c 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.

Now, the phenomena of electrolysis have, for a long time, forced upon
us an almost necessary image