198 Specific Gravity of Soluble Salts by 



to replace the water by a hydrocarbon or mineral oil. The 

 objections to the use of this liquid are numerous, especially 

 when the salt, the specific gravity of which it is desired to 

 determine, is rare or costly. Moreover, to judge by the want 

 of agreement among the values of the specific gravity of the 

 same salt found by different chemists, there is greater uncer- 

 tainty about the numerical results than there should be. One 

 reason for this may be that the salts are not insoluble, but only 

 sparingly soluble in the oil, and that sufficient attention has 

 not been given to this point. 



There is one liquid in which every soluble ^alt is quite in- 

 soluble, and that is its own mother-liquor at the temperature 

 at which the one parted from the other. By immersing the 

 salt in its own mother-liquor at the temperature of what we 

 may call its birth, and by making the maintenance of this 

 temperature a conditio sine qua non of every manipulation 

 during which the two are brought together again, errors due 

 to uncertain solubility are eliminated, and contamination of 

 valuable preparations is avoided. It is therefore by the 

 immersion of each salt in its own mother-liquor that I determine 

 its displacement; and this, combined with the weight of the 

 salt and the specific gravity of the mother-liquor, gives the 

 specific gravity of the salt. 



It is obvious that the method is applicable only to salts 

 which have a mother-liquor, such as KG; RbBr; CaCl 2 ,- 

 6H 2 O; BaCl 2 2H 2 O; it is inapplicable to salts such as CaCl 2 ; 

 BaCl 2 ; and the like, which have no legitimate mother-liquor: 



The anticyclonic meteorological conditions which prevailed 

 during the greater part of July and August 1904 were very 

 favourable to this class of work. The anticyclone began to 

 give way when the work was nearly finished, and it was evident 

 that, in the absence of artificial arrangements for the preser- 

 vation of a constant laboratory temperature, this class of work 

 cannot be carried on easily or satisfactorily except in the 

 hottest summer weather. 



It is an essential condition of success that the work be 

 carried on in a room, for the time being, especially devoted to 

 the purpose, and occupied by one investigator. He must have 



