ELECTROKINETICS 



359 



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Methods. — Electroendosmotic flow can be readily demon- 

 strated by the simple apparatus shown in Fig. 155. A sack made 

 of collodion is filled with and immersed in water. Two electrodes 

 — the anode without and the cathode within the sack — connect 

 the system to a source of potential. The sack is tightly corked, 

 with an outlet for a glass tube. After current is applied, water 

 very slowly rises in the tube because 

 of electroendosmotic flow through the 

 pores (capillaries) of the coflodion 

 membrane. 



Cataphoretic flow may be observed 

 by fastening two metal strips to a 

 microscope slide, putting several drops 

 of a colloidal suspension, e.g., diluted 

 milk, between the metal electrodes, 

 and then placing a glass cover slip on 

 top; but it is just this simple experi- 

 ment which led so many earlier workers 

 in the field, and still leads beginners, 

 astray. In such a simply made 

 chamber the cataphoretic flow is com- 

 pletely disturbed by the electroen- 

 dosmotic flow. Results purporting to 



find cells positively charged are due to Fig- 155. — Demonstration 



the use of such a primitive chamber. "Lt^Xf^^^T'^^ll 



A U tube, into the ends of which pro- the outer positive pole, 



ject two electrodes, suffices for direct tZt.tl^r^rL'^^l 



macroscopic observation of cataphoresis pole; rise of the water in the 

 U, Fig. 156). The clouding up of capillary tube indicates flow. 



the colloidal solution near one electrode in one end of the 

 tube indicates the direction of flow. If a negatively charged 

 arsenic trisulphide suspension is placed in a U tube, and 

 a direct current of 110 volts applied through electrodes, 

 within an hour the anode (positive) end of the tube will show a 

 more dense color, indicating a higher concentration of the colloid, 

 while the opposite negative pole has become clear, the water here 

 having been robbed of its colloidal particles. 



Similar in principle is the cataphoretic tube of Pauli (5, 

 Fig. 156). It was designed primarily for the study of protein 

 migration. Acid or alkaline protein is put in the lower part of the 



