Thus the freezing-point of a normal solution, containing a molecule gramme (that is, the number of grammes equal to the figure representing the molecular mass) of alcohol or sugar in water, falls 1.85 deg.  C. If the laws of solution were identically the same for a solution of sea-salt, the same depression should be noticed in a saline solution also containing 1 molecule per litre.  In fact, the fall reaches 3.26 deg., and the solution behaves as if it contained, not 1, but 1.75 normal molecules per litre.  The consideration of the osmotic pressures would lead to similar observations, but we know that the experiment would be more difficult and less precise.

We may wonder whether anything really analogous to this can be met with in the case of a gas, and we are thus led to consider the phenomena of dissociation.[18] If we heat a body which, in a gaseous state, is capable of dissociation—­hydriodic acid, for example—­at a given temperature, an equilibrium is established between three gaseous bodies, the acid, the iodine, and the hydrogen.  The total mass will follow with fair closeness Mariotte’s law, but the characteristic constant will no longer be the same as in the case of a non-dissociated gas.  We here no longer have to do with a single molecule, since each molecule is in part dissociated.

[Footnote 18:  Dissociation must be distinguished from decomposition, which is what occurs when the whole of a particle (compound, molecule, atom, etc.) breaks up into its component parts.  In dissociation the breaking up is only partial, and the resultant consists of a mixture of decomposed and undecomposed parts.  See Ganot’s Physics, 17th English edition, Sec. 395, for examples.—­ED.]

The comparison of the two cases leads to the employment of a new image for representing the phenomenon which has been produced throughout the saline solution.  We have introduced a single molecule of salt, and everything occurs as if there were 1.75 molecules.  May it not really be said that the number is 1.75, because the sea-salt is partly dissociated, and a molecule has become transformed into 0.75 molecule of sodium, 0.75 of chlorium, and 0.25 of salt?

This is a way of speaking which seems, at first sight, strangely contradicted by experiment.  Professor Van t’ Hoff, like other chemists, would certainly have rejected—­in fact, he did so at first—­ such a conception, if, about the same time, an illustrious Swedish scholar, M. Arrhenius, had not been brought to the same idea by another road, and, had not by stating it precisely and modifying it, presented it in an acceptable form.

A brief examination will easily show that all the substances which are exceptions to the laws of Van t’Hoff are precisely those which are capable of conducting electricity when undergoing decomposition—­that is to say, are electrolytes.  The coincidence is absolute, and cannot be simply due to chance.