138 
MR. B. D. STP:ELE ON THE MEASUREMENT OF IONIC VELOCITIES 
It is only with the weaker ions that this tendency to the formation of stable 
complexes is manifested, and according to the theoiy put forward by Abegg and 
Bodlander, this tendency is a measure of the electro-affinity or the ability of an atom 
to combine with unit charge of electricity to form an ion. “ The weaker an ion is, 
the more it seeks to form complexes by the addition of a neutral part.” The nature 
of the complex that will be formed, whether cation or anion, depends on the question 
whetlier the simple anion or cation has the greater electro-affinity. 
“ The weaker of the two ions will combine with the neutral molecule to foi'm the 
complex. ” 
Thus the complexes that are formed in solutions of the chlorides of the alkaline 
earth group are anions, the comparatively strong cations showing less tendency to 
combine with the neutral salt than does tlie chlorine. On the other hand, in solu¬ 
tions of silver nitrate, where the cation is a very weak one, and the anion one of the 
strongest known, complexes, if formed at all, should be cations, and, as a matter of 
fact, Hittorf’s measurements show" that for this salt changes in the opposite 
direction to that for the other salts considered. 
With the very strong ions K and Na there is shown no tendency towards the 
formation of such complex bodies, and consequently no change in transport number 
with increasing concentration, whereas w"ith the weaker ions of the second group, a 
very marked change in p occurs, and for the still weaker ions, such as Cu and Zn, 
the change in p is still more marked. 
If now we apply this explanation to our three difficulties, we shall find that it is 
able to atibrd at least a qualitative explanation without involving any improbable 
conclusions. 
If we assume that in a given solution dissociation takes place in such a manner 
that there are formed in addition to the simple ions only one species of complex (for 
example, complex anions) then, if c is the ionic concentration of the cations, and 
c that of the complex anions, (c — c') is the ionic concentration of the simple anions, 
and if u, v, and v' are the specific ionic velocities of the cation, simple anion, and 
complex anion respectively, the total anu^unt of current carried is proportional te 
cu + (c — c') V “b c'v', 
the amount of current carried by the anions to the anode is equal to 
and hence 
(c — c')v cv, 
P = 
(c — c') r + c'v' 
cu -f {c — c)v -}- c'v' 
cv -f- c' {v' — V) 
c {u -f v) -f c {v' — f) " 
Since V wall be most probably greater than v\ this e([nation seems to shoAv that, by 
tlie formation of complexes, p sliould become diminished and not increased ; but in the 
Hittorfian experiments, whilst the current, as determined by the silver voltameter, is 
