OF NITI;O<;KN WITH HYI>RO<;FA AND OXYMK.V 265 



it. The first portion of the nitric acid thus distilled boils at 86, has a 

 specific gravity at 15 of 1*526, and solidifies at -50 ; it is very 

 unstable at higher temperatures. This is the normal hydrate, HNO 3 , 

 which corresponds with the salts, NMO 3 , of nitric acid. When diluted 

 with water nitric acid presents a higher boiling point, not only as 

 compared with that of the nitric acid itself, but also with that of water ; 

 so that, if very dilute nitric acid be distilled, the first portions passing 

 over will consist of almost pure water, until the boiling point in the 

 vapours reaches 121. At this temperature a compound of nitric acid 

 with water, containing about 70 p.c. of nitric acid, 31 distils over ; its 

 specific gravity at 15 = 1-521. If the solution contain less than 

 25 p.c. of water, then, the specific gravity of the solution being above 

 1*44, HNO 3 evaporates off and fumes in the air, forming the above 

 hydrate, whose vapour tension is less than that of water. Such solu- 

 tions form fuming nitric acid. On distilling it gives monohydrated 

 acid, 32 HNO 3 ; it is a hydrate boiling at 121, so that it is obtained 

 from both weak and strong solutions. Fuming nitric acid, under the 

 action not only of organic substances, but even of heat, loses a portion 



31 Dalton, Smith, Bineau, and others considered that the hydrate of constant boiling 

 point (see Chapter I. Note 60) for nitric acid was the compound 2HNO 3 ,3H. 2 O, but Eoscoe 

 showed that its composition changes with a variation of the pressure and temperature 

 under which the distillation proceeds. Thus, at a pressure of 1 atmosphere the solution 

 of constant boiling point contains 68'6 p.c., and at one-tenth atmosphere 66'8 p.c. 

 Judging from what has been said concerning solutions of hydrochloric acid, and from the 

 variation of specific gravity, I think that the comparatively large decrease of the 

 tensions of the vapours depends on the formation of a hydrate, NHO 5 ,2H 2 O ( = 63'6 p.c.). 

 Such a hydrate may be expressed by N(HO) 5 , that is as NH 4 (HO) in which all the 

 equivalents of hydrogen are replaced by hydroxyl. The constant boiling point will 

 then be the temperature of the decomposition of this hydrate. 



Besides which, judging by the variation of the specific gravity (see my work cited in 

 Chapter I. Note 29), at least one more hydrate, NHO S ,5H 2 O (41'2 p.c. HNO 3 ), must be 

 acknowledged. Starting from water (p = 0) to this hydrate, the specific gravities of the 

 solutions at 15 is well expressed by s = 9992 + 57'4p + 0. 16p*, if water = 10000 at 4. 

 For example, when p = 30 p.c., s = 11860. For more concentrated solutions, at least, the 

 above-mentioned hydrate, HNO 3 ,2H.>O, must be taken, up to which the specific gravity 

 s = 9570 + 84-18p 0'240p*; but perhaps (the results of observations of the specific gravity 

 of the solutions are not in sufficient agreement to make a decision) the hydrate 

 HNO 5 ,8H.jO should be recognised, as is indicated by many nitrates (Al, Mg, Co., &c.), 

 which crystallise with this amount of water of crystallisation. From HNOs^H^O to 

 HNO 3 the specific gravity of the solutions (at 15) s = 10652 + W08p-0-I60p*. The 

 ]>fiitahydrated hydrate is recognised by Berthelot on the basis of the thermo-chemical 

 data for solutions of nitric acid, because on approaching to this composition there is a 

 rapid change in the amount of heat evolved by mixing nitric acid with water. This 

 hydrate solidifies at about - 19. One would think that a more detailed study of the 

 reactions of hydrated nitric acid would show the existence of change in the process and 

 rapidity of reaction in approaching these hydrates. 



52 The normal hydrate HNO 3 , corresponding with the ordinary salts, may be termed 

 the monohydrated acid, because the anhydride N.,O 5 with water forms this normal nitric- 

 acid. In thi> M-HSI; the hydrate HNO 3 ,2HoO is the pentahydrated acid. 



