Dr Ogden on Saline Crystallization. 317 



been in such. In one case the vessel was an open dish four inches 

 wide. 



But there is another phenomenon in the habitudes of sulphate 

 of soda with water, which, so far as I know, has not been no- 

 ticed by any writer. Under certain circumstances, a cold super- 

 saturated solution has the power to dissolve an additional quan- 

 tity of the crystallized salt. Not only does it dissolve it, but 

 the solution is greatly facilitated by agitation, unless, by some 

 capricious incident, the agitation should excite it to crystallize. 

 To illustrate this point, four ounces of sulphate of soda may be 

 dissolved in four ounces and a half of hot distilled water in a 

 glass-flask. The superfluous salt must be allowed to crystallize, 

 and the vessel containing both salt and mother-water is to be 

 placed in a dish containing sand, and exposed to a temperature 

 of 1 20° or 130° F. in a common kitchen-oven. When all the 

 salt, with the exception of about a drachm, is dissolved, the 

 flask is to be removed and carefully cooled. If this is success- 

 fully done, it is not accompanied by any deposition of crystals. 

 In this stage, the flask contains a cold supersaturated solution, 

 along with the portion of salt which remained undissolved by 

 the heat of the sand-bath *. It is now to be gently inclined to 

 one side, so as to elevate the undissolved crystals into the supe- 

 rior part of the liquid. After standing an hour or two in this 

 position, the most elevated part of the salt will have been dis- ' 

 solved ; and, the vessel being inclined in another direction^ an- 

 other part of the salt is in its turn raised to the superior part of 

 the liquid, and there dissolved. 



In repeating the experiment, I have generally shaken the 

 vessel briskly, and found the crystals to dissolve with much 

 greater rapidity in consequence ; though this very agitation has 

 sometimes induced crystallization before the solution had been 

 completed. 



Thus, a solution of sulphate of soda more than saturated, and 

 which has stood two, three, or four days in a cool room, actual- 

 ly continues to exert a solvent power on salt of its own kind. 



It was suggested to me, that the undissolved salt might not 

 be sulphate of soda, but some accidental impurity, soluble in 



• Furnishing another proof that the presence of crystals in a supersaturated 

 solution does not necessarily determine crystallization. 



