Page:Elementary Text-book of Physics (Anthony, 1897).djvu/228

314 the bottle and admitting air, crystallization commences, and spreads rapidly through the mass, accompanied by a considerable rise of temperature. If the amount of salt dissolved in the water be not too great, the solution will remain liquid when cooled in the open air, and it may even suffer considerable disturbance by foreign bodies without crystallization; but crystallization begins immediately upon contact with the smallest crystal of the same salt.

197. Freezing-point of Solutions.—It has been long known that the freezing-point of a solution of salt and water is lower than that of pure water. The relation of the lowering of the freezing-point to the concentration of the solution was investigated by Blagden, who found that for dilute solutions the lowering of the freezing-point was proportional to the concentration. This matter has been investigated by Raoult, who established some most important generalizations. Raoult showed that, for indifferent solutions, that is, for solutions which are not electrolytes, provided they are very dilute, the lowering of the freezing-point is very closely proportional to the concentration; its amount differs for different solvents. He further showed that, for any one solvent, the lowering of the freezing-point is the same whatever be the dissolved substance, provided that the solutions are equimolecular, that is, contain the same number of molecules of the dissolved substance in unit volume of the solution. It may be shown on theoretical grounds that the change in the freezing-point depends upon the osmotic pressure, the freezing-point, the heat of fusion, and the density of the solution.

Solutions which are electrolytes, or are not indifferent, also exhibit a lowering of the freezing-point proportional to the concentration, but the amount of change is greater than in indifferent solutions. This difference is explained by assuming a partial or complete dissociation of the molecules of the dissolved substances into their constituent ions (§ 285).

198. Heat Equivalent of Fusion.— Some facts that have appeared in the above account of the phenomena of fusion and solidification require further study. It has been seen that, however rapidly the temperature of a solid may be rising, the moment fusion