358 PROTOPLASM 



Suppose, now, that the particles are held still— we can imagine 

 them clinging together and forming a porous disk — and an 

 electric field is again applied; the water will then flow through the 

 capillaries of the porous mass. This is known as eledroendosmosis, 

 shortened to eledrosmose. If, again, matters are reversed, and 

 the water is forced through the capillaries by mechanical means 

 (hydrostatic pressure), a potential is produced. The electric 

 force thus generated is known as a stream potential. It is the 

 reverse of electroendosmosis. 



Colloidal phenomena such as we have been considering are 

 grouped by FreundUch under the term electrokinetics. To 

 recapitulate: (1) Electroendosmosis (electrosmose) is the flow- 

 ing of a liquid through capillary tubes under the influence of 

 an applied electromotive force; (2) cataphoresis (electrophoresis) 

 is the migration of soUd particles in a liquid in response to an 

 appUed external electromotive force; (3) stream potential is the 

 electromotive force set up by the mechanically impelled move- 

 ment of a Hquid through capiUaries; and (4) the Dorn effect is 

 the potential produced by the motion (falling) of solid particles 

 through a hquid. Electroendosmosis and cataphoresis are forms 

 of motion caused by an appHed electromotive force, while stream 

 potentials and the Dorn effect are forms of motion that produce 

 an electromotive force. 



Of these four phenomena, cataphoresis is the one that has been 

 most studied, but electroendosmosis first held the attention of the 

 physicists. Both were simultaneously discovered in one experi- 

 ment more than a century ago, though it is doubtful if the experi- 

 menter understood the full significance of his findings. In 1807, 

 the Russian physicist Reuss placed two glass tubes in a piece of 

 wet clay, filled the tubes with water, and connected them to a 

 voltaic cell. He observed that at the positive pole the water 

 became milky because of the migration of the fine clay particles. 

 At the negative pole, the water remained clear but increased in 

 volume. The first observation was one of cataphoresis; and the 

 second, of electroendosmosis. When the movement of colloidal 

 particles in an electric field takes place in capillaries, a certain 

 amount of electroendosmotic flow usually accompanies cata- 

 phoretic migration, because the same electric force that causes the 

 colloidal particles to move will also cause the water, where it 

 touches a wall or membrane, to flow. 



