276 PRINCIPLES OF CHEMISTRY 



evident that at the ordinary temperature there is a partially dis- 

 sociated system or mixture of nitrogen peroxide, N 2 O 4 , and nitrogen 

 dioxide, NO 2 . In the brown liquid boiling at 22 probably a portion 

 of the N 2 O, has already passed into NO 2 , and it is only the colourless 

 liquid and crystalline substance at 10 that can be considered as pure 

 nitrogen peroxide. 47 



The above explains the action of nitrogen peroxide on water at low 

 temperatures. N 2 O 4 then acts on water like a mixture of the anhy- 

 drides of nitrous and nitric acids. The first, X 2 O 3 , may be looked on 

 as water in which the two atoms of hydrogen are replaced by the radicle 

 NO, while in the second the hydrogen is replaced by the radicle NO 2 , 

 proper to nitric acid ; and in nitrogen peroxide one atom of the 

 hydrogen of water is replaced by NO and the other by NO.,, as is seen 

 from the formulae 



H) . N0j . N0 2 ) . NO ) . 



Hj> N0i> N0 2) > N0 2 , r > 



or H 2 ; N 2 O :J ; N 2 O 5 ; N 2 O 4 . 



In fact, nitrogen peroxide at low temperatures gives with water (ice) 

 both nitric, HNO 3 , and nitrous, HNO 2 , acids. The latter, as we shall 

 afterwards see, splits up into water and the anhydride, N 2 O 3 . If, how- 

 ever, warm water act on nitrogen peroxide, only nitric acid and oxide 

 of nitrogen are formed : 3NO 2 + H 2 O=NO + 2NHO 3 . 



Although NO.; is not decomposed into N and O even at 500, 

 still in many cases it acts as an oxidising agent. Thus, for instance, 

 it oxidises mercury, converting it into mercurous nitrate, 2NO 2 + 

 Hg=HgNO 3 -|-NOj it being itself deoxidised into nitric oxide, into 



47 The fact that the presence of a portion of dioxide, NO.,,, must be acknowledged in 

 liquid nitrogen peroxide, N 2 O 4 , at temperatures of from to 22, is not only of great 

 significance for the theory which regards solutions as liquid systems of equilibrium, consist- 

 ing of combined and decomposed substances, but it also shows the nature of solutions of 

 gaseous substances, because the NO 2 must be regarded as a gas dissolved in the volatile 

 liquid N 2 O 4 . 



Liquid nitrogen peroxide is said by Geuther to boil at 22-26, and to have a sp. gr. 

 at = 1-494 and at 15 = 1'474. It is evident that, in the liquid as in the gaseous state, 

 the variation of density with the temperature depends not only on physical, but also on 

 chemical changes, as the amount of N 2 O 4 decreases and the amount of NO. 2 increases with 

 the temperature, and they (as polymeric substances) should have different densities, as we 

 find, for instance, in the hydrocarbons C 5 H 10 and C 10 H.,o. 



It may not be superfluous to here mention that the measurement of the specific heat 

 of a mixture of the vapours of N 2 O 4 and NO 2 enabled Berthelot to determine that the 

 transformation of 2NO 2 into N 2 O 4 is accompanied by the evolution of about 18000 units 

 of heat, and as the reaction proceeds with equal facility in either direction, it will be 

 exothermal in the one direction and endothermal in the other ; and this clearly demon- 

 strates the possibility of reactions of both aspects proceeding in either direction, although, 

 as a rule, reactions evolving heat proceed with greater ease. 



