]kIR. W. C. D. WHETHAM IONIC VELOCITIES. 
341 
The agreement between Dr. Lodge’s observed value for the velocity of hydrogen 
and that deduced by Kohleausch is too close to be accidental, but some further 
experiments in which the velocities of various ions were determined by watching the 
formation of precipitates as they travelled along did not give such good results. 
Thus, Dr. Lodge found Ba = ‘00012, Sr = -00015, and Cl = Br = I = -00024 
centims. per second, while Kohleausch gives Ba = -00033, Cl = -00053, I = -00060 
(‘Wied. Ann.,’ vol. 6, p. 206). 
There are several disadvantages in this method. The greatest appears to be the 
impossibility of keeping the solutions of a definite strength^—the mere fact of pro- 
duciug a precipitate must withdraw a certain amount of salt from the sphere of action, 
and so reduce the strength of the solution. This will increase the resistance of that 
portion of the tube in which the precipitate has been formed, and the potential 
Fig. 1. 
gradient will cease to be uniform and calculable. When jelly is used, the experiments 
are easy to make, but it has a considerable conductivity of its own, and its great 
viscosity must offer a high resistance to the passage of the ions through it. More¬ 
over, it gradually exudes from the-tube in a direction opposite to that of the current 
—perhaps owing to electric endosmose. If ordinary liquid solutions are employed, 
the precipitates cause serious mechanical disturbances which prevent good results, 
while the use of indicators restricts the experiments to the cases of acids and alkalis 
and introduces a foreign substance which may itself serve to convey some of the 
current. 
In order to get over these difficulties, and to submit Kohleausch’s important and 
beautiful theory to adequate experimental tests, I undertook a series of experiments 
m which the substances used acted as their own indicators. The solutions can then 
be kept at a definite strength, no third substance need be introduced, no precipitate 
