212 



On Supersaturated Saline Solutions. [Mar. 13, 



answer my present purpose — namely, to show that some supersaturated 

 saline solutions really do crystallize under the action of other nuclei than 

 a salt of the same kind as that of the solution operated on. 



My experiments were performed in the open air, at temperatures be- 

 tween 30° and 50° F. At comparatively low temperatures the shaking of 

 the flask containing the solution sometimes produced a copious liberation 

 of anhydrous salt, which rapidly combines with water and forms the 

 7-atoin hydrate*; but on allowing the flask to rest for some time, 

 it generally happens that the solution becomes solid, and the 7-atom salt 

 opaque white. 



I may perhaps be allowed to state that my experiments, conducted as 

 they were in the open air, were delayed by the rainy weather of last year. 

 There is, however, this advantage in wet weather, that the saline particles 

 said to exist in the air are washed down and brought into solution, in 

 which condition they are not nuclear, as I have already shown in the case 

 of sodic sulphate, alum, and one or two other salts f. On the other hand, 

 fine weather has its advantages ; not only are the surfaces of the solu- 

 tions more active and the evaporative force stronger, but hydrated salts, 

 said to exist in the air, part with their water of crystallization so readily 

 as to reduce them to the non-nuclear condition. This is especially the 

 case with sodic sulphate. I have already shown that supersaturated 

 solutions of this salt may be exposed to the air, both in fine and wet 

 weather, for a long time without crystallizing + as well as the fact just 

 noticed §, that a solution of a salt does not act as a nucleus to its super- 

 saturated solution. Moreover, if sodic sulphate exist in the dusty air of 

 a room, it cannot retain the hydrated form during many minutes, but 

 must rapidly pass into the anhydrous, in which it is no longer a nucleus. 



* The view adopted by me that the supersaturated solution of sodic sulphate con- 

 tains the anhydrous salt in solution, and that this is first thrown down on lowering the 

 temperature, agitating, See., has been objected to, on the ground that, according to 

 Lowel, it is the 7-atom hydrate that is really in solution and is deposited on cooling. 

 There are numerous proofs that it is the anhydrous salt which is really in solution ; 

 these I have collected in two papers, contained in the ' Chemical News' of 3rd and 10th 

 December 1869. That the 7-atom hydrate is built up on the anhydrous salt may, I 

 think, be shown by an experiment. Two flasks containing a solidified solution of sodic 

 sulphate of the same strength are heated over a spirit-lamp ; one of the flasks is con- 

 stantly turned round on the ring of the retort-stand until the whole of the salt has 

 entered into solution ; it is then boiled, closed, and set aside. The other flask, during 

 the heating, is allowed to remain at rest oyer the flame for a short time, so as to 

 liberate a portion of the anhydrous salt. This flask is also boiled, although the ope- 

 ration is interrupted by violent bumpings. It is closed and placed by the side of 

 the other flask, under the same conditions. When both flasks are cooled down to the 

 temperature of the air (say about 50°), the flask containing the anhydrous salt will con- 

 tain a crop of the 7-atom hydrate, built upon the anhydrous deposit. The other Cask 

 will have no crystalline deposit at all. 



t Chemical News, February 4, 1870, p. 52. 

 . % Proceedings of the Eoyal Society, 1871, p. 41. 

 § Chemical News, February 4, 1870, p. 52. 



