Aqueous Solutions of Hydrogen Chloride, &c. 



115 



he gives his results as the differences between the indices of his solu- 

 tions and that of water ; his Table I* shows that the values for the 

 potassium chloride solutions were always lower than those for the cor- 

 responding sodium chloride solutions. 



The refractive index of the solution of a salt clearly depends on the 

 influence both of the solvent and of the salt in solution. In order that 

 the values obtained with different salts should be comparable, it is 

 necessary that the solutions should be similar, i.e. t that in unit volume 

 there should be the same weight of solvent and of salt, or the same 

 weight of solvent and weights of salt bearing the same relation to the 

 molecular weights of the salts. 



Such is seldom, if ever, the case with aqueous solutions at least. A 

 litre of a normal solution contains, together with the gram-molecular 

 weight of the salt, a weight of water which is different for different 

 salts. 



Hence the refractive indices of the solutions of any two salts cannot 

 be taken as a measure of the influence which the several salts exert on 

 the velocity of light, unless it has been shown that the unit volumes of 

 the solutions contain equal weights of water, together with either 

 equal weights of the salts, or weights which bear the same ratio to the 

 molecular weights. 



Table III gives the weight in grams of the water contained in 1000 c.c, 

 of the solutions whose refractive indices and densities are given in 

 Table II, obtained by subtracting from the weight of 1000 c.c. of the 

 solution the weight of salt contained in that volume. 



Table III. 



The table shows that in equal volumes of the solutions of the chlo- 

 rides of hydrogen, lithium, and sodium nearly equal weights of water 

 are present, but that such is not the case with the solutions of the two 

 other chlorides. 



In a litre of a normal solution of potassium chloride there are about 

 10 grams, and in a semi-normal solution about 5 grams less water than 



* Loo cit., p. 333. 



