xx DISSOCIATION 525 



Deville and Troost found (Comptes rendus, 1867, 64, 

 237-243) for the pure vapour : 



Temp. ... 267 49-6 60-2 8o'6 ioo'i 121-5 I 35 l $4 C" 

 Density... 2*65 2-27 2*08 i'8o i'68 1-62 i'6o 1-58 



The values calculated by Playfair and Wanklyn were 

 3*179 for N 2 O 4 , 1*589 for NO 2 . The percentage com- 

 position of the vapour is therefore : 



At 267 C ............................ 80% 20% 



At 49-6 .............................. 60 40 



At 100-1 ............................. ii 89 



the dissociation being complete from 1 54 C. upwards. 



Playfair and Wanklyn concluded "that peroxide of 

 nitrogen exists in two states ; that there are two bodies having 

 the same percentage composition as peroxide of nitrogen, 

 but which are polymeric." The change of molecular state 

 was associated by them with " the wonderful deepening of 

 colour which peroxide of nitrogen undergoes on being 

 heated," and which had been noted by Priestley ninety years 

 before (p. 76). 



The two changes are undoubtedly due to a common 

 cause, namely, the dissociation of a dense colourless 

 tetroxide, N 2 O 4 , into a deeply-coloured dioxide, NO 2 , of 

 lower density. The liquid, boiling at 22 C., which Gay- 

 Lussac prepared in 1816 by heating lead nitrate and cooling 

 the vapours by means of ice and salt (Ann. Chim. Phys.^ 

 1816, 1, 405), is pale yellow; it probably consists of N 2 O 4 

 with only a trace of NO 2 . The solid, melting at 10 C., 

 is a colourless ice (Deville and Troost, Comptes rendus^ 1867, 

 64, 238, footnote), and must be regarded as the pure 

 tetroxide, free from all traces of dioxide. 



The dissociation of nitrogen peroxide, 



N 2 4 2 N0 2 , 



cannot be proved by chilling the brown dioxide, which loses 



