There is probably no subject in science that is more involved in contradiction than that of supersaturation. All the phenomena connected with it seem to behave differently in the hands of different inquirers, so that the facts affirmed by one writer are simply denied by another; and the same theory which seems to have been disproved by one is again and again brought forward by another. Take one point by way of example, namely, the nuclear action of bodies in producing the sudden crystallization of a supersaturated saline solution. Ziz, in 1809, stated that not only air, but solids, act best as nuclei when dry: if wet, or boiled with the solution, or thrown into it while hot and allowed to cool with it, they are inactive. Löwel (1850-57) denies that air, whether wet or dry, has any nuclear action; but he admits that solids exposed to the air become active, and that alcohol is always active. Selmi and Goskynski, in 1851, assert that dry air is nuclear, and acts by getting rid of water at the surface, and producing small crystals there which continue the action. This seems to be a revival of Gay Lussac’s theory, namely, that air is absorbed at the surface of the solution and precipitates a portion of the salt in the same way that one salt may precipitate another, and this precipitate continues the crystallization. Lieben, in 1854, states that soot is a nucleus, also platinum black whether ignited or not; that pounded glass heated in sulphuric acid produces sudden crystallization, but that platinum sponge and precipitated sulphate of baryta after being heated have no action. Schröder, in 1859, remarks that it is always a matter of chance whether such or such a substance produces crystallization. “Such facts,” he says, "singularly increase the difficulty of interpreting theoretically the phenomena of supersaturation.” He concludes that the only general rule that can be admitted in the presence of so many opposed and contradictory results is that bodies act on supersaturated solutions only after having been exposed to the air. In 1866 Gernez and Viollette and in 1868 Schiff are satisfied that there is only one nucleus for a supersaturated solution, and that is a salt of the same kind as the one in solution or one isomeric therewith. In 1866 Jeannel opposes this theory of pancrystallography, as he calls it, on the ground that it. cannot be supposed that crystals, often of rare salts, are to be found waiting in the atmosphere, ready to enter our flasks as soon as they are uncovered. Pellogio also, in 1875, “gives proofs that the phenomena of supersaturation are not so simple as the French physicists would imply, namely, that the only nucleus is a salt of the same kind,” seeing that some supersaturated solutions, such as those of hyposulphite of soda, acetate of lead, acetate of soda, &c., may be exposed to the air, in places where the air is any thing but still, for fifteen or twenty days without the formation of crystals. He states further that porous bodies are active, such as common sponge, platinum black, iron reduced by hydrogen, and carbon. For example, carbon was raised to a red heat, quenched under mercury, and introduced into a solution of 100 sodic sulphate to 102 of water: it fell to the bottom and disengaged gas for sometime; crystallization then set in and spread all through the mass. Viollette, on the contrary, finds that bodies greedy of water and capable of being hydrated, such as the fused sulphates of copper and of iron, and porous bodies recently calcined, such as carbon, have no action on supersaturated saline solutions.
VII. On supersaturated saline solutions.—Part II
