by Displacement in their own Mother-liquor. 



27 



water, of the temperature of the air. The salt is then grad- 

 ually added and the mixture stirred with the thermometer. 

 In the case of every one of these salts the temperature falls 

 rapidly and by as much as from 15° to 20°. The salt is added 

 as rapidly as it is taken up by the water. "When the fall of 

 temperature slackens, a minimum is soon reached, while some 

 salt still remains undissolved at the bottom of the vessel. It 

 is then continually stirred ; the temperature rises slowly while 

 the salt gradually passes into solution, until, at a certain tem- 

 perature the amount of salt remaining undissolved is such that 

 a further rise of one degree of temperature will evidently 

 cause it to disappear. The vessel is now weighed and, as 

 result, we have the weight of salt dissolved in 25 grams of 

 water at about the last observed temperature. With a little 

 care it is easy to arrange that this temperature shall be in the 

 neighborhood of that of the air. The vessel with its contents 

 is now heated, and salt added by degrees, while the tempera- 

 ture rises and finally reaches the boiling point or whatever 

 other temperature may have been determined on. Salt is 

 added until the liquid is saturated at this temperature. The 

 vessel is again weighed and the salt dissolved at the higher 

 temperature is ascertained. These simple experiments, which 

 are completed in very few minutes, furnish all the information 

 that is required for the economical employment of the material. 

 In the absence of more detailed information the following- 

 results obtained in the above way are worth quoting: 



100 Grams op Water Dissolve 



Grams 



98 



164 



264 



225 



51 



157 



9.S 



121 



156 



222 



of 



RbBr 



Rbl 



Rbl 



CsCl 



Csl 



Csl 



CsBr 



CsBr 



CsBr 



CsBr 



at °C 



12 



20 



boiling 



25 



12 



107 7-5 



24-5 



50 



93-5 



"With this information there is no difficulty in preparing the 

 solution which shall, after allowing for unavoidable loss in 

 preparation, give the required amount of mother-liquor and of 

 crystals. The water is warmed and the pure salt is added 

 while the temperature is raised to that of ebullition, or to any 

 lower temperature that may have been selected. "When the 

 salt has all passed into solution, the liquid is poured into a flat 

 crystallizing dish and crystallization begins immediately. The 

 area of the dish should be such that the layer of solution shall 

 not be more than half a centimeter thick. The mother-liquor 

 is then everywhere in close touch with the crystals. The dish 

 is then put away in a cupboard for the night. 



In the morning, the temperature of the contents of the 

 crystallizing dish and that of the air were taken very carefully. 

 The mother-liquor was then poured off clear into a stoppered 



