MR. W. C. D. WHETHAM ON THE VELOCITIES OF THE IONS. 509 
the junction tube. A precipitate formed at the surface of contact between the 
two solutions, and, when a current was passed from one platinum electrode to the 
other, the precipitate gradually spread upward or downwards with a velocity which 
could be measured on the scale by means of a telescope. 
Fig. 1. 



TOTTI 





If the potential gradient at the junction is dV/dz, we have 
AV /da = yr'/A, 
where y represents the total current, 7 the specific resistance of the solution, and A 
the area of cross-section of the tube. 
If v be the observed velocity, the specific velocity for unit potential gradient is 
given by 
v vA. 
(ae dVjde yr” 
A is determined by filling a known length of the tube with water or mercury, y is 
read off on a galvanometer previously graduated, and is determined by KoHLRAUSCH’S 
method of a Wheatstone’s bridge with alternating currents. In order that the 
method should be a success, it is necessary that the solutions should be of nearly 
equal specific resistance, so that a mean value of r may be taken. The shght 
disturbing effect of any small difference is shown in the former paper to be eliminated 
if measurements be made when the current is passing in both directions, and the 
mean taken. But, unlike the colour boundary method, the formation of a precipitate 
is an irreversible process, so that measurements of the velocity can only be made when 
the current passes in one direction. All that can be done is to choose solutions of 
