44 SECTIONAL ADDRESSES. 
into the ions of elements with a great affinity for one another; and 
secondly, how could the ions remain in presence of water without chemical 
action ? 
The next phase is almost too well known to need recalling. In 1883 
Arrhenius, working on dilute aqueous solutions, concluded that the 
equivalent conductivity increases with dilution because the proportion of 
conducting molecules increases. He then found a correspondence between 
the conductivities of acids and their strength as determined thermo- 
chemically by Berthelot, or by the method of displacement. This suggested 
to him that the molecules which are active as regards conductivity are 
active chemically, and it occurred to him that these active molecules are 
dissociated, since this would explain why the heats of neutralisation of all 
strong acids and bases are the same. He communicated this idea to 
Ostwald, who was then working on the catalytic activity of different acids, 
and they found that the catalytic activities of these acids was roughly 
proportional to their conductivities. In 1886 van’t Hoff published his 
memoir on the analogy between dilute solutions and gases, in which he 
drew attention to Raoult’s measurements of the freezing-points of aqueous 
solutions, which showed that the influence of one molecule of an electrolyte 
like potassium chloride was double that of a molecule of a non-electrolyte 
such as alcohol. ‘ After reading this memoir,’ writes Arrhenius, ‘it was 
quite clear to me that I might dare to say that all those substances which 
are active, that is all electrolytes, consist of two molecules and not of one ; 
that is, sodium chloride is composed of two molecules, the sodium ion and 
the chlorine ion. Then the theory of electrolytic dissociation was 
expressed without any restriction (1887). I had then a threefold basis 
for my conclusion, the chemical one and the electrical one, and then the 
thermodynamical one, regarding the freezing-point. On a foundation of 
three points you may construct a very solid building.’ 
It is interesting that both Planck and van ’t Hoff put to Arrhenius the 
difficulty that if a salt is partially dissociated into ions, then the equilibrium 
between the ions and molecules should be in agreement with Guldberg and 
Waage’s Law of Mass Action, while they found by calculating the degree 
of dissociation by Arrhenius’s formula « = A that this was not the case. 
a) 
Arrhenius suggested that a better test would be made by considering the 
dissociation of weak acids which varied over a much wider range and the 
results, as we know, confirmed his theory, and were embodied in Ostwald’s 
Dilution Law. 
On its publication in 1887 the ionic theory met with violent opposition, 
but, in the hands of Arrhenius, Ostwald, van ’t Hoff and Nernst, its value 
was quickly established in the most varied fields: the theory of concentra- 
tion cells and of liquid junction potentials, the behaviour of weak acids 
and bases and the hydrolysis of salts, the properties of indicators, the 
dissociation of water, and the theory of qualitative and quantitative 
analysis. Entirely fresh light was thrown on all these problems, and in 
many instances they admitted of quantitative explanation for the first 
time. It is true that the reason for ionisation and for the stability of ions 
remained unknown for many years, until the discoveries of Rutherford 
and Moseley had made possible Bohr’s application of the quantum theory 
