitre, and the sodium salt NaNO 3 

 Chili saltpetre, or cubic nitre. Nitres are formed when a nitrogenous 

 substance is slowly oxidised in the presence of an alkali by means 

 of the oxygen of the atmosphere. In nature there are very fre- 

 quent instances of such oxidation. For this reason, certain soils and 



starting points, and if the compounds formed be also gaseous, the following amounts 

 of heat, expressed in thousands of heat units, are absorbed (hence a minus sign) : 



N 2 N 2 2 N,0 3 N 2 4 N 2 5 



-21 -43 -22 -5 -1 



-22 +21 +17 + 4 



The difference is given in the lower line. For example, if N.,, or 28 grams of nitrogen, 

 combine with O that is, with 16 grams of oxygen then 21000 units of heat are absorbed, 

 or sufficient heat is assimilated to heat 21000 grams of water through 1. Naturally, 

 direct observations are impossible in this case ; but if charcoal, phosphorus, or similar 

 substances, are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed 

 in both cases, then the difference (more heat will be evolved in burning in nitrous oxide) 

 gives the figures required. If, then, N.,O 2 , by combining with O.,, gives N.,,O 4 , then, as is 

 seen from the table, heat should be developed, namely 88000 units of heat, or NO + O = 

 19000 units of heat. The differences given in the table show that the maximum absorp- 

 tion of heat corresponds with nitric oxide, and that the higher oxides are formed from it 

 with evolution of heat. If liquid nitric acid, NHO 3 , were decomposed into N + O 3 + H, 

 then 41000 heat units would be required ; that is, an evolution of heat takes place in its 

 formation from the gases. It should be observed that the formation of ammonia, NH 5 , 

 from the gases N + H 3 evolves 12'2 thousand heat units. 



