4 54 DK. R. B. DENISON AND DR. B. D. STEELE ON THE 



placed in a bath with plate-glass sides. The framework of the bath was of copper and 

 provided with flanged edges, on which the plate-glass sides were clamped. Rubber 

 tubing was placed between the copper flanges and the glass, and in this way a perfectly 

 water-tight junction was made and the use of putty rendered unnecessary, a con- 

 siderable advantage if the bath is to be heated. The base of the bath was provided 

 with two holes for the insertion and support of the apparatus shown in fig. 2. 



In putting together the apparatus for an experiment, the electrode vessels E are 

 first filled and the electrodes placed in position, care being taken that none of the 

 electrode liquid passes over into B or A.* The two halves of the apparatus are then 

 connected by rubber tubing and the tubes A are filled just to the shoulder with the 

 solution to be measured. The rubber cork carrying the tube K and the membrane is 

 then placed in position! and pressed on the shoulder, care being taken not to 

 imprison any air-bubbles below the membrane. The electrical connections having 

 been made, the current is started by pouring the indicator through F (fig. 1) or L 

 (fig. 2), after which the plugs P and P' are inserted. The appearance of the margins 

 in the tubes generally occupies only a few minutes, and as soon as they have 

 advanced about 1 centim. into the tubes the membranes are removed by lifting K or 

 lowering K'. In this manner electric endosmose is entirely prevented. The velocities 

 of the two margins are measured by observing the distance moved in stated intervals 

 of time, the observations being made by means of a telescope with cross wires and a 

 glass scale placed immediately behind the tubes A and A'. 



One other point requires notice. The tubes K in apparatus fig. 1 and P' in 

 apparatus fig. 2 must not have a greater diameter than about 1 millim. The reason 

 for this is as follows : Suppose that we are measuring the transport number of 

 sodium chloride with lithium chloride and sodium acetate as indicators, both solutions 

 being lighter than that of sodium chloride. In one measuring tube we have the 

 solutions lithium chloride over sodium chloride, and in the other sodium acetate over 

 sodium chloride, and the relative length of the columns of lithium chloride and 

 sodium acetate depends on the position of the margins. Now the sodium ion travels 

 more slowly than the chlorine ion, with the result that a longer column of sodium 

 acetate than of lithium chloride is formed. These solutions have different densities, 

 and so there is a difference of hydrostatic pressure tending to move the whole column 

 of liquid up one measuring tube and down the other, and the level in the tubes K 

 and P becomes slightly altered during the progress of the experiment. This change 

 of level accelerates one margin and retards the other, and false values for the 

 velocities and transport number are obtained. In order to reduce the effect of the 

 change of level, the diameters of K and of P are made as small as possible, and hence, 

 since the total change of level amounts to only a fraction of a millimetre, the 



* In the final form of apparatus the two halves were sealed together. 



t In the case of the apparatus (fig. 2) used for heavy indicators, the tube K is placed in position before 

 putting the solution into the tube A. 



