JOHN H. NORTHROP 



789 



ity was closely connected with the potential. The experiments were then carefully 

 carried out by Powis/ who made accurate measurements of the p.d. between the oil 

 drops and surrounding Hquid in a series of electrolyte solutions and found that when- 

 ever the potential between the drops and the surrounding liquid was reduced below 

 about 30 mv., the particles collected into larger aggregates. A summary of Powis' 

 results is given in Table I. They leave little doubt that in this case the potential is the 

 decisive factor. It will be noted, however, that it becomes necessary to assume a criti- 

 cal P.D. instead of Hardy's zero potential. Powis found later that in the case of arsenic 

 sulphide suspensions this critical p.d. was different for different salts. (This effect 

 will be discussed below in connection with work on bacterial agglutinations.) A num- 

 ber of experiments were carried out by the writer on suspensions of typhoid bacteria. 



-i9 



10-& 



10-5 



10-'^ ^ ^ 10-i 10-2 



Salt Conccntra+ion, equivalents per liter 



10- 



1.0 



Fig. I. — Effect of various electrolytes on the agglutination of typhoid bacteria sensitized with 

 immune serum; complete agglutination; partial or no agglutination. 



It was found that when these had been treated with an excess of antiserum they be- 

 haved in the same way as Powis' oil drops, i.e., agglutination always occurred when- 

 ever the potential was reduced below a critical value of about 15 mv. A summary of 

 these experiments is shown in Figure i. 



A similar series of experiments was carried out by Loeb^ on suspensions of collo- 

 dion particles. The results are summarized in Table II. Here again there is no doubt 

 that agglutination occurs whenever the p.d. is reduced below about 15 mv. The same 

 result was obtained when the particles were previously treated with egg albumin, 

 and with particles of denatured egg albumin.^ These experiments leave little doubt 

 that in these cases the potential is the determining factor for the stability of the sus- 

 pension. It is possible to predict from a measurement of the cataphoretic p.d. alone 

 whether or not the suspension will remain stable. It also follows that the cataphoretic 

 potential is the decisive one for the prevention of agglutination and, further, that this 

 potential must be directly proportional to the rate of migration in the electric field as 

 predicted by the Lamb-Helmholtz formula. 



^ Powis, F.: ibid., 89, 91, 179, 186. 1914-15. 



*Loeb, J.: op. cit., p. 12^. 1922-23. sLoeb, J.: ibid., p. 485. 1922-23. 



