JOHN H. NORTHROP AND PAUL H. DE KRUEF 641 



these measurements in conjunction with the measurements of the 

 potential difference, it has been found that whenever the potential 

 difference between the surface of the bacteria and the solution is less 

 than about 15 millivolts the bacteria agglutinate, provided the cohesive 

 force is not afected. If the cohesive force is decreased, this critical 

 potential is decreased, and if the cohesive force is made very small, 

 no agglutination occurs even though the potential be reduced to zero. 

 It was further found that all electrolytes tested in concentrations less 

 than 0.01 to 0.1 N affect primarily the potential, while in concentra- 

 tions greater than 0.1 n the effect is principally on the cohesive force. 

 In the case of bacteria sensitized with immune serum, the cohesive 

 force remains constant and the agglutination can be predicted solely 

 from the measurement of the potential. 



Experimental Methods. 



Measurement of the Potential. — The potential was determined from the rate 

 of migration as described in the preceding papers.^ The U-tube method was used 

 for the experiments with the bacillus of rabbit septicemia and the microscopic 

 method with B. typhostis. 



Measurement of the Cohesive Force. — A piece of thick glass slide was covered 

 with a thin film of very heavy suspension of washed organisms {B. typhosus), 

 the film allowed to dry and then heated to 60° for a few minutes. This causes 

 the bacteria to adhere firmly to the glass. A heavy (No. 3) cover-slip was simi- 

 larly prepared. The cover-slip was suspended by means of a fine platinum wire 

 from the lever of the du Noiiy^ surface tension apparatus. The glass slide was 

 immersed in a dish containing the solution to be studied and the cover-slip allowed 

 to rest on it with its own weight for 1 minute. The force required to pull the cover- 

 sHp from the slide was then determined. It was found that if the measurement 

 was made immediately after the two surfaces came in contact, the value obtained 

 depended on the force with which the two had been pressed together. If the slip 

 had been pressed down firmly a much greater force was required than if it had 

 simply been allowed to rest on the slide. After a short time interval, however, 

 this difference became less, and eventually the same reading was obtained in both 

 cases. This is due presumably to the fact that the distance apart of the two sur- 

 faces is regulated by capillary forces and comes to the same point from either side. 

 The same smear was used as long as the same value was obtained on replacing the 

 preparation in distilled water. The value obtained becomes less after ten or 



8 Northrop, J. H., and CuUen, G. E., /. Gen. Physiol., 1921-22, iv, 635. Nor- 

 throp, J. H., J. Gen. Physiol., 1921-22, iv, 629. 



9 du Noiiy, P. L., J. Gen. Physiol, 1918-19, i, 521. 



