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Conditions necessary and precautions required in 

 making the experiment . . . . . .199 



The salts used were those of the Ennead of the Alka- 

 line Halides, namely, the Chlorides, Bromides and 

 Iodides of Potassium, Rubidium and Caesium respec- 

 tively. 



Solubility of these salts 200 



Preparation of the crystallizing mixture. 



Description of Table I in which the experimental 

 details are given in the case of one salt, namely, 

 Caesium Chloride . . . . . . .201 



Table I 202 



Precautions to be observed when bringing the 

 crystals together with the mother-liquor in the pyk- 

 nometer 203 



Given T, the temperature of equilibrium, the per- 

 missible latitude of deviation from it is specified as being 

 from T- 0-01 to T+ 0-1. 



The reason why it is not permissible for the tempera- 

 ture of the mixture, during adjustment, to fall ever so 

 little below the temperature of equilibrium, while a rise 

 of 0-1 above it can be tolerated is the following. When 

 the salt is immersed in its mother-liquor at the tem- 

 perature of the system when crystallisation was com- 

 plete, T, the equilibrium between crystals and solution 

 is stable. If the temperature falls by, say, 0-1 below 

 that of equilibrium the solution, in the absence of 

 crystals, would remain unaltered; but, in presence of 

 crystals in abundance, the state of supersaturation is 

 impossible and equilibrium at the temperature T - 0-1 

 is quickly established. If the temperature of the mix- 

 ture be now raised to T the solution, though not quite 

 saturated, is very nearly so, and the crystals of the salt 

 are so nearly insoluble in it that the re-establishment of 

 equilibrium at T could not be assumed to have taken 

 place except after an exceedingly long interval of time. 



A distinction must be made between the insolubility 

 of a salt in, for instance, a hydro-carbon which, when 

 saturated, contains only a fraction of one per cent, of the 

 salt, and that of the same salt in a solution of itself 

 which is so nearly saturated as to be able to take up no 

 more than the same percentage of the salt which would 

 completely saturate the same volume of hydro-carbon. 



