CIILHKIDE-BERTHOLLET'S LAWS 447 



water, the sporitir gravity at 1 (or the differential ' j rises with a 



rise of temperature, for the difference between 15 and *S' ir ,~.S' j(1 

 (i.e. the difference between the specific gravities at 0, 15, and 30) 

 varies as follows : " 



]>= 5 10 15 20 



,S' -,S',- = 7-1 23 38 52 64 



S lt ,-S 80 =33-9 42 50 59 67 



Whilst for solutions which contain 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 15 =88 

 and 1.5 3 o=87 (according to Marigiiac's data). In the case of 

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



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

 HC1,6H 2 O, between hydrochloric acid and water may be accepted 

 upon the basis of many facts. But both of them, inasmuch as 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- 



uniform, then the modulus of expansion must be taken as ds/dtS ] attains a magnitude 

 ii (1(10147 at about 48, it might be thought that at 48 all solutions of hydrochloric acid 

 would have the same modulus of expansion, but in reality this is not the case. At low 

 and the ordinary temperatures the modulus of expansion of aqueous solutions is greater 

 than that of water, and the greater it is the greater the amount of the substance dissolved 

 (for this reason the temperature of maximum density, when the modulus = 0, is lowered 

 by solution). But for water the modulus of expansion ( ds/dtS,,} rises rapidly with the 

 temperature, whilst for solutions the increase is much slower (or even falls, as for 

 sulphuric acid or fuming hydrochloric acid), and therefore at a certain temperature t the 

 modulus of solutions becomes equal to that of water. This temperature may be termed 

 the ' characteristic temperature.' For solutions of sodium chloride it is about 58, for 

 lithium chloride 80, for potassium nitrate about 80, for weak solutions of sulphuric 

 acid about 68. In order to illustrate this by an example I will insert the magni- 

 tudes of denomination of expansion (multiplied by 10000) of solutions of sodium 

 chloride : 



20 50 60 80 100 



Water . . . -0(55 2'07 8'64 5'11 6'25 7'<>'.) 

 10 p.c. NaCl . 2'8 8-4 4'8 5'0 5'7 6'8 



20 p.c. . . . 8-(5 4-0 4'5 5'0 5'4 5'8 



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, because we 

 may take S, = S,> - t (A-Bt). Thus, knowing that at 15 the specific gravity of a 

 in p.c. solution of hydrochloric acid = 10492, we find that at f it = 10580 - 

 Whence also may be found the modulus of expansion (Note 40). 



