ELECTROKINETICS 363 



are at 0.21 and 0.79 of the total depth, from the top or bottom of 

 the chamber. 



The effect of the electroendosmotic flow of the water on the 

 migration of the particles is nicely illustrated in the case of mush- 

 room spores, the specific gravity of which is just great enough to 

 cause them to fall slowly; in so doing, they pass through the 

 successive layers of flow (electroendosmotic and hydrodynamic) 

 of the water. When the spores, in falling, reach the upper layer 

 of no flow of water, they, being negative, move to the anode. 

 This represents motion due to their own electric charge, which 

 is weakly negative (in a salt solution of pH 8.2). As they fall 



+•__+ + + ^-f + » 



CL- 



± ± ± ± ± ± ± ± ± ± 



Fig. 158.— An enlarged longitudinal section (with depth exaggerated) of a 

 cataphoresis chamber showing endosmotic flow of water, and the two quiet 

 levels (dotted lines). 



farther, they first pass through water which flows with them (in 

 the center of the chamber) and increases their speed. They then 

 reach the lower, quiet layer of water and finally the water at the 

 bottom of the chamber, which opposes them by its greater flow in 

 the opposite direction to such an extent as to reverse the direction 

 of their migration. 



Theory. — If water fills a glass capillary tube or a porous clay 

 disk (which is essentially a mass of capillary tubes), and if 

 two wires coming from a source of current are placed one in each 

 end of the tube or one on each side of the porous disk, the water 

 will move through the tube or through the disk toward the 

 cathode or negative pole. The German physicist Quincke was 

 the first to suggest that this extraordinary electroendosmotic 

 migration of a liquid through capillaries under the influence of an 

 applied potential is due to an electric charge which lines the walls 

 of the capillary in such a way that electricity of one sign is in 

 immediate contact with the solid wall, while electricity of the 

 opposite sign, equal in quantity to the first, forms a second layer 

 lying near the first. The German physicist Helmholtz offered 



