278 DAVID R. BRIGGS 



solution within the cell caused by the heat effects of the large electric currents 

 that will pass at higher conductivities of the solution. 



(4) The shape, size, and orientation of the particles may be directly 

 observed and the mobilities of the various particles may be compared by 

 watching them move simultaneously. 



(5) Small quantities of disperse phase are required for a determination; 

 in fact too high a density of particle population must generally be avoided in 

 order to prevent confusion in the microscope field. 



(6) It may be adapted to electrophoresis measurements in nonaqueous 

 mediums. 



(7) Measurements may be made through a wide range of pH, with a 

 minimum of time consumed. 



(8) A very distinct advantage lies in the fact that individual determina- 

 tions require only a few minutes. 



(9) The apparatus is inexpensive and relatively simple to set up and to 

 use. 



An important extension in the applicability of the microelectro- 

 phoresis method occurs in the case of soluble materials of such small 

 dimensions as to be microscopically invisible themselves but which, 

 being surface active, will accumulate on the surfaces of visible par- 

 ticles and impart to the surfaces of such particles the electrophoretic 

 properties of the adsorbed substance. Particles of nonionogenic 

 solids, such as finely ground quartz or finely dispersed collodion, 

 which show a relatively high interfacial tension in contact with water, 

 will adsorb many water-soluble, surface-active substances such as 

 albumins, gums, soaps, etc. to such an extent that the electrical prop- 

 erties of the original solid-water interface are completely masked 

 by those of the newly formed, adsorbed substance-water interface. 

 The mobihties of such covered particles can then be determined micro- 

 electrophoretically and have repeatedly been found to agree closely 

 with the mobilities of the same materials when in solution as measured 

 by the moving-boundary method. This is an experimental fact that 

 is not easily explained on the basis of electrokinetic theory, but it is a 

 fortunate situation insofar as the experimenter is concerned since it 

 makes possible the determination of the mobility properties of many 

 substances by use of the relatively simple micro technique which 

 otherwise could only be studied by the moving-boundary method, 

 notably a more time-consuming operation. Isoelectric points of 

 soluble, purified protein preparations are usually determined by this 

 procedure. The investigator should be warned, however, that where 

 mixtures of materials of varying degrees of surface activity are being 



