42 SECTIONAL ADDRESSES. 
experimentally Faraday’s error about the existence of a suboxide and 
sulphide of antimony, and he pointed out the difficulty respecting elements 
with more than one valency. But his main reason for criticising Faraday’s 
Laws was the improbability that the same current would decompose 
equivalent quantities of substances such as the oxides of silver and of 
potassium which differ so greatly in chemical affinity. Here Berzelius 
falls into the error of confusing what Faraday called the quantity and 
intensity of electricity, and we see how much clearer a view Faraday had 
of the relations between electricity and chemical action. It is amazing 
how Faraday’s instinct guided him and kept him to the right path, 
enabling him to seize on the relevant evidence and neglect the apparent 
exceptions. SBerzelius sees all the difficulties, all the apparent anomalies, 
detects infallibly any experimental errors, but fails to grasp the essential 
truth of Faraday’s ideas which stand to-day without modification. 
There is something uncanny in Faraday’s avoidance of pitfalls and 
his recognition of fundamental truths. As Kohlrausch said of him, ‘ Er 
riecht die Wahrheit’—he smells the truth: or in Tyndall’s words, ‘ Faraday 
was more than a philosopher: he was a prophet.’ 
During the century that has elapsed since Faraday’s discoveries, the 
conduction of electricity in solutions has remained one of the central 
problems of chemistry and physics, and I propose now to trace briefly 
the steps by which we have arrived at our present position and to recall 
to you the chief landmarks in the history of the ionic theory. There 
have been three main phases in the development of the problem—Firstly, 
the discovery of the general relationships between the conductivity of 
solutions and the concentration and nature of the dissolved substances, 
due mainly to the work of Hittorf and Kohlrausch : secondly, the recog- 
nition of the relations of these facts to the general theories of chemistry 
in the classical ionic theory of Arrhenius ; and, lastly, the quantitative 
explanation of the properties of electrolytes in the mathematical theory 
due to Milner, and to Debye and Hiickel. 
The direct successor to Faraday was Daniell, who studied in detail 
the changes in the concentration of electrolytes at the electrodes produced 
by electrolysis, which had first been observed by Faraday. His papers 
were published in three letters to Faraday in 1840-44, and their most 
important result was to show that the current in aqueous solutions is 
actually carried by the ions of the solute and not, as Faraday had supposed, 
by the ions of hydrogen and oxygen. Daniell failed to find any reasonable 
explanation of his transference data. 
Ten years later this aspect of electrolysis was the subject of a classical 
investigation by Hittorf on ‘ The Migration of Ions during Electrolysis,’ 
which was of the greatest significance for the theory of solutions. Hittorf 
realised that the changes in concentration round the electrodes could 
only be explamed on the assumption that the ions move with different 
velocities, and he showed how their relative speeds could be calculated 
from the change in concentration, a very remarkable achievement in 1853. 
Not only did Hittorf show great theoretical acumen, but he was an out- 
standing experimentalist. He devised the methods by which transport 
numbers are still determined, and so accurate and comprehensive were 
his results, that until recently they were the main source of our knowledge 
