106 
MIL B. 1). STEELE ON THE xMEASUEEMENT OE IONIC YELOCTTTES 
change is differeut in difterent cases; for lithinin chloride, and the chlorides and 
sulphates of tlie alkaline earth metals, i? diminishes with increasing dilution, whilst 
for silver nitrate, the opposite is the case. The measurements of the same salt which 
have been made by different investigators, although they shoAv considerable differences 
amongst themselves, nevertheless all point to the same general conclusion; namely, 
that it is only for a very limited number of salts that the transport number is 
independent of concentration. These differences are probably due in part to the 
difficulties of the method, and the very great influence of a small experimental error 
in the determination of small changes of concentration, and partly also to the fact 
tliat, following Hittoef, many rvorkers have employed membranes of one kind or 
another to separate difterent portions of the solution, and thus preA'ent mixing by 
convection currents, whilst others again have altogether avoided their use. Beix 
(‘Zeit. Phys. Chem.,’ 28, 439, 1898) has shown that the use of certain membranes 
aflects in a very remarkable manner the value of the transport number, and it is 
probable that a considerable number of the discrepancies are to be traced to this 
cause. Another serious difficulty consists in the fact that an experiment inirst only 
be carried on for such a time that no change in concentration can take place in the 
middle part of the solution, and for many salts the formation of hydrogen and 
hydroxyl ions at the electrodes further diminishes this time on account of the great 
velocity of these ions. The danger from hydrogen ions has been minimised by the 
use of a cadmium anode, and quite recently Noyes (‘ Zeit. Phys. Chem.,’ 36, 63, 
1901) has completely overcome this difficulty by the device of adding small quantities 
of acid and alkali to the solution in the neighbourhood of the anode and cathode 
I'espectively. 
The measurements of Noyes are probably the most accurate that have been made, 
and his results again confirm the statement made above. 
Tlie importance of and the need for a measurement t)f the resistance of aqueous 
solutions of electrolytes was repeatedly referred to by Hittoef ; but it was not until 
the development by Kohleausch of his Avell-knoAvn method—in Avhich, by the use t>f 
an alternating current, the polarisation eflect is neutralised—that this problem 
could be successfully attacked. 
From the measurement of the conductivities of salt solutions, Kohleausch (AVied. 
Ann.,’ 6, 1, 26, 213) deduced the law of “the independent Avandei'ing of the ions,” 
which states that the imdecular conductivity of an aqueous solution of an electrolyte 
is the sum of two constants, of which one depends only on the nature of the cation, 
and the other only on the nature of the anion ; and he further assumed that these 
coirstants are proportional to the velocities of the ions, g oc U + A’. The values of L 
and A" are obtained from the molecular conductivity, which is the sum, and the trans¬ 
port number, which is the ratio, of tlie two velocities, or \Jx (1 — p)g and Yx pji. 
It is found further, tliat with increasing dilution the molecular conductivity 
increases, until it tinallv reaches a con.stant value at very great dilutions. This is 
