204 Specific Gravity of Soluble Salts by 



When the first weighing has been completed, about 20 or 

 25 c.c. of the clear mother-liquor are drawn off and the second 

 charge of dry salt is added and mixed, after which the level is 

 adjusted, and the weight determined. In the absence of 

 experience it might be thought that it would be difficult to 

 draw off so much of the liquid without some of the solid salt ; 

 but no matter how much they may be stirred up, these crys- 

 tallised salts settle at once and completely to the bottom when 

 immersed in their saturated solutions, and the operation 

 presents no difficulty. It was at first intended to make a 

 series of three determinations with each salt, but two were 

 found to be sufficient. During all these manipulations the 

 temperature of the air in the laboratory never differed from 

 that of crystallisation (T = 23-1 C.) by more than one or 

 two tenths of a degree, and it is only in such conditions that 

 operations of this kind can be carried out successfully. 



Before bringing the crystals together with the mother- 

 liquor in the specific gravity bottle, the operator must realise 

 that their common temperature when mixed is to be as nearly 

 as possible exactly that of crystallisation or equilibrium (T) ; 

 and he must take such measures as his experience dictates to 

 arrive at this end. Preliminary experiments on a somewhat 

 extensive scale are absolutely necessary, and the success of an 

 operation depends almost entirely on the competence of the 

 operator and on the trouble that he is prepared to take. 



Table II gives for each salt, MR, the temperature, T, of 

 equilibrium between crystals and mother-liquor, and, in con- 

 densed form, the experimental data of the determination of S, the 

 specific gravity at T of the mother-liquor, that of water at the 

 same temperature being unity ; of m, the concentration of the 

 mother-liquor in gram-molecules salt per 1000 grammes water, 

 and of D lf D 2 , D 3 , the three observed values, as well as D, the 

 finally accepted value, of the specific gravity of the salt, all at 

 T and referred to that of water at the same temperature as 

 unity. 



The letters and suffixes have the same significance as in 

 Table I. 



The figures in line T show how uniform the temperature 



