IX. ELECTROPHORESIS 279 



studied, the mobility properties observed ma,y apply to only one (the 

 most surface-active) component present or it may apply to no one 

 component but to the equilibrium mixture of components (if all are of 

 comparable adsorbability). The relative effectiveness of the various 

 components may well vary also with such conditions as pH and ionic 

 strength or with the nature of the solid particle employed. Such 

 possible limitations of the method must always be considered in 

 drawing conclusions from observations made on native mixtures. 



2. Method of Observation 



For the above reasons the microelectrophoresis method will usu- 

 ally be preferred for the study of the electrophoretic mobilities of 

 solid or liquid particles of microscopic size, of monocellular organisms, 

 or body cells. It will find frequent application in studies on purified 

 preparations of proteins or other soluble high molecular substances 

 that are strongly adsorbable on microscopically visible particles. A 

 variety of apparatus has been devised for this purpose. The essen- 

 tial features, which are common to all, of these apparatus, are: 

 (a) an observation cell in which the solution containing the particles 

 is placed and which is of such construction that a microscope can be 

 focused upon the particles contained therein ; (6) the ends of this ob- 

 servation cell are connected to electrodes through which an electric 

 field can be introduced to the solution in the cell ; and (c) some pro- 

 vision is usually made by which the cell can be filled and emptied 

 without disturbing the assembly after it is mounted on the micro- 

 scope stage. These apparatus vary primarily as to shape (flat or 

 cylindrical) and size of lumen of the cell, in the nature of the electrode 

 systems employed, in the manner of assembly, and in the manipula- 

 tion required during measurements. 



In choosing a cell for microelectrophoresis work, a number of im- 

 portant characteristics that will determine the ease and accuracy 

 with which measurements may be made need to be considered. The 

 mobility (cm.Vv. sec.) of a particle is obtained from the simultaneous 

 measurements of two quantities: (a) the velocity (cm. /sec.) of the 

 particle and (6) the field strength (v./cm.) acting on the particle. 

 The velocity of the particle is obtained by observing with the aid of a 

 calibrated eyepiece micrometer the displacement experienced by the 

 particle in unit time as caused by the applied electric field. It is es- 

 sential, however, that this measurement of particle velocity be made 



