206 MR J. Y. BUCHANAN ON THE 



specific gravity bottle. The wide end was closed with a glass stopper, and the narrow 

 end with a small india-rubber cork. 



It was the custom to work so as to have about 15 c.c. of dry 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 deter- 

 mined 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 487449 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. 



§ 124. Owing to the readiness with which these salts crystallise 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 001", nor should it be 

 allowed, even momentarily, to rise above it by more than O"!". 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 millimetre 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 c.c. of the clear mother- 

 liquor are drawn off and the second charge of dry salt is added and mixed, after winch 

 the level is adjusted, and the weight determined. In the absence of experience it might 

 be thought that it would be difficult to draw oft' so much of the liquid without some of 

 the solid salt ; but no matter how much they may be stirred up, these crystallised 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'l°) by more than one or two tenths of a degree, and, 

 when the solubility of the salt is great, 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 ivhen mixed is to be 



