s the sum of three values, of which one
concerns the positive ion, a second the negative ion, and the third
the solvent. The properties of the solutions would then be what are
called additive properties. Numerous verifications may be attempted by
very different roads. They generally succeed very well; and whether we
measure the electric conductivity, the density, the specific heats,
the index of refraction, the power of rotatory polarization, the
colour, or the absorption spectrum, the additive property will
everywhere be found in the solution.

The hypothesis, so contested at the outset by the chemists, is,
moreover, assuring its triumph by important conquests in the domain of
chemistry itself. It permits us to give a vivid explanation of
chemical reaction, and for the old motto of the chemists, "Corpora non
agunt, nisi soluta," it substitutes a modern one, "It is especially
the ions which react." Thus, for example, all salts of iron, which
contain iron in the state of ions, give similar reactions; but salts
such as ferrocyanide of potassium, in which iron does not play the
part of an ion, never give the characteristic reactions of iron.

Professor Ostwald and his pupils have drawn from the hypothesis of
Arrhenius manifold consequences which have been the cause of
considerable progress in physical chemistry. Professor Ostwald has
shown, in particular, how this hypothesis permits the quantitative
calculation of the conditions of equilibrium of electrolytes and
solutions, and especially of the phenomena of neutralization. If a
dissolved salt is partly dissociated into ions, this solution must be
limited by an equilibrium between the non-dissociated molecule and the
two ions resulting from the dissociation; and, assimilating the
phenomenon to the case of gases, we may take for its study the laws of
Gibbs and of Guldberg and Waage. The results are generally very
satisfactory, and new researches daily furnish new checks.

Professor Nernst, who before gave, as has been said, a remarkable
interpretation of the diffusion of electrolytes, has, in the direction
pointed out by M. Arrhenius, developed a theory of the entire
phenomena of electrolysis, which, in particular, furnishes a striking
explanation of the mechanism of the production of electromotive force
in galvanic batteries.

Extending the analogy, already so happily invoked, between the
phenomena met with in solutions and those produced in gases, Professor
Nernst su