about these free ions
in solution, and they asked to see this chlorine and this sodium which
swam about the water in a state of liberty. But in science, as
elsewhere, irony is not argument, and it soon had to be acknowledged
that the hypothesis of M. Arrhenius showed itself singularly fertile
and had to be regarded, at all events, as a very expressive image, if
not, indeed, entirely in conformity with reality.

It would certainly be contrary to all experience, and even to common
sense itself, to suppose that in dissolved chloride of sodium there is
really free sodium, if we suppose these atoms of sodium to be
absolutely identical with ordinary atoms. But there is a great
difference. In the one case the atoms are electrified, and carry a
relatively considerable positive charge, inseparable from their state
as ions, while in the other they are in the neutral state. We may
suppose that the presence of this charge brings about modifications as
extensive as one pleases in the chemical properties of the atom. Thus
the hypothesis will be removed from all discussion of a chemical
order, since it will have been made plastic enough beforehand to adapt
itself to all the known facts; and if we object that sodium cannot
subsist in water because it instantaneously decomposes the latter, the
answer is simply that the sodium ion does not decompose water as does
ordinary sodium.

Still, other objections might be raised which could not be so easily
refuted. One, to which chemists not unreasonably attached great
importance, was this:--If a certain quantity of chloride of sodium is
dissociated into chlorine and sodium, it should be possible, by
diffusion, for example, which brings out plainly the phenomena of
dissociation in gases, to extract from the solution a part either of
the chlorine or of the sodium, while the corresponding part of the
other compound would remain. This result would be in flagrant
contradiction with the fact that, everywhere and always, a solution of
salt contains strictly the same proportions of its component elements.

M. Arrhenius answers to this that the electrical forces in ordinary
conditions prevent separation by diffusion or by any other process.
Professor Nernst goes further, and has shown that the concentration
currents which are produced when two electrodes of the same substance
are plunged into two unequally concentrated solutions may be
interpreted by the hypothesis that, in these particular condition