The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS known to every one, but at the same time they represent two quite distinct classes. So far, no reference has been made to this distinction between sugar and salt, but it is one which has led to much controversy and practical work on the part of modem chemists, and as such deserves our attention. There are various good grounds for believing that the molecule of cane sugar is nearly six times as heavy as the molecule of sodium chloride (common salt). If, then, we took equal weights of the two substances, we should have six times as many molecules of salt as of sugar. Since, as already stated, the extent to which the freezing-point is lowered—the “depression,” as we may call it—is proportional to the number of molecules or dissolved units, it follows that salt ought to produce a depression six times as great as that caused by an equal weight of sugar, provided that each is dissolved in the same quantity of water. Expectations, however, are not realised in this case, and the salt gives a depres- sion about eleven times as great as that due to the sugar. The salt, in the process of solution, seems to have yielded nearly twice as many dissolved units as we should expect. What interpretation can be given of this extraordinary behaviour ? Before any explanation is attempted, attention must be directed to another point of distinction between salt and sugar. If two wires connected with an electric battery were immersed in pure water, only an infinitesi- mally small current would pass; the water offers an enormous resistance to the passage of the current, and may be described as practically a non-conductor. The moment, however, a pinch of salt is dissolved in the water, all this is changed, and the current experiences 310