Page:Proceedings of the Royal Society of London Vol 69.djvu/104

96 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.

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


 * ’Chem. Soc. Journ.,' 1861 (i), p. 147.