314 Dr Ogden on Saline Crystallization. 



the salts in supersaturated solutions continue to exert chemical 

 elective attraction, without necessarily reverting to the state of 

 ordinary saturation. This might have been considered doubt- 

 ful, in consequence of the feeble affinity by which the salt is 

 held in solution. A drop of alcohol, by combining with the 

 water, separates a nucleus, and thus crystallizes the whole. 



It occurred to me that the peculiar constitution of super- 

 saturated solutions, on which depends their power of resisting 

 crystallization, might be completely changed by the admixture 

 of a portion of mother-water of the same salt, which had af- 

 forded crystals ; — as the fermentative process is established by 

 means of a ferment, in a liquid capable of undergoing fermen- 

 tation. 



9. With this view I again prepared the supersaturated solu- 

 tion of muriate of lime, and poured into it a drachm of mother- 

 water of the same salt. The vessel was briskly agitated, but no 

 crystallization succeeded. 



Hence, whatever may be the molecular state or constitution 

 of a supersaturated solution, it is quite compatible in mixture, 

 or in combination, with a solution simply saturated. 



In some supersaturated solutions crystallization proceeds slow- 

 ly ; in others, especially if concentrated, it proceeds so rapidly 

 when a nucleus is obtained, as to give the idea that the disposi- 

 tion to crystallize is communicated instantaneously, like an elec- 

 tric shock, to the whole mass. The following experiment, how- 

 ever, proves that this is not the case. 



10. An evaporating dish was prepared with a supersaturated 

 solution of acetate of soda, and left for twenty-four hours in a 

 state of perfect quiescence. A crystal was then carefully dropped 

 in at one side. When about one-fourth of the mass had crystal- 

 lized, a portion of the remaining liquid was poured into another 

 vessel, where it continued fluid until the next day ; it also was 

 then crystallized by the same means. 'nrwii, 



11. A vessel containing supersujphate of .'dumina and potash^ 

 which had already been the subject of experiment, was heated 

 in a sand-bath until all the salt was redissolved. On cooling, 

 about one-third of it crystallized ; the rest remained fluid until 

 it was shaken. 



