Mr. C. Tomlinson on Supersaturated Saline Solutions. 2.25 



it is contaminated with minute portions of the salt itself, which M. 

 Gernez believes to exist in the air, not only of towns, but in the coun- 

 try. According to this view, the supersaturated solution of any other 

 salt can only be crystallized by a saline nucleus of its own kind. But, 

 as M. Jeannel* has pointed out, if this theory be true, we must have 

 floating in the air specimens of all kinds of salts that form supersatu- 

 rated solutions, and crystallize by the introduction of a solid nucleus ; 

 whereas there are some such salts which cannot exist in the presence 

 of the oxygen or of the ammonia of the air. M. Jeannel shows that 

 a few drops of an ordinary solution of a salt will induce crystalliza- 

 tion in a supersaturated solution of the same salt without contact 

 of air. 



3. On the Action of Nuclei. — With respect to the action of nuclei 

 on saline supersaturated solutions generally, the author refers to a 

 theory of hisf which seems to account for the liberation of gases 

 from their supersaturated solutions (soda-water, seltzer-water, cham- 

 pagne, &c) when a solid nucleus that had been exposed to the air is 

 immersed in them ; while such nucleus becomes inactive if kept long 

 in water, or passed through flame &c, and dried or cooled out of 

 contact with air. The action of nuclei is referred to adhesion. Nuclei 

 are active in inducing crystallization, or they are inactive, according to 

 the state of chemical purity of their surfaces. In the case of a su- 

 persaturated saline solution, the sides of the vessel may act as nuclei, 

 or any solid, and some liquid, bodies brought into contact with it. 

 Now suppose the inner surface of the vessel to be made chemically 

 clean, either by well washing it with strong sulphuric acid, or caustic 

 alkali, or spirits of wine, or, as often happens, by boiling the saline 

 solution in the vessel in which it is intended to be kept. In such 

 cases there is perfect adhesion between the sides and the solution, and 

 no salt will be liberated ; the sides may in fact be regarded as merely 

 a continuation of the liquid itself, and no salt can be formed there, any 

 more than in the central parts of the liquid. But suppose the sides 

 to be not chemically clean — to be more or less dirty, in fact ; in such 

 cases adhesion is diminished or destroyed, and the surface of the liquid 

 next to such sides is virtually as free as its upper surface. Salt will 

 be deposited there, other circumstances being favourable, really from 

 want of adhesion between the side and the liquid that holds the 

 salt in solution. Now apply this to the case of a so-called "ady- 

 namic," "non-catalytic," or "inactive" glass rod, or coin, or frag- 

 ment of glass or of flint, &c. A glass rod placed in the solution does 

 nothing more than form new sides, as it were, to the vessel ; and its 

 effect is merely that of the sides. If chemically clean, the rod will 

 form no crystals about it, and hence it is "inactive" because its 

 adhesion is perfect. If dirty, the surface of the solution in contact 

 with it will be as free, or almost so, as the upper surface. It requires 

 special means to produce a chemically clean surface ; and when pro- 

 duced, it is not easy to maintain it. A short exposure to the air, or 

 a mere touch, will suffice to cover it with an organic film, or with 



* Ann. cle Chim. et de Pliys. 4th ser. vol. vi.p. 166 ; and Comptes Rendas, Jan. 

 2,1866. 



t Phi!. Mag. July and August 1867. 

 Phil. Mag. S. 4. Vol. 36. No. 242. Sept. 18G8. Q 



