2OO Specific Gravity of Soluble Salts by 



hood of that of the air. The vessel with its contents is now 

 heated, and salt added by degrees, while the temperature rises 

 and finally reaches the boiling point or whatever other tem- 

 perature 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 GRAMMES OF WATER DISSOLVE 



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 

 crystallising dish and crystallisation begins immediately. The 

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

 not be more than half a centimetre 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 

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

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

 bottle, while the crystals were collected, allowed to drain, 

 and dried in the ordinary way. The temperature which the 

 mixture had when separated is noted as that at which the 

 crystals and the mother-liquor were in equilibrium; and it is 

 exactly at this temperature that they have to be brought 

 together again in order to determine the specific gravity of the 



