454 PRINCIPLES OF CHEMISTRY 



rise of specific gravity with an increase of percentage (or the differential 

 - J reaches a maximum at about 25 p.c. 39 The intermediate solution, 



HC1,6H 2 O, is further distinguished by the fact that the variation of 

 the specific gravity with the variation of temperature is a constant 

 quantity, so that the specific gravity of this solution is equal to 

 11,3527 (1 - 0-000447*), where 0-000447 is the coefficient of expansion 

 of the solution. 40 In the case of more dilute solutions, as with water, 



the specific gravity per 1 (or the differential -- J rises with a rise of tem- 

 perature. 41 



p= 5 10' 15 20 



So-S,5 = 7-2 23 38 52 64 



S 15 -S 3 o = 34-l 42 50 59 67 



Whilst for solutions which cdntain a greater proportion of hydrogen 

 chloride than HC1,6H 2 O, these coefficients decrease with a rise of 

 temperature ; for instance, for 30 p.c. of hydrogen chloride S 15 

 = 88 and S ir> 30 =87 (according to Marignac's data). In the cas~ 

 of HC1,6H 2 O these differences are constant, and equal 76. 



Thus the formation of two definite hydrates, HC1,2H 2 O and 

 HCl,6HjO, between hydrochloric acid and water may be accepted 

 upon the basis of many facts. But both of them, if they occur in a 

 liquid state, dissociate with great facility into hydrogen chloride and 

 water, and are completely decomposed when distilled. 



All solutions of hydrochloric acid present the properties of an 

 energetic acid. They not only transform blue vegetable colouring 

 matter into red, and disengage carbonic acid gas from carbonates, &c., 

 but they also entirely saturate bases, even such energetic ones as pot- 

 ash, lime, &c. In a dry state, however, hydrochloric acid does not alter 



59 If it be admitted that the maximum of the differential corresponds with HC1,6H 2 O, 

 then it might be thought that the specific gravity is expressed by a parabola of the third 

 order ; but such an admission does not give expressions in accordance with fact. This 

 is all more fully considered in my work mentioned in Chapter I., Note 19. 



40 As in water, the coefficient of expansion (or the quantity Jc in the expression 

 Qt=& -tcSot, or Vt = l/(l-M) attains a magnitude 0'000447 at about 48, it might be 

 thought that at 48 all solutions of hydrochloric acid would have the same coefficient 

 of expansion, but in reality this is not the case. At low and at the ordinary temperatures 

 the coefficient of expansion of aqueous solutions is greater than that of water, and 

 increases with the amount of substance dissolved. 



41 The figures cited above may serve for the direct determination of the variation of 

 the specific gravity of solutions of hydrochloric acid with the temperature. Thus, 

 knowing that at 15 the specific gravity of a 10 p.c. solution of hydrochloric acid = 10,492, 

 we find that at t 3 it = 10,580 - <(2'1S -f 0'027). Whence also may be found the coefficient 

 of expansion (Note 40). 



