JOHN H. NORTHROP 785 



however, owing to the irregular size of the clumps and the uncertainty as to the sur- 

 faces. In any case most bacteria are so large that at equilibrium the individual bac- 

 teria as well as the clumps would be collected on the bottom of the vessel. The theory 

 may be of importance, however, in the case of the filterable viruses. 



FACTORS CONTROLLING THE FORMATION OF AGGREGATES 



If a Stable suspension is observed under the microscope, it may be seen that, al- 

 though the particles approach one another, they do not actually collide. If some sub- 

 stance is now added which precipitates the suspension, the particles then collide and 

 stick together. Since they sometimes adhere to one another and sometimes remain 

 separate, there must evidently be a force which tends to keep them apart and another 

 force which holds them together. If the repulsive force is greater than the cohesive 

 force, or greater than the momentum of the particles due to their movement, the 

 particles will remain separate, whereas if it is less they will adhere into larger aggre- 

 gates. 



It has long been known that particles or surfaces in contact with liquids are elec- 

 trically charged, since under the influence of an external e.m.f. the particles move. If 

 the liquid moves while the solid is kept stationary, the phenomenon is known as "elec- 

 tro-endosmosis," whereas if the particles move through the liquid, it is known as 

 "cataphoresis."' It was early suggested that it was this repulsion, due to the charge 

 carried by the particles, which prevented their touching one another, and practically 

 all theories of the stability of suspension depend in some way on this potential dif- 

 ference between the particles and the surrounding liquid. In order to trace the con- 

 nection between this property and the behavior of the particles, it is necessary to 

 touch somewhat on the nature and origin of this potential difference. 



METHOD OF MEASUREMENT AND PROBABLE NATURE AND ORIGIN OF THE 

 CHARGES OF COLLOIDAL SUSPENSIONS 



All methods of measurement of the charges carried by colloidal suspensions de- 

 pend on determining the motion of the particles in an external electric field. This may 

 be done either by noting the movement of the boundary of the suspension as a whole 

 in a U-tube^ or by following the motion of a single particle under the microscope or 

 ultramicroscope. (A convenient type of apparatus for these measurements has been 

 described by the writer.)^ 



The potential between the surface of the particle and the surrounding film of 

 liquid may then be calculated by the Lamb-Helmholtz formula, 



47/u7r 



P.D.=-^ 



in which tj is the viscosity, K the dielectric constant of the surface layer, i; the velocity 

 in centimeters per second, and A' the potential gradient. All electrical units are elec- 



' Cf. Freundlich, H.: op. cil., p. 326. Leipzig, 1922; Burton, E. F.: op. ciL, p. 125. 1921. 



"Burton, E. F.: op. cit., p. 131. 1921. 



3 Northrop, J. H.: /. General Physiol., 4, 629. 1921-22; Northrop, J. H., and Cullen, G. E.: 

 ibid., p. 635. 1921-22; Northrop, J. H., and Kunitz, M.: ibid., 7, 729. 1925. 



