Dr Ogden on, Saline Crystallization, 81 S 



This experiment shews that the presence of crystals in a 

 supersaturated solution does not necessarily determine crystal, 

 lization. 



12. There does not appear any limitation to the time which 

 a supersaturated solution may retain its fluidity. Phosphate of 

 soda was kept in that state for the space of eleven days, then 

 shaken for several minutes, and poured into another vessel. It 

 continued perfectly fluid until a crystal was dropped into it. 



I have already stated, that with some salts I have never suc- 

 ceeded in forming supersaturated solutions, the excess being 

 always deposited on cooling. The mode of determining this 

 property may be varied in many ways ; that which I generally 

 adopted, and found most convenient, was the following. 



Any given quantity of a salt is dissolved in a quantity of 

 distilled water less than is requisite to form a saturated solution 

 when cold, but greater than is requisite to form a hot saturated 

 solution. The operation is performed in a glass-flask over a 

 charcoal-fire, and after being kept a few seconds at a boiling 

 heat, the solution is filtered through paper into another clean 

 flask, in which it is again brought to a boiling heat. The fire 

 is then covered with an iron-plate, by which it is slowly and 

 gradually extinguished. The vessel containing the solution is 

 suspended over the iron-plate, and within an inch of it ; every 

 thing is then left undisturbed for ten or twelve hours. When 

 cold, if the salt is found to have crystallized, it is owing to one 

 of two causes; either it is a failure of the experiment, the salt 

 being capable of forming a supersaturated solution ; or the salt 

 is one of those which are incapable of existing in that state. 

 To remove the doubt, the vessel and its contents must be again 

 heated and cooled ; this process must be frequently repeated ; 

 and if there is uniformly a deposition of the suf)erabundant salt, 

 it may be concluded that it is incapable of forming a supersatu- 

 rated solution. 



The first of the following tables exhibits those salts with which 

 I have formed supersaturated solutions; the second, those which 

 appear incapable of that state : — 



