406 Mr. C. Tomlinson on Supersaturated Saline Solutions. [May 28, 



or tliey are inactive, according to the state of chemical purity of their sur- 

 faces. In the case of a supersaturated sahne 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 cen- 

 tral parts of the hquid. 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 "adynamic," " non- catalytic," or inactive" glass rod, or coin, or 

 fragment 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 per- 

 fect. 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 produced, 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 motes or dust that prevent or lessen adhesion between 

 it and the aqueous part of the solution, and apparently render an inactive 

 solid active. When a glass rod &c. has been kept in water or passed 

 through flame and dried, or cooled out of contact with the air, it is more or 

 less chemically clean, and remains so while being sheltered. When Ziz found 

 a knitting-needle active on one solution, and by passing it through the cork 

 which confined a similar solution it became inactive, he simply made the 

 wire chemically clean by the friction. Air is not a nucleus, and when it 

 appears to act as such, it is simply as a carrier of some solid particle not 

 chemically clean. Hence narrow-necked flasks when opened retain their 

 solutions liquid longer than wide-necked ones, as the former are less likely 

 to catch motes &c. from the air than the latter. Supersaturated solutions 

 are best preserved by plugging the necks of the flasks &c. with cotton-wool, 

 since in cooling down the air is filtered in passing through the plug, and 

 motes and dust are thus kept back. 



Tubes made chemically clean by the action of strong sulphuric acid may 

 be filled with a strong solution of sodic sulphate, and when cold the tubes 

 may be placed in a freezing-mixture at 10° F. without any separation of 

 the salt. Hence the author difl'ers from M. Lowel's theor}-, which supposes 

 a molecular change to take place when strong solutions of the salt are 



