J. Y. Buchanan — Specific Gravity of Soluble Salts. 25 



Art. II. — On a Method of Determining the' Specific Gravity 

 of Soluble Salts by Displacement in their own Mother- 

 liquor ' and its Application in the case of the Alkaline 

 Halides /* by J. Y. Buchanan. 



During the summer of 1904 I was occupied with the 

 determination of the specific gravity of various saline solutions 

 by the hydrometric method, which I designed for use on board 

 the " Challenger " and have perfected in the course of years, 

 since that date. The most important condition of success with 

 this method is to operate always at the same temperature, and 

 during an operation to keep that temperature perfectly con- 

 stant. The temperature which I used was 19*5°, both because 

 a great quantity of similar work has been done at this tem- 

 perature and because, in the room which it was my privilege 

 to occupy in the Davy-Faraday Laboratory, this temperature is 

 one which is very easily produced and kept constant, so long 

 as the temperature of the air outside does not exceed it. This 

 work was put a stop to by the arrival of the great anticyclone 

 of the summer of 1904 which persisted over northern Europe 

 for nearly six weeks and produced tropical conditions, which 

 were evidenced alike by the high temperature of the air and 

 by its insignificant diurnal variation. 



In these circumstances I decided to make use of the time 

 by putting into practice a method of determining the specific 

 gravity of soluble salts which I have long intended to try. 



The specific gravity of an insoluble substance is determined 

 by the amount of distilled water which a known weight of it 

 displaces. In the case of soluble salts it has been the custom 

 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 agree- 

 ment among the values of the specific gravity of the same salt 

 found by different chemists, there is greater uncertainty 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 salt is quite 

 insoluble, 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 



*Read at the meeting of the Chemical Society of London on 6th April, 

 1905. 



