640 STABILITY or BACTERIAL SUSPENSIONS. II 



tion of suspensions are governed by entirely distinct forces. In the 

 case of oil emulsions Powis^ was able to show that agglutination oc- 

 curred whenever the potential became less than a certain critical 

 value, in this case about 30 millivolts. Powis' experiments leave 

 little doubt that the potential at the oil-water surface is the deter- 

 mining factor in the agglutination of oil emulsions. Burton^ also 

 found that metallic suspensions coagulate in the zone where the 

 potential is small, although he did not find such a definite critical 

 value. 



In the case of bacteria, however, the results have been much less 

 satisfactory. It was found by Bechhold,^ Arkwright,^ Teague and 

 Buxton,^ and others that bacteria were always negatively charged 

 whether or not they were agglutinated. These authors concluded, 

 therefore, that the potential carried by the organisms could not ac- 

 count for the phenomena. Putter^ was able to show some qualitative 

 agreement between the potential and agglutination of Bacillus 

 typhosus. 



Results of the Present Experiments. 



It is evident that in order to test the hypothesis outlined above, 

 it is necessary to measure both the force which tends to cause the 

 particles to adhere as well as that which keeps them apart, since if 

 both forces are affected by the conditions of the experiment but only 

 one is measured, it will be impossible to interpret the results. The 

 potential may be conveniently measured by the rate of migration in an 

 electric field. The attractive forces, however, have usually been 

 assumed to remain constant and no attempt has been made to mea- 

 sure them. It was found, in the course of the present experiments, 

 that a comparative measure of the attractive forces between the 

 organisms could be obtained by measuring the force required to tear 

 apart two glass plates covered with a film of the bacteria and im- 

 mersed in the solution which was under investigation. As a result of 



2 Powis, F., Z. physik. Chem., 1914, Ixxxix, 186. 



4 Bechhold, H., Z. physik. Chem., 1904, xlviii, 385. 



^ Arkwright, J. A., /. Hygiene, 1914, xiv, 261. 



^ Teague, O., and Buxton, B. H., Z. physik. Chem., 1907, Ivii, 76. 



^ Putter, E., Z. Immunitatsforsch., 1 leAbL, Orig., 1921, xxxii, 538. 



