274 PRINCIPLES OF CHEMISTRY 



oxide, oxygen gas, and nitrogen peroxide are formed. The latter, in a 

 strongly cooled vessel, condenses into a brown liquid, which boils at 

 about 22. The purest peroxide of nitrogen, solidifying at 9, is 

 obtained when dry oxygen is mixed in a freezing-mixture with twice 

 its volume of dry nitric oxide, NO, when transparent prisms of nitrogen 

 peroxide are formed in the receiver ; they melt into a colourless liquid 

 at about 10. When the temperature of the receiver is above 

 9, the crystals melt, 44 and at give a reddish-yellow liquid, like 

 that obtained in the decomposition of lead nitrate. The vapours of 

 nitrogen peroxide have a characteristic odour, and at the ordinary 

 temperature are of a dark-brown colour, but at lower temperatures the 

 colour of the vapour is much fainter. When heated, especially above 

 50, the colour becomes a very dark brown, so that the vapours almost 

 lose their transparency. 



The causes of these peculiarities of nitrogen peroxide were not 

 clearly understood until Deville and Trooste determined the density 

 and dissociation of the vapour of this substance at different temperatures, 

 and showed that the density varies. If the density be referred to that 

 of hydrogen at the same temperature and pressure, then it is found to 

 vary from 38 at the boiling point, or about 27, to 23 at 135, after 

 which the density remains constant up to those high temperatures at 

 which the oxides of nitrogen are decomposed. As, on the basis of the 

 laws enunciated in the following chapter, the density 23 corresponds 

 with the compound N0 2 (because the weight corresponding with this 

 molecular formula= 46, and the density referred to hydrogen as unity is 

 equal to half the molecular weight), therefore at temperatures above 1 35 

 the existence of nitrogen dioxide only must be recognised. It is this 

 gas which is of a brown colour. At a lower temperature it forms 

 nitrogen peroxide, N 2 O 4 , whose molecular weight, and therefore density, 

 is twice that of the dioxide. This substance, which is isomeric with 

 nitrogen dioxide, as ozone is isomeric with oxygen, and has twice as 

 great a vapour density (46 referred to hydrogen), is formed in greater 

 quantity the lower the temperature, and crystallises at 10. The 

 reasons both of the variation of the colour of the gas (N. 2 4 gives 

 colourless and transparent vapours, whilst those of NO 2 are brown and 

 opaque) and the variation of the vapour density with the variation of 



44 According to certain investigations, if a brown liquid is formed from the melted 

 crystals by heating above 9, then they no longer solidify at 10, probably because a 

 certain amount of N 2 O 5 (and oxygen) is formed, and this substance remains liquid at 

 80, or it may be that the passage from 2NO 2 into N 2 O4 is not so easily accomplished 

 as the passage from N 2 O 4 into 2NO 2 . 



Liquid nitrogen peroxide (that is, a mixture of NO 2 and N 2 O 4 ) is employed in admix- 

 ture with hydrocarbons as an explosive. 



