JOHN H. NORTHROP 



633 



only those particles that are sharply in focus. This error could be 

 lessened by using a deeper cell. It was found, however, that in a cell 

 2 mm, deep there was irregular drifting of the particles. Since the 

 final value depends as a rule on the difference of two experi- 

 mental values, the percentage error is larger the smaller the velocity. 



TABLE I. 



Rate of Migration of B. typhosus Suspension in Distilled Water. 

 Potential gradient = 4.5 volts per cm. 



Average n per second at all depths —11.9 



Average n per second calculated for potential gradient of 1 volt per cm. — 2 . 65 



Potential bacteria water, —33.5 millivolts. 



* The sign refers to the apparent sign of charge of the particle, i.e. + indi- 

 cates migration to the cathode. 



Apparent Reversal of the Charge on the Glass. — It was noted that 

 the direction of migration of the water reversed at times under the 

 same conditions that caused a reversal in the motion of the particles. 

 It is known that the sign of the charge on glass cannot be reversed so 

 that this result is at first sight anomalous. The explanation is simple, 

 however, since microscopic examination of the cell shows that it 

 becomes more or less coated with the organisms, which adhere firmly 

 to the glass. The cell wall is, therefore, no longer glass but is partially 

 composed of the same material as the suspension and therefore reverses 

 its charge under the same conditions as does the suspension. 



