792 



THE MECHANISM OF AGGLUTINATION 



ranges instead of agglutinating and that this effect is also due to the potential. A pro- 

 tective colloid and precipitating colloid can, therefore, not be separated.' The addi- 

 tion of proteins, etc., sometimes has a marked effect on the properties of a suspension 

 by affecting the critical potential. This effect will be discussed more fully below. 



As far as the writer is aware, there are no complete potential measurements in 

 the case of the mutual precipitation of two oppositely charged suspensions, so that 

 it cannot be determined whether or not this is also purely a question of lowering the 

 potential below the critical value. Linder and Picton, however, showed that in the 

 presence of an excess of one of the components the entire suspension was stable and 

 had the charge (qualitatively) of the excess suspension, and that the flocculated sus- 

 pension had a lower p.d. than either of the suspensions alone. 



8 31.2 



Fig. 3. — The effect of the pH on the combination of antibody and bacteria and on the potential 

 and quantity of serum required to cause flocculation. 



THE COHESIVE FORCE 



In the experiments discussed so far it has been possible to predict whether or not 

 the suspension would precipitate simply by measuring the potential after the substance 

 under investigation had been added. There are a number of cases, however, where 

 this is not the case, and it is necessary to measure another variable. This has always 

 been a marked property of bacterial suspensions. It was noted by Neisser and Friede- 

 mann^ that suspensions of t>7)hoid bacilli were not agglutinated by mono- and divalent 

 salts as were suspensions of the organism treated by immune serum. This observa- 

 tion was confirmed by Buxton.-' The experiments were repeated by the writer,'' and 

 the potential measurements were made. The results were the same as those found by 

 Neisser and Friedemann. The experiments are shown graphically in Figures 4 and 5. 

 Figure 5 shows that in all cases where the potential was lowered to less than about 

 13 mv. by a concentration of salt of less than o.oi m, agglutination occurred, as would 



» Freundlich, H., and Loning, E.: Fest. Kaiser Wilhelm Gcs. Forderung Wiss. (10. Jahr. Jub.), 

 p. 82. 1921. 



2 Neisser, M., and Friedemann, U.: Miinchen. nied. Wchnschr., No. 19. 1904. 



3 Buxton, B. H.: Ztschr. phys. Chemie, 57, 47. 1907. 



4 Northrop, J. H., and de Kruif, P. H.: J. General Physiol., 4, 639. 1921-22. 



