IX. ELECTROPHORESIS 297 



strength, type of buffer salts, and concentration of colloid compo- 

 nents. Such variations will probably be due to interactions of the 

 colloid components with each other or with salt components in the 

 solution. In order for the analytical patterns to yield true values, 

 i.e., in order for the aieas under the various peaks to truly denote 

 relative concentrations of the various components, it may be neces- 

 sary to search for conditions under which such interactions are mini- 

 mized. While no generalizations concerning this phenomenon can 

 be given as yet, it is mentioned here as a precaution against a too 

 ready acceptance of any given pattern as representing a definite and 

 invariable analytical relationshi]) for the mixture involved. 



In view of the importance of electrosmosis effects in the micro- 

 electrophoresis method, in which measurement of electrophoresis must 

 be made at a definite depth in the observation cell in order to avoid 

 errors due to electrosmotic displacement of the liquid along the wall 

 of the cell, it might be expected that some precautions would be re- 

 quired in the moving-boundary method in order to take similar elec- 

 trosmotic effects into account. While such displacements of the 

 fluids along the walls of the U tube undoubtedly occur, experiments 

 all seem to indicate that they cause no observable effects upon the 

 positions of the boundaries. This probably results from the circum- 

 stance that the density differences that exist across the boundary 

 serves to make this plane in the fluid system limiting with respect to 

 electrosmotic movements that will occur within either solution meet- 

 ing at the boundary. That is, circulation of the fluid in either solu- 

 tion arising from electrosmosis does not penetrate into the body of 

 the other solution but is stopped there, the return flow in each being 

 restricted to that solution. This circulation, with the boundary as 

 the limiting plane, may promote a slight mixing there so that both 

 rising and descending boundaries broaden out faster under the in- 

 fluence of the electric field than would result from the diffusion process 

 alone. It is probable, also, that in any case in which the difference 

 in density of the two solutions is small {e.g., dilute colloid solutions) 

 disturbances due to electrosmosis would cause inordinately high de- 

 grees of mixing of the solutions and could entirely invalidate the 

 method. Experience has indicated that for this reason, as well as 

 for the reason that the refractive index method for boundary detec- 

 tion requires appreciable concentration differences at the boundary, 

 the moving boundary method should not be employed on solutions of 

 less than about 0.25% of colloid component. 



