NITROGEN PENTOXIDE. 181 



We have, therefore, neglecting the differences between the 

 specific heats of the body under its various states 



N a -f 5 + H 2 solid = 2HN0 8 solid (at low temperature) ... + 7-0 



=2HN0 8 ... F ...... + 42-2 



gas =2HN0 8 gas ......... - 0-1 



= 2HNO, ...... +34-4 



8. Add at different degrees of concentration. To pass to nitric 

 acid regarded under different states of concentration, it is 

 sufficient to know its heat of dilution, under its various states, 

 and to add it to the foregoing figures. It has been measured 

 for the whole scale of dilutions, and the results will be found 

 in the "Annales de Chimie et de Physique," 5 e se*rie, torn. iv. 

 p. 446. We confine ourselves to giving here the heat of 

 formation of the hydrate HN0 3 , 2H 2 0, which represents 

 approximately the acid of commerce. 



HN0 3 + 2H 2 = HN0 3 '2H 2 liquid, liberates + 5-0 Cal. 

 Engravers' aqua fortis approaches the formula 



HN0 3 + 6-5H 2 0. 

 The formation of such a hydrate, 



HN0 8 + 6'5H 2 = HN0 3 6'5H 2 0, liberates + 7'0. 



9. Nitrogen pentoxide, N 2 5 . Preparation. It is well known 

 that this body was discovered by Sainte-Claire Deville in the 

 reaction of chlorine on silver nitrate. A more simple method 

 was devised by the author. The principle of the process 

 was discovered by Weber in 1872, 1 and consists in dehydrating 

 nitric acid by phosphoric anhydride. Nitric acid, cooled by a 

 freezing mixture, is mixed with pulverulent phosphoric oxide in 

 small portions at a time, taking care to avoid any elevation of 

 temperature. The temperature of the mass should never 

 exceed 0. When a little more than its weight of phosphoric 

 oxide has been added to the nitric acid, the mass becomes of 

 the consistency of a jelly. It is then placed in a roomy 

 tubulated retort and distilled with extreme slowness. The 

 products are condensed in receivers with ground stoppers, 

 immersed in ice. In this way large, brilliant, colourless crystals 

 of nitrogen pentoxide are obtained, which are perfectly pure. 

 From 150 grms. of nitric acid nearly 80 grms. of the crystals 

 were obtained. This substance is non-explosive either as a 

 solid or in vapour. It decomposes, however, very easily, and 

 this at the ordinary temperature, as Deville has observed, into 

 nitric peroxide and oxygen. It should not be preserved in 

 hermetically sealed vessels. It keeps well in good glass- 

 stoppered bottles placed under a bell glass with sulphuric acid. 

 In the air the crystals evaporate slowly, evolving abundant 

 vapours, but not liquefying. Hence they can be weighed 



1 " Ann. Pogg.," torn, cxlvii. p. 113. 



