THE STABILITY OF BACTERIAL SUSPENSIONS. 



I. A Convenient Cell for Microscopic Cataphoresis 

 Experiments. 



By JOHN H. NORTHROP. 

 (From the Lahoralories of The Rockefeller Institute for Medical Research.) 



(Received for publication, May 10, 1922.) 



Measurements of cataphoresis of particles in an electric field are 

 complicated by the fact that in addition to the motion of the particles 

 relative to the water, the water itself moves at the surface of the cell. 

 In macroscopic measurements the cell is large enough so that the 

 movement of the water as a whole is negligible. If the measurements 

 are made in a narrow cell, however, the motion of the water is very- 

 great and must be corrected for. The theory and technique of such 

 measurements have been thoroughly discussed by Ellis^ and Powis.^ 

 As Ellis pointed out, the total motion of the water in a closed cell 

 must be zero, since the water which moves one way at the surface of 

 the glass must return in the opposite direction in the center of the cell. 

 The average observed motion of the particles at all depths relative to 

 the cell must, therefore, be the true motion of the particles relative to 

 the water, which is the desired value. It is also necessary to use some 

 form of non-polarizable electrodes in order to avoid the formation of 

 gas bubbles. The cell devised by Powis answers the requirements 

 but is troublesome to use if a large number of experiments are made. 

 The cell shown in Fig. 1 has been found very convenient. 



Conslriiction of the Cell. — The cell itself is made of a thin slide 

 resting on strips of glass about 0.8 mm. thick cemented to a thick 

 glass slide. Two blocks of thick glass are cemented on top of the cover- 

 slide at each end of the cell. The ends of the cell are then ground 

 smooth on an emery wheel. A piece of thick walled glass tubing is 

 widened and flattened at one end so as to cover the end opening of the 



1 Ellis, R., Z. physik. Ghent., 1911, Ixxviii, 321; 1912, L\xx, 597. 

 ^ Powis, F., Z. physik. Chetn., 1914, Ixxxix, 91. 



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