452 PRINCIPLES OF CHEMISTEY 



chloric acid gas is expelled with only a slight admixture of aqueous 

 vapour. But it is impossible to entirely separate the whole of the 

 hydrochloric acid from the water by this means, as could be done in 

 the case of an ammoniacal solution. The temperature required for the 

 evolution of the gas- rises and reaches 110-111 , and after this remains 

 constant that is, a solution having a constant boiling point is obtained 

 (as with HNO 3 ), which, however, does not (Roscoe and Dittmar) 

 present a constant composition under different pressures, because the 

 hydrate is decomposed in distillation, as is seen from the determinations 

 of its vapour density (Bineau). 'Judging from the facts (1) that with 

 decrease of the pressure under which the distillation proceeds the 

 solution of constant boiling point approaches to a composition of 25 p.c. 

 of hydrochloric acid, 35 (2) that by passing a stream of dry air through 

 a solution of hydrochloric acid there is obtained in the residue a solution 

 which -also approaches to 25 p.c. of acid, and more nearly as the tempera- 

 ture falls, 36 (3) that many of the properties of solutions of hydrochloric 

 acid vary distinctly according as they contain more or less than 25 p.c. 

 of hydrochloric acid (for instance, antimonious sulphide gives hydro- 

 gen sulphide with a stronger acid, but is not acted on by a weaker 

 solution, also a stronger solution fumes in the air, &c.), and (4) that the 

 composition HC1,6H 2 corresponds with 25'26 p.c. HC1 judging from 

 all these data, and also from the loss of tension which occurs in the 

 combination of hydrochloric acid with water, it may be said that they 

 form a definite hydrate of the composition HC1,6H 2 O. Besides this 

 hydrate there exists also a crystallo-hydrate, HC1,2H 2 O, 37 which is 

 formed by the absorption of hydrochloric acid by a saturated solution 

 at a temperature Of - 23. -It crystallises and melts at - 18. 38 



The mean specific gravities at 15, taking water at its maximum 



34 According to Roscoe and Dittmar at a pressure of three atmospheres the solution 

 of constant boiling point contains 18 p.c. of hydrogen chloride, and at a pressure of one- 

 tenth atmosphere 23 p.c. The percentage is intermediate at medium pressures. 



56 At 25 p.c., at 100 20'7 p.c. ; Roscoe and Dittmar. 



37 This crystallo-hydrate (obtained by Pierre and Puchot, and investigated by Rooze- 

 boom) is analogous to NaCl,2H 2 O. The crystals HC1,2H 2 O afr- -22 have a specific 

 gravity 1*46 ; the vapour tension (under dissociation) of the solution having a composition 

 HC1,2H 2 O at -24 = 760, at -19 = 1,010, at -18 = 1,057, at -17 = 1,112 mm. of mer- 

 cury. In a, solid state the crystallo-hydrato at I? 1 ? has the same tension, whilst at 

 lower temperatures it is much less: at 24 about 150, at 19 about 580 mm. A 

 mixture of fuming hydrochloric acid with snow reduces the temperature to -88, II 

 another equivalent of water be added to the hydrate HC1,2H 2 O at 18, the temperature 

 of solidification falls to -25, and the hydrate HC1,8H 2 O is .formed (Pickering, 1898). 



38 According to Roscoe at one hundred grams of water a a pressure p (in millimetres 

 of. mercury) dissolves 



p = 100 200 800 500 700 1,000 



Crams HC1 65-7 70'7 78'8 78-2 81'7 65'ft 



