ELECTROKINETICS 



361 



U tube. The central stopcocks are closed, and the remainder of 

 the apparatus rinsed and filled with acid or alkali (of the same 

 concentration). The stopcocks are then cautiously opened, and 

 current applied to the two electrodes. Within an hour (using 

 70 to 75 volts) , enough protein will have migrated toward one or 

 the other pole to give a precipitate when sulphosalicyUc acid is 

 added. If the protein is acid, it will migrate to the negative pole; 

 if alkaline, to the positive pole. Electrolyte-free albumin has a 

 very weak negative charge and therefore migrates to the anode. 



Most cataphoretic work involves the observation of individual 

 microscopic particles. An original apparatus for such work 



Fig. 157. — The Kruyt cataphoresis chamber. 



is that shown in Fig. 157. In America, the Northrop-Kunitz 

 model is extensively used (C, Fig. 156). In all cases, the appara- 

 tus consists of two electrodes and a central glass chamber. 

 The latter rests under the microscope objective. Each end of the 

 chamber is joined to glass tubes which lead to zinc electrodes 

 through two-way stopcocks. When an external electromotive 

 force is applied to the electrodes, the particles in suspension are 

 seen to migrate in one or the other direction. The rate of 

 migration is determined by timing a particle across a scale 

 inserted in the microscope ocular. Comparative results may be 

 expressed in terms of rate of movement in microns per second 

 per volt per centimeter. 



A number of factors are involved in the migration of a colloidal 

 particle in an electric field. One of these is the electrokinetic 



