JOHN H. NORTHEOP AND PAUL H. DE KRUIF 663 



The effect of dialyzed normal and of powerful antityphoid horse 

 serum on the potential and agglutination of Bacillus typhosus is given 

 in Fig. 5, The two upper curves are the results in 0.3 n NaCl. There 

 was no complete agglutination in the absence of salt and no marked 

 difference between the normal and immune serum, although both 

 affect the potential. (The serum was prepared by dialysis against 

 distilled water and then dissolved by the addition of a small amount 

 of NaOH. Conductivity measurements showed that the total con- 

 centration of salt was less than 0.001 n; i.e., too small to cause the 

 noted effect on the potential.) In the presence of salt, on the other 

 hand, there is no effect on the potential but agglutination occurs in 

 very high dilution with the immune serum and to a much less extent 

 with the normal serum. This experiment shows that the effect of the 

 serum on a suspension of bacteria in concentrated salt solution is not 

 primarily on the charge but on the cohesive force. The serum raises 

 the cohesive force and hence the critical charge to a value greater 

 than the potential carried by the organism and they therefore aggluti- 

 nate. The effect of the serum on the cohesion is shown in Fig. 6. 

 The upper part of the figure shows that the addition of serum raises 

 the cohesion to the value in distilled water; i.e., it prevents the salt 

 from decreasing the attractive force and thereby lowering the critical 

 potential. The lower part of the figure shows the converse experi- 

 ment; i.e., the effect of salt on a film of washed, and of sensitized 

 organisms. The salt decreases the cohesion of the washed organisms 

 very markedly but has no effect on the cohesion of the film sensitized 

 with serum. 



The effect of varying both the salt and the serum concentration on 

 the agglutination is shown in Table V. As the serum concentration 

 is increased, the salt concentration in which complete agglutination 

 occurs widens on both sides from 0.10 n. The lower limiting concen- 

 tration of salt remains at about 0.01 n, however, and does not con- 

 tinue decreasing as the serum increases. In other words, the effect 

 is not additive, but there is a critical concentration of serum beyond 

 which there is little or no effect on the concentration of salt needed to 

 agglutinate. This "critical" salt concentration corresponds to the 

 point at which the charge on the organisms is about 10 millivolts; i.e., 

 just under the critical potential. This is the result expected if the 



