PROFESSOR O. MASSON ON IONIC VELOCITIES. 
333 
the transport numbers determined in dilute solutions are aj^proximately correct for 
extreme dilution, and that they may be legitimately so far modified as to lead to 
constant specific velocities, lu and lu, in all combinations of the corresponding ions. 
Kohlrausch’s values are thus obtained. 
In general, electrolytes composed of monad ions, such as the chlorides of the alkali 
metals, are those which yield the best and most consistent results, while compounds 
containing divalent and polyvalent ions, such as Zn or SO 4 , do not behave so conform¬ 
ably with the law represented by the equation. This is shown both by the great 
alteration in the value of the transport numbers produced l)y change of concentration 
in many such cases, and by the fact that polyvalent ions do not appear to afford 
constant values of lu and Voo in their different comlfinations, on which account tliey 
are excluded from Kohlrausch’s tables of specific ionic velocities. Whatever may 
be the full explanation of these apparent contradictions of theory, it is probable that 
in these cases the ionization is not of such a character that the nature and number of 
ions of either kind can be deduced direct from the known composition of the solution ; 
in other words, the number of active ions of either kind is not related in the usual 
simple manner to n. 
The third chapter in the history of the study of ionic velocities is that which deals 
Avith their direct observation and measurement, and was begun by LodC4E in 1885 
(‘ Brit. Ass. Beports,’ 1886, p. 389). The great Audue of his Avork lay in the ingenious 
conception of the possibility of actually Avatching the advance of ions AAdiose colour 
renders their progress through an otherwise colourless solution visible, and of ions 
Avhich, though themselves colourless, may be detected in progress by their interaction 
with indicators ; and, further, in the introduction for these purposes of solid jellies in 
place of ordinary aqueous solutions, and the avoidance by this means of various 
sources of error, such as convection currents due to gravity and to temperature 
changes. The actual experiments were, however, of a pioneer character ; and the 
interpretation of them seems to have been Autiated by a misunderstanding of the 
mechanism of the process on Avhich they depended. It is necessary to point this out; 
first, because the author’s experiments cannot be properly discussed unless this be 
done, and secondly, because most of the recent text-books dealing Avith electrolysis 
quote Lodge’s experiment on the velocity of the hydrogen ion as afibrding the first 
and chief direct experimental verification of Kohlrausch’s theory, and do not direct 
attention to the difficulty in question. 
From the general equation and the explanations already given it is obvious that 
mere direct measurement of U and V, or of both, cannot by itself give results of 
exact value for comparison with calculated velocities [xu and xv). At best it can 
afford only an indication of whether something like the right order of magnitude has 
been arrived at by such calculation. To obtain data for exact comparison not only 
U or V, or both, hut also the potential slope 77, causing U or V, must be correctly 
measured. But the work of Lodge does not show that this latter Avas determined in 
