202 MR J. Y. BUCHANAN ON THE 



Section XVI. — The Determination of the Specific G-ravity of the Crystals of 

 A Soluble Salt by Displacement in its own Mother-Liquor, and the 

 Volumetric Relations between the Crystals and the Mother-Liquor 



WHICH ARE established BY THE EXPERIMENT.* 



§ 120. The work on the specific gravity of dilute solutions at 19 "5° C. reported in the 

 early part of this memoir was interrupted by the arrival of the great anticyclone or heat- 

 wave of the summer of 1904, during which observations at a temperature of 19"5° were 

 quite impossible. Indeed, the temperature of the laboratory, whether by night or day, 

 hardly ever fell below 23° C. or rose above 25° C. It 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 had long 

 intended to try. I took it up at first merely as a tour de force in experimentation 

 with which to occupy myself during the hot weather, but it turned out to be a valuable 

 method of research, and the duration of the spell of hot weather enabled me to prove 

 and to use it. 



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 agreement 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 may call 

 its birth, and by making the maintenance of this temperature a conditio sine qud 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 determined its displacement ; and this, combined with the weight of the salt and the 

 specific gravity of the mother-liquor, gave the specific gravity of the salt. 



* This formed the subject of a paper which was read at tlie meeting of the Chemical Society of London on 6th 

 April 1905, but it was not published by the Society. I owe it to the courtesy of Professor E. S. Dana that the 

 liospitality of the pages of the American Journal of Hcience was extended to it. It appeared in the January 

 number of 1906, vol. xxi. jx 25, under the title: — "Ou a Method of Determining the SiJecific 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." 



