310 Dr Ogden on Saline Crystallization. 



In other cases, when the solution has been preserved from 

 atmospheric pressure, the experiment has nevertheless failed, 

 and the salt has crystallized. It requires some delicacy to con- 

 duct the experiment successfully, and mere protection from the 

 atmosphere is no security against its failing. And when it suc- 

 ceeds, and a supersaturated solution is thus obtained in a well- 

 closed flask or phial, I have found it as easily excited to crystal- 

 lize by agitation without removing the cork, as when it is con- 

 tained in an open vessel. 



Atmospheric pressure, then, I do not consider as a condition 

 at all facilitating crystallization in a supersaturated solution. 

 Nor have I been able to determine what conditions are necessa- 

 ry. CrystaUization often occurs without any obvious cause. 

 The slightest agitation will sometimes produce it ; even that oc- 

 casioned by stepping across the room has appeared sufficient. 

 Sometimes, again, the solution will bear to be briskly agitated 

 several times daily, for many successive days, without crystalli- 

 zing. A crystal of the same salt as that in solution will gene- 

 rally bring on crystallization, when placed in contact with the 

 liquid; frequently, however, it fails to do so. As a general 

 rule, the stronger the solution is, the more readily it is excited 

 to crystallize by either of these means. 



Sulphate of soda is the salt most frequently quoted ds pos- 

 sessing the property of resisting crystallization ; but it is by no 

 means confined to it, or to any genus of salts in particular, alka- 

 line, earthy, or metallic. Yet there are many salts of which I 

 have never been able to form supersaturated solutions ; if there 

 was the slightest excess over the quantity soluble in cold water, 

 it was invariably deposited on cooling, however carefully the 

 experiment was conducted, and whether or not the solution was 

 protected from atmospheric pressure. To this peculiarity I 

 shall subsequently revert. 



In stating that crystallization is quite independent of atmo- 

 spheric pressure, I do not advance it as a new discovery, but 

 rather as a point requiring additional investigation. The ear- 

 liest mention I find of it, is in Gay-Lussac's paper published in 

 1813 ; but he seems to have considered some protection neces- 

 sary, and employed for this purpose a stratum of oil of turpen- 

 tine. Gay-Lussac also formed supersaturated solutions with 



