662 ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 
accompanying this action. By cautiously fusing the salt after each experiment, these 
phenomena may be repeated with the same solution a number of times. It is very pos- 
sible, however, that the tube may cease to retain its condition of chemical purity perfect ; 
in which case the ordinary phenomena consequent on reduction of temperature will take 
place. It is not easy to maintain a chemically clean surface. A short exposure to the 
air, or a mere touch, will often suffice to impart a greasy film, or motes or dust, which 
prevent or lessen adhesion, and render a so-called “ inactive ” solid “ active.” When 
a glass rod has been kept in water or passed through flame and dried, or cooled not in 
contact with the air, it is more or less chemically clean, and remains so while being 
sheltered. When Herr Ziz found that a knitting-needle was “ active ” in one solution, 
and that by passing it through the cork which confined another solution it became 
inactive, he simply made the wire chemically clean by the friction. 
Some liquids, such as spirits of wine, act as nuclei by combining with a portion of the 
water of the solution, and liberating a little salt which acts as a nucleus. 
My experiments lead me to the conclusion that air, in its pure state, is not a nucleus ; 
for not only may a supersaturated solution be briskly shaken up in a bottle half-full of 
air *, but air if previously passed through water may be made to bubble through a super- 
saturated solution without producing any separation of the salt. 
When air appears to act as a nucleus, I believe that it is simply performing the part 
of a carrier of some solid particle not chemically clean. If a boiling solution be poured 
into a bottle, which is then tightly corked, or the stopper inserted, the solution in cooling 
down will have a partial vacuum over it. If, now, the cork be loosened, air will rush in, 
and almost certainly drag in with it a solid nucleus. But if the flask be plugged with 
cotton-wool, motes &c. will be kept back while the air streams in as the solution cools, 
and the plug may be removed without always producing immediate crystallization. 
Thus a solution opened in a garden, on a fine clear still day, is less likely to crystallize 
than if opened in a room, where a step across the carpet raises a cloud of solid particles. 
So also a narrow-necked bottle is less likely to catch nuclei than a wide one. Lowel 
found that he could keep his solutions from crystallizing longest in tubes of narrow 
bore. Those in tubes of from ^ to f-inch bore, crystallized on being opened ; in those 
of less than half an inch the solutions remained longer in the liquid state, and they could 
be kept for weeks exposed to the air in tubes of j-inch bore. All this seems intelligible 
without recurring to any mysterious undiscovered force, such as M. Lowel so frequently 
refers to. 
By using chemically clean vessels and thermometers, we get rid of various anomalous 
cases of crystallization referred to by previous observers. And not only so ; but we 
reduce most of the phenomena of supersaturation to the case of nucleus or no nucleus. 
In chemically clean vessels there is no nucleus, and the modified salt, if formed at all, 
* In such a case it is a long time before the air becomes disengaged and the solution clear — an effect which 
belongs to many saline solutions a familiar example of which is afforded by the long white foaming trail left 
behind a steamer in sea- water. This effect is not produced in river-water. 
