CHLOBIDE BEBTHOLLET^ LAWS 445 



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 Jtydrtite of the composition 

 HC1,6H 2 0. The conclusions to be drawn from the densities of solu- 

 tions of hydrochloric acid also point to the same fact, as will presently 

 be explained. Besides this hydrate there exists also a crystallo-hyclrate 

 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. :w 



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

 l)()oin i is analogous to XaCl,2H.,O. The crystals HC1,2H.,O at -22 have a specific gravity 

 T4C), the vapour tension (dissociation) of the solution having a composition HC1,2H 2 O 

 at -24 =7(50, at -19 =1010, at -18 = 1057, at -17 = 1112 mm. of mercury. In a solid 

 -tatt- tin- crystallo-hydrate at 17'7 has the same tension, whilst at lower temperatures 

 it is much less : at 24 about 150, at 19 about 580 mm., at 17*2 about 10 atmo 

 >pheres; at 13 about 150 atmospheres. A mixture of fuming hydrochloric acid with 

 snow reduces the temperature to 38. 



~ 8 According to Roscoe at a hundred grams of water at a pressure^ (in millimetres 

 of mercury) dissolves 



p = 100 200 300 500 700 1000 



Grams HC1 65'7 70'7 73'8 7*-2 Hl'7 s.Vf, 



At a pressure of 7<>0 millimetres and temperature t, a hundred grams of water dissolves 



t = 8 16 -24 3 40 60 



Grams HC1 82'5 78'3 74'2 70'0 (i3'3 5<M 



Roozeboom (1886) showed that at t solutions containing c grams of hydrogen chloride per 

 100 grams of water may (with the variation of the pressure p) be formed together with 

 the crystallo-hydrate HCl,2H.jO : 



t = -28-8 -21 -19 18 -l- 17'7 



C = 84-2 8(5-8 92'(5 !)8'4 101'4 



p = -334 580 900 1073 mm. 



The last composition answers to the melted crystallo-hydrate HC1,2H.O, which splits up 

 at temperatures above 17'7, and at a constant atmospheric pressure when there are 

 no crystals 



t = -24 21 -18 10 



c = .101-2 98-8 95-7 89'8 84'2 



From these data it is seen that the hydrate HC1,2H._,O can exist in a liquid state, which 

 is not the case for the hydrates of carbonic and sulphurous anhydrides, chlorine, &c. 



According to Marignac, the specific heat c of a solution HC1 + wH 2 O (at about 30, 

 taking the specific heat of water = 1) is given by the expression 



C(36'5 + w!8) = 18/rc - 28'89 + 140/w - 2<>8/ w 2 

 if in be not less than 6'25. For example, for HC1 + 25H->O C = 0'877. 



According to Thomsen's data, the amount of heat Q, expressed in thousands of calories, 

 evolved in the solution of 3(5'5 grams of gaseous hydrochloric acid in ?H 2 O or 18m 

 grains of water 



m =2 4 10 50 400 



Q = 11-4 14-3 16-2 17-1 17'3 



In these quantities the latent heat of liquefaction is included, which, judging by 

 analogy (page 821), must be taken as 5-9 thousand calories per molecular quantity of 

 hydrogen chloride. 



The researches of Scheffer (1888) on the rate of diffusion (in water) of solutions of 



