through the apparatus in order to remove all traces of volatile substances (liquid oxides of nitrogen) adhering to the crystals of nitric anhydride. These form a voluminous mass of rhombic crystals (density 1·64), which sometimes are of rather large size; they melt at about 30° and distil at about 47°. In distilling, a portion of the substance is decomposed. With water these crystals give nitric acid. Nitric anhydride is also obtained by the action of phosphoric anhydride, P 2 O 5 , on cold pure nitric acid (below 0°). During the very careful distillation of equal parts by weight of these two substances a portion of the acid decomposes, giving a liquid compound, H 2 O,2N 2 O 5 = N 2 O 5 ,2HNO 3 , whilst the greater part of the nitric acid gives the anhydride according to the equation 2NHO 3 + P 2 O 5 = 2PHO 3 + N 2 O 5 . On heating, nitric anhydride decomposes with an explosion, or gradually, into nitric peroxide and oxygen, N 2 O 5 = N 2 O 4 + O.
Nitrogen peroxide, N2O4, and nitrogen dioxide, NO2, express one and the same composition, but they should be distinguished like ordinary oxygen and ozone, although in this case their mutual conversion is more easily effected and takes place on vaporisation; also, O3 loses heat in passing into O2, whilst N2O4 absorbs heat in forming NO2.
Nitric acid in acting on tin and on many organic substances (for example, starch) gives brown vapours, consisting of a mixture of N2O3 and NO2.
A purer product is obtained by the decomposition of lead nitrate by heat, Pb(NO3)2 = 2NO2 + O + PbO, when non-volatile lead oxide, oxygen gas, and nitrogen peroxide are formed. The latter condenses, in a well-cooled vessel, to a brown liquid, which boils at about 22°.
The purest peroxide of nitrogen, solidifying at -9°, is obtained by mixing dry oxygen 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,51 and at 0° 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 Troost 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 NO 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 135° the existence of nitrogen dioxide only must be recognised.