MR. W. C. D. WHETHAM lOXIC VELOCITIES. 
347 
Now ill this equation A can be determined once for all, y can be read off in a 
graduated galvanometer of proper sensitiveness, r can be measured bj separate 
experiments for the solution in question, and v, the velocity of the junction, is directly 
observed. Hence the specific ionic velocity, can be deduced without estimating 
the potential gradient directly, and introducing all the consequent uncertainties. 
The method was first employed for the case of copper and ammonium chlorides, 
where the differences in specific resistance are not great, so that the velocities in 
opposite directions are aj^proximately equal, and the correction for discontinuity is 
not important. The area of cross-section of the tube was determined as follows. 
Fig. 3. 
The longer limb was corked up so as to be air-tight, and filled with water till the 
surface stood just above the beginning of the narrow part of the junction tube. Its 
level was read off on the kadhetorneter. A flask containing’ water was then weighed, 
and liquid from it poured into the junction-tube through a weighed thistle-tube, till 
the surface nearly reached the top of the narrow part. The level was then again read 
off, giving the length of the column of liquid added, and the volume of water used 
vvas estimated by weighing the flask again, a correction for the small cjuantity 
retained on the walls of the thistle-tube being obtained by noticing the increase in 
its weight. Two determinations of the area of cross-sections by this section gave 
(1) 0’4300 sq. centim. and (2) 0’4297 sq. centim. Mean 0’4299, or correct to three 
places, 0’430 sq. centim. 
The solutions were prepared by weighing out 0'670 grm. of pure cupric chloride 
and 0’534 grm. of ammonium chloride into two 100 cub. centims. flasks, and dis- 
2 Y 2 
