JOHN H. NORTHROP 797 



been reduced to 6 mv. The action of antiserum is therefore the same as that of other 

 substances except that in the case of the antibody the reaction takes place only with 

 the bacteria used to produce it. The action of the other substances is not so specific. 

 Shibley found in a study of diffuse and granular strains of streptococci that here 

 also the difference was due to the cohesive force rather than to a difference in poten- 

 tial.' In the autoagglu finable strains agglutination occurred whenever the potential 

 was reduced below a critical value, and measurements of the cohesive force by the 

 method described above showed that this was not reduced in concentrated salt. The 

 diffuse strains are not agglutinated by concentrated salt even though the potential 

 is lowered, and measurements of the cohesive force show that this also is lowered by 

 the salt. The autoagglutinable strains of these bacteria therefore differ from the dif- 

 fuse just as sensitized B. typhosus differs from the unsensitized. 



EFFECT OF ADDED SUBSTANCES ON THE PROPERTIES 

 OF THE SUSPENSION 



In the cases just considered the critical potential was increased by the addition 

 of a second substance. The reverse condition in which the critical potential is lowered 

 and the suspension thereby made more stable is also well known, and substances which 

 cause this effect are classed as "protective colloids." 



Meyer and Lottermoser^ stated that a suspension acquires the properties of the 

 added substance, and this statement has been confirmed, in general, by all subsequent 

 investigators. There is little doubt that this is due to the formation of a surface film. 

 It was noted by Bredig,^ for instance, that molds grow on the surface of gold particles 

 which had been treated with gelatin. Arkwright-* found that a suspension of B. coli 

 in water containing soluble substances from B. typhosus was agglutinated by anti- 

 B. typhosus serum. This is a very sensitive test since the action of immune serum is 

 strictly specific. 



An equally striking result has been obtained by Jones^ who has found that collo- 

 dion particles treated with egg albumin are agglutinated with anti-egg-albumin 

 serum. This experiment is the more interesting since it suggests that the "precipitin" 

 reaction is essentially the same as agglutination. In the experiment noted the anti- 

 egg-albumin serum would produce the ordinary precipitin reaction if mixed with egg- 

 albumin solutions, but when added to the egg-albumin-coated collodion particles it 

 causes agglutination. . The same experiment may be performed with bacteria treated 

 with cow serum and washed. These bacteria are then agglutinated by anti-cow serum. 

 It has also been found by a number of workers that the isoelectric point of a suspen- 

 sion changes to that of the added substance.'' 



' Shibley, G.: /. Exper. Med., 39, 245. 1924. 



^ Lottermoser: Anorganische Colloidc, p. 50. igoi. 



3 Bredig, G.: Anorganische Fermente, p. 15. Leipzig, 1901. 



"I Arkwright, J. A.: J. Hyg., 14, 261. 1914. 



5 Jones, F. S.: J. Exper. Med., 44, 625. 1926. 



* Arkwright, J. A.: loc. cit.; Coulter, C. B.: J. General Physiol., 3, 317. 1920; 4, 403. 1921-22; 

 Loeb, J.: op. cit., p. 109. 1922-23; Northrop, J. H., and de Kruif, P. H.: op. cit., p. 655. 1921-22; 

 Eggerth, A. H., and Bellows, M.: loc. cit. 



