JOHN H. NORTHROP 795 



concentrations. It is found in this experiment that the suspension at one end of the 

 series has a low positive potential and at the other end a low negative potential. There 

 must, then, be a place in between where there is no potential, even though there is a 

 very large error in the method. Nevertheless, no agglutination occurs in any tube in 

 the presence of concentrated sodium chloride. This is shown in Figure 6. The salt 

 evidently acts as though it prevented the particles from sticking together, even 

 though there is no force to hold them apart. It occurred to the writer that it might be 

 possible to measure this sticking or cohesive force by determining the force required to 

 separate two films of the suspension. This turned out to be the case. The measure- 

 ment was made by coating two pieces of glass with a thick smear of the suspension. 

 The glass was warmed slightly in order to cause the particles to adhere to it, and the 

 two films were allowed to rest together in the solution to be studied. The force re- 

 quired to tear the films apart was then determined by a torsion balance. The measure- 

 ment is rough and the conditions very different from those existing in the original 

 suspension, but nevertheless the results are surprisingly reproducible and show a very 

 marked effect of concentrated salt solutions on this cohesive force. Furthermore, 

 the effect is in the range of salt concentrations where the potential measurements fail 

 to predict the agglutination. The results of some of these experiments are shown in 

 Figure 7. It will be noted that the only solution which shows a second rise in this 

 value is hydrochloric acid, and this agrees with a second agglutination zone in this 

 acid (cf. Fig. 5). It is evident then that so-called "irregular series" are not always due 

 solely to the potential changes.' 



It was stated above that the addition of immune serum to a suspension of bacteria 

 caused them to act like collodion or oil particles, i.e., they agglutinate whenever the 

 p.D. is below the critical value. It might be expected, then, that the addition of serum 

 to bacteria whose potential was already below the critical value would cause aggluti- 

 nation without any change in the potential, but an increase in the cohesive force. This 

 is the result obtained, as is shown in Figure 8. The figure also shows that the cohesive 

 force of a smear treated with immune serum is not affected by the salt concentration. 



Under the ordinary conditions of bacteriological agglutination in .075 m salt, there- 

 fore, immune serum causes agglutination by increasing the cohesive force between 

 the organisms rather than by decreasing the potential. This effect on the cohesive 

 force is not shown by normal serum nor by ordinary protein solutions. It is possible, 

 however, to agglutinate bacteria either with normal serum or protein solutions pro- 

 vided the solution is at or near the isoelectric point of the protein used.^ Under these 

 conditions agglutination may be caused by a very minute amount of the protein or 

 normal serum. The agglutination is caused, however, by a change in the potential 

 of the suspension instead of a change in the cohesive force as is the case in immune- 

 serum agglutination. 



The increase in the critical potential of a suspension by the addition of another 

 substance is not confined to immune serum but has been found to occur with various 



'In saturated salt solutions there is again an agglutination effect (cf. Porges, O.: Centralhl. f. 

 Bakteriol., Abt. I, Orig., 40, 133. 1905). This is probably a true solubility effect similar to that 

 found by Loeb in the case of gelatin-coated particles. 



^ Eggerth, A. H., and Bellows, M.: /. General Physiol., 4, 669. 1922. 



