346 
PEOFESSOR O. MASSON ON IONIC VELOCITIES. 
experiment are not comparable, those of C referring to current at given instants and 
those of U and V to average velocities for a long period of time; and it has been 
already told how both current and velocity steadily diminish at a rate determined 
by the nature and strength of the jelly. Either, then, an average current value must 
be obtained, or velocity values at dehnite points. Both methods have been used. 
For the former, since the quantities involved were too small to admit of the use of a 
voltameter, the average current was calculated from the area of the time-current 
curve. This was done for all the experiments except those with lithium and 
magnesium sulphates; and it was found in all cases that the average current value 
was attained at a point very close to that at which the indicators had travelled half 
their distance. With lithium and magnesium sulphates the current and velocities 
were so nearly constant that an appreciable error could hardly occur by assuming 
this as the correct point. The numbers given in the table Avere calculated in this 
manner. 
The other mode of calculation, which is a more rigid test of the experimental method, 
was applied to only two cases, but with satisfactory results. The values of U -h V at 
intervals throughout the exjjeriment Avere deduced Iw draAving tangents to a cuiwe 
got by plotting tlie added lengths of the coloured portions of the tube against the 
time. The current-time curA^e gaA^e the corresponding current values. Numerous 
C 
values of the ratio Avere thus obtained, which in the case of half-normal KCl 
I -t- V 
Avere constant to Avithin I per cent, of their mean Amine, and in the case of tAvice- 
normal KCl AAmre nearly, but not quite, so consistent. These mean Amines corresponded 
* A rather trequeut occm’rence in the.se e.\;periuieuts may be mentioned here. It is one AA’hieh, at first 
sight, looks as if it must be fatal to them, Imt AA’hich has been proA^ed to be really unimportant. FlaAvs, 
haAung the appearance of small bulddes or cracks, are apt to appear in the jelly tOAvards the end of an 
experiment. They seldom occui- till the boundaries haA'e done most of their journey, and they generally 
aj)pear near the centre of the tube. In many experiments they do not appear at all, but Avhen a Ahaa’ does 
come it is apt to e.xtend in a rather curious manner. This occurrence of flatA'S at once causes a reduction 
of current, and it is easy to locate their first appearance in the time-current ciuve, et'en Axere it not noted 
at the time, as Avas alAA'a3AS done. It has, hoAA'eA'er, A'ery little, if aiqq effect on the A’elocities of the 
l>ottndaries; and the ratio of these (itir) is absolutely unaft’ected. It is indeed striking to AA-atch the calm 
inditterence AA-ith AA’hich these flaAA's are treated Iq^ the traA’elling colour boundaries, and to contrast the 
behaA iour of the latter A\-ith that of the gah'anometer needle. The facts are explicable on the assumption 
that the fiaAA^s reduce the current by reducing the A’ahte of A, leaA'ing the current density (C/A), and 
therefore the actual A'elocities, as aa’cII as their ratio, unaffected. It is obA'ious that, in such a case, the 
e.xperiment must be regarded as finished as soon as flaAA-s appear, AAEere the object is to test the truth of 
the ecpiation involving the original, and only knoAvn, A-alue of A; but that, for the determination of ujr 
(or of jj), the experiment may be continued as usual till the boundaries meet. These rules haA'e been 
obsei'A'ed in all cases. The occurrence of flaAvs of the kind described AA'as noticed by Lodge {Ioc. cit.). 
Their cause has not been ascertained. Whether the\' Avill occur or not in any given experiment almost 
seems to be decided Ity caprice. 
