334 
PEOFESSOR 0. MASSON ON IONIC \T:L0CIT1ES. 
any of his experiments; or rather it shows that the value of tt was mcorrectly 
assumed. Thus he assumed, in the hydrogen experiment, that the potential slope 
causing the observed hydrogen velocity was 1 volt per centim., because the tube was 
40 centims. long, and there was a difference of potential of 40 volts between the 
electrodes. It is obvious, however, that this may be a very misleading assumption 
where the value of tt is required in one particular part of a tube which contains quite 
different electrolytic solutions in different portions of its length. There is, therefore, 
no exact information to be obtained by comparing Lodge’s observed velocity of from 
•0024 to ‘0029 centim. per second with Kohleausch’s calculated value •0032. 
Considerations of temperature and of concentration, though important, are less so 
than that of correct potential slope, and therefore may be passed over. 
Perhaps the most striking of all Lodge’s experiments were those in which he 
observed the velocities of Cl, Br, or I entering and travelling through a jelly tube 
from the cathode end, while Sr or Ba travelled in the opposite direction. The 
original jelly was charged with, among other things, a small pro 23 ortion of Ag ions to 
act by j)artial jDrecipitation as an indicator of the j^rogTess of the halogen, and with 
SO 4 ions to l^lay a similar ^Dart towards the new cations. The observed velocities of 
the former were in all cases aiDjDroximately double those of the latter; whence Lodge 
concluded that Cl, Br, and I are, as ions, naturally twice as fast as Sr and Ba. Here 
again, however, the fact that the observed velocities were caused by unknovm, and 
jjresumably different, jootential slopes necessarily vitiates the conclusion dra^vn. It 
will be sho\vn in the sequel that what really determined the interesting and simple 
velocity ratios observed in this set of experunents was not the specific character of 
the ions under insiDection, but the composition of the intermediate solution into which 
the Cl and Ba, or similar ions, had not yet ^Jenetrated. As, however, this was a 
mixture, and the indications given of its composition are rather qualitative than 
quantitative, no results of theoretical value can be deduced. 
Whetham’s method {loc. cit.) rendered the use of gelatine unnecessary, as he 
avoided gravity currents, at all events, by emjDloying a vertical tube in which to 
observe the rate of migration of the boundary between a coloured solution and a 
colourless one during the passage of a current, the lighter solution lying above. He 
also avoided the occurrence of difierent and unknovm potential slopes in difierent 
joarts of the column by selecting for each experiment a pair of solutions of, as nearly 
as joossible, equal sjDecific resistance. The results so obtained were in very good 
accord with the calculated velocities {xu or xv) of the same ions in similar solutions 
of the same concentration, and thus afforded the first exact confirmation of the 
Kohleausch theory. But, from the very nature of the method, its application was 
restricted to a very few cases, as it is obviously not easy to find solutions suitable in 
all resjDects.’^ 
* Whetham’s determination of the velocity of the copper ion, and his comparison of the results with 
the calciUated number, are open to the objection that what he observed was not the copper ion at all, but 
