660 ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 
these three temperatures the saturated solutions are about equally rich in salt. The 
7-atom hydrate and the anhydrous salt can only maintain their molecular constitution 
when in contact with the mother-liquor in closed vessels, in which they are sheltered 
from the air and from other bodies that act upon them as nuclei. No sooner are they 
exposed to the air than they become opaque and warm, and assume the molecular 
constitution of the 10-atom hydrate as well as its solubility. Hence the conclusion is 
that supersaturated solutions of the sodic sulphate are not really so, since they hold a 
salt of much greater solubility at common temperatures than the ordinary salt. It is 
also stated that the solutions of the sodic carbonate and of the magnesic sulphate form 
salts of a lower degree of hydration, and of greater solubility than the normal salts. 
Bowel’s general conclusion is that all cases of supersaturation are such in appearance 
only, and not in fact. As to the function of nuclei and the inner sides of the flasks in 
determining crystallization, he regards it as the effect of one of those mysterious contact 
actions known as catalytic, of which science has not yet been able to give a satisfactory 
explanation. Bodies that appear to be active in inducing crystallization are designated 
catalytic or dynamic ; while bodies that are apparently inactive are termed non-catalytic 
or adynamic. “ It appears certain,” he says, “ that but for the mysterious action 
which the air and other bodies exert on supersaturated solutions, we should obtain sul- 
phate of soda only in the modified state ; that is, crystallized with seven equivalents of 
water, and possessing at ordinary temperatures of the air a much greater solubility than 
that of the ordinary 10-atom salt.” 
Later inquirers have endeavoured to explain the action of nuclei. Gernez* tried 220 
solids, and of these he selected thirty-nine that were active in inducing crystallization, 
of which eighteen were insoluble : these were carefully washed in distilled water, and 
dried out of contact with air. When dry they were found to be inactive. The twenty- 
one soluble substances were purified by recrystallization, and they all became inactive. 
Hence it is concluded that sulphate of soda is the only nucleus for solutions of the same 
salt. That is to say, whenever a glass rod or other body acts as a nucleus, it is conta- 
minated with minute portions of the salt itself, which is supposed to exist in the air, 
not only of towns, but in the country. According to this view, the supersaturated 
solution of any other salt can only be crystallized by a saline nucleus of its own kind. 
II. Definitions. 
In the present memoir the terms chemically clean or chemically unclean, or simply 
clean and unclean , are to be taken with the following limitations : — 
1. A chemically clean surface is one that has not a greater attraction for the dissolved 
matter than for the liquid which dissolves it. A supersaturated aqueous solution, for 
example, whether of a salt or of a gas, will adhere perfectly to such surface, and there 
will be no separation of salt or of gas. 
2. But as there is little or no adhesion between water and a fatty, oily, or greasy 
* Comptes Eendus, vol. lx. p. 833. See also Schiff, Ann. der Chem. und Pharm. vol. cxi. p. 68. 
