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



salt to be added in two charges to the specific gravity bottle. 

 These charges were intended to be nearly, though not quite, 

 equal. The available supply was distributed between two 

 weighing tubes by approximate weight, after which the exact 

 weight of each portion was determined in the usual way. The 

 two portions of caesium chloride weighed respectively 22*1229 

 and 26*6220 grams, so that in the first determination of specific 

 gravity 22*1229 grams and in the second 48*7449 grams were 

 concerned. It is not immaterial whether the first portion is 

 charged into the empty specific gravity bottle and the mother- 

 liquor poured over the dry powder, or is charged into the 

 bottle which is already about half full of mother-liquor. In 

 the former case the elimination of the entangled air is difficult 

 and takes time, during which it is not easy to prevent the 

 temperature getting out of hand. By the latter process very 

 little air is carried past the surface of the liquid and very little 

 stirring with the thermometer, which is required on other 

 grounds, suffices to eliminate it. 



Owing to the readiness with which these salts crystallize 

 and to the slowness with which all salts dissolve in an almost 

 saturated solution, the temperature of the mixture of salt and 

 mother-liquor, during the adjustment of level in the specific 

 gravity bottle, must on no account be permitted to fall below 

 T by even 0*01°, nor should it be allowed to rise above it by 

 more than 0*1°. The regulation of temperature was effected 

 entirely with a standard thermometer divided into tenths of a 

 degree, each tenth occupying a length of rather more than 

 one millimeter on the stem. The thermometer which forms 

 part of the specific gravity bottle is used chiefly as a stopper 

 of convenient form. So soon as the level of the liquid has 

 been adjusted in the bottle, it is weighed. The temperature 

 and pressure of the air are kept account of for the reduction 

 of all weights to the vacuum. 



When the first weighing has been completed, about 20 or 

 25 cc 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- 

 tallized salts settle at once and completely to the bottom when 

 immersed in their saturated solutions, and the operation pre- 

 sents 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 crystal- 

 lization (T = 23*l°) by more than one or two tenths of a degree, 



