96 Messrs. V. H. Veley and J. J. Maiiley. 



and some, if not all, of the critical points would be eliminated ; 

 unfortunately for the demonstration of such a probability, nitric 

 acid of 100 per cent, concentration or thereabouts cannot be heated 

 tinder ordinary conditions much above 25 C. without increasing 

 considerably the risk of decomposition, which would render such an 

 acid useless as a standard. 



VIII . Discussion. 



If the minor variation at 4 per cent, be dismissed as inconsequent, 

 it will be observed that the principal critical points are, in all three 

 cases of temperature examined, in the vicinity of 20, 33, 42, 54, 63, 

 70, and 78 per cent. ; the variation at 94 per cent, will be discussed 

 separately. The first of the series has already been observed by 

 Pickering in plotting out Berthelot's results for the heat of dissolution, 

 and those of Kolb for the contractions (Joe. dt.\ and would correspond 

 approximately to a hydrate, HN0314ILO (20'31 per cent.), the 

 second to the composition of a hydrate, HN0 3 .7H-20 (33 - 33 per cent.), 

 the third to that of the hydrate HN0 3 .4HoO (4M8 per cent.), the 

 fourth to that of the hydrate HN0 3 .3HoO (53'93 per cent.), the fifth 

 to that of the hydrate HN0 3 .2H.,0 (63'63 per cent.), the sixth to that 

 of the hydrate 2HNOs.3rLO (70 per cent.), and the seventh to that of 

 the hydrate HN0 3 .H.,0 (77"77 per cent.). Of these the second, third, 

 fourth, fifth, and seventh have frequently been alluded to in recent 

 literature (vide summary in previous paper, lor,. dt.\ so that it will be 

 unnecessary to recapitulate the evidence. As regards the sixth point 

 at 70 per cent., there appears to have been a considerable diversity of 

 opinion. Since the acid obtained by the distillation, whether of more 

 concentrated or more dilute acid, has an approximate composition of 

 68 per cent., older writers regarded the acid as consisting of the 

 hydrate 2HNO. r 3H.,0 deprived of a small proportion of acid by 

 volatilisation. But as Koscoe* showed that the composition of the 

 distillate varied with the conditions of pressure, the former view was 

 abandoned, and it was held that the constancy of the distillate was 

 merely dependent upon the particular conditions of atmospheric 

 pressure under which the greater proportion of* mankind lives. 

 More recent writers (Mendeleeff, Erdmann) have been impressed 

 with the theoretical significance of the hydrate HNO ;J .2HoO as the 

 ultimate possible combination of nitrogen with hydroxyl, namely, 

 N(OH) 5 , and therefore regard the acid of constant composition men- 

 tioned above as consisting of the hydrate deprived of a small pro- 

 portion of water by volatilisation. These writers adduce as evidence 

 that if dry air at 15 C. be passed, whether through more concentrated 

 or more dilute acid, the final product in either case contains 64 per 



* < Cliem. Soc. Journ.,' 1861 (i), p. 147. 



