IX. ELECTROPHORESIS ' 281 



sharply at the correct depth and to realize that the greater the di- 

 ameter of the particle being observed the greater the probability that 

 it will overlap above or below this exact depth and thus be given a 

 plus or minus viscous drag by the moving lamellae of liquid above or 

 below that of zero liquid flow. The larger the diameter of the parti- 

 cles being observed the thicker should be the lumen of the cell for 

 greatest accuracy. 



The proper cross-sectional depth of the flat cell lumen and the 

 optimal thickness of the glass plates making up the top and bottom of 

 the cell will be determined roughly by the size range of the particles 

 to be studied. It is always necessary to be able to focus the micro- 

 scope both on the top and on the bottom of the cell lumen in order 

 that the 0.21 or 0.79 depths can be found. The maximum optical 

 distance from the bottom of the cell lumen to the top of the top plate 

 must be less than the focal length of the microscope objective that 

 may be required to resolve the particles. Where very small particles 

 are being followed and water or oil immersion objectives are needed, 

 the lumen depth and cell wall thickness must be held at a minimum. 

 Cells made from polished glass slides 0.5 to 0.6 mm. in thickness and 

 assembled with a cell lumen depth of the same order of magnitude 

 will be found usable with the smallest visible particles and will ap- 

 proach the limits for construction of such cells. The cell should be 

 of one piece of glass in order that the cleaning (with the alkaU and 

 chromic acid solutions) needed to render the cell walls free of contam- 

 inants may be easily carried out. The all-glass cell also eliminates 

 troublesome movements of liquid in the cell due to the vibrational or 

 otherwise chance disturbances resulting from elastic distortions of rub- 

 ber or other flexible tube connections. 



The electrodes through which the electric field is applied across 

 the ends of the cell must be nonpolarizing and of sufficient current- 

 carrying capacity so that no gassing will occur. The cell must be so 

 constructed that no contaminating electrolyte from the electrodes 

 will reach the point in the cell where measurements are being made, 

 since even infinitesimal amounts of such electrolytes by changing the 

 ionic strength of the solution will have profound effects in reducing 

 the mobilities of particles, particularly when measurements are 

 being made at low ionic strengths. The cell should be so constructed 

 that quick and easy change of the contents (of the flat portion of the 

 cell where actual measurement is to be made) can be accomplished 

 mthout danger of washing in electrolytes from the electrode regions. 



