Now ill this equation can be determined once for all, can be read off in a graduated galvanometer of proper sensitiveness, can be measured by separate experiments for the solution in question, and , 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 approximately 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 kathetometer. 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 was estimated by weighing the flask again, a correction for the small quantity 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-
