B.—CHEMISTRY. 45 
to the problem of atomic structure. Then the connection between 
electricity and chemical affinity, which chemists had been seeking since 
the days of Berzelius and Faraday, suddenly became clear, and we learnt 
the cause of ionisation and of the stability of ions in solution. Metallic 
sodium reacts with water in order to give up an electron; the sodium ion 
having already lost an electron is no longer reactive. 
But in spite of the many triumphs of the ionic theory, and its success 
so far as weak electrolytes are concerned, the discrepancy between the 
behaviour of strong electrolytes and the mass law, pointed out by Planck 
and vant Hoff to Arrhenius, still remained. Kohlrausch had shown that 
in very dilute solutions the relationship between the equivalent con- 
ductivity and concentration of salts was expressed by the equation 
Ae = No — ket (k = constant) 
which is incompatible with the masslaw. At the beginning of this century 
the outstanding problem of the ionic theory was this so-called anomaly of 
strong electrolytes. Up to this point the whole development of the ionic 
theory had come from the experimental side, and every advance had 
originated in some new discovery. But the solution of the final problem 
came not from experiment but from the mathematical physicists, who 
thus repaid to chemistry the debt which physics owed to Faraday. 
Faraday had often drawn attention to ‘ the enormous electric power of 
each particle or atom of matter,’ 7.e. the large size of the ionic charge. 
~ Helmholtz in 1881, in his Faraday lecture, made a calculation showing that 
the attractive force between the electrical charges associated with 
hydrogen and oxygen is 71,000 billion times greater than the gravitational 
attraction between their masses. It would seem obvious that forces such 
as these must affect the behaviour of ions, and from time to time sugges- 
tions came from chemists—van Laar in 1900, Bjerrum in 1906, Suther- 
land in 1907—that strong electrolytes were completely dissociated, and 
that the variations of equivalent conductivity with concentration were 
due not to a change in the degree of dissociation, but to the varying effect 
of the interionic forces. Bjerrum had found that the molecular colour of 
chromium salts was independent of dilution in the absence of complex 
ions, and explained this on the basis of complete dissociation. 
One of the earliest supporters of the ionic theory was Nernst, who 
made very substantial contributions to it in his theory of concentration 
cells and of diffusion potentials. Milner, working in his laboratory at 
Gottingen in 1898, was attracted by the problem of strong electrolytes, 
and attacked it from the point of view of the interionic forces. The 
mathematical difficulties were great, and it was not until 1909 that they 
had been overcome sufficiently to admit of the calculation of the change 
of total internal energy with dilution. Milner was the first to realise that 
the ions cannot be distributed at random in a solution since, owing to 
the Coulomb forces, there must be an excess of positive ions in the 
neighbourhood of a negative ion and vice versa. Thus each ion can be 
considered as surrounded by a spherical ionic atmosphere, the density of 
which decreases with the distance from the ion. The electrical potential 
at the surface of the central ion will therefore be affected by the ionic 
atmosphere, and by taking into account the changes of potential with 
