As already stated, alcohol is miscible with water in all proportions, but Wroth and Reid2 have devised an ingenious method of estimating the " ideal " solubility, based upon the value of the partition-coefficient of alcohol between water and cotton-seed oil.

If Sa and Sb represent respectively the solubilities of alcohol in water and in the oil, Ca and Cb the concentrations in the same two solvents at equilibrium, and k the partition-coefficient, then SaSb = k = Ca/Cb.

By determining experimentally the proportion of alcohol in the aqueous layer and in the oily layer after the mixture has been shaken up and allowed to separate, the values of Ca and Cb are found, and therefore the value of k is known. It was found to be 28.3 at 25°. Then, by determining the value of Sb experimentally, the value of Sa can be calculated. Thus ethyl alcohol was found to dissolve in the oil to the extent of 212 grams per 100 c.c, whence Sa = kSb = 283 x 212 = 600 grams per 100 c.c.

This represents the "ideal" solubility of ethyl alcohol in water at 25°, which could be realised if water and alcohol were separated by a semi-permeable membrane through which only the alcohol molecules could pass. For methyl alcohol, the corresponding number is 501,3 the value of k being 103.6 and the solubility in cotton-seed oil 4.84 grams per 100 c.c.

In the foregoing experiments, the mixtures were shaken for an hour in a bath kept at a constant temperature of 25°. About three weeks' standing, or six hours' centrifuging, were required before the layers became clear.