JOHN H. NORTHROP . 787 



to that of prot?in particles and that the observed migration agreed quahtativcly with 

 that expected on the basis of a Donnan equihbrium.' 



This theory accounts satisfactorily for the observation that the sign of charge 

 changes at the isoelectric point of the particle when the latter is amphoteric, and ac- 

 counts also for the relation between the ionization and the sign of charge. It is known, 

 for instance, that substances which tend to dissociate as acids are usually negative, 

 and basic substances positive. 



Von Hevesy- considers the particles as analogous to large ions. It is true that the 

 rate of migration is about the same. There would seem to be a definite difference, 

 however, between the mechanism by which the solution as a whole is kept electrically 

 neutral. In the case of an ion there is always an equal number of ions of the opposite 

 charge in the solution, whereas, according to the Lamb-Helmholtz theory, each par- 

 ticle as a whole is electrically neutral. In any case, it is certain that the p.d. of the 

 particles is closely connected with the presence of electrolytes in the solution. It has 

 been found by numerous workers that the more carefully the solution was freed from 

 electrolytes the lower the potential and the more unstable the suspension.^ 



On the other hand, Laing has been able to account for the electrical migration 

 properties of soap solutions on the assumption that the micelles are, fundamentally, 

 large ions differing from ordinary ions merely in that they possess a high valence. This 

 point of view has been accepted by McBain,^ and there is undoubtedly considerable 

 evidence in its favor. It appears to the writer more probable than the Lamb- 

 Helmholtz theory. In view of this fact, it is evident that the potential calculated in 

 the ordinary way from the Lamb-Helmholtz formula is of very uncertain meaning 

 and may have no real physical significance. It is, however, probably proportional to 

 the charge carried by the particle, and since it serves as a convenient unit it will be 

 retained in the following discussion. If the particles are analogous to large ions then 

 an increase in the rate of migration does not mean an increase in the potential between 

 the surface of the particle and the surrounding liquid but merely an increase in the 

 number of charges per unit of surface. It also follows that according to this theory 

 the charge on the particle should increase by steps corresponding to the loss or gain 

 of one electron while on the basis of the Donnan or Lamb-Helmholtz theory the 

 charge might be expected to increase gradually since it is essentially a statistical re- 

 sult. Unfortunately, the expected change in migration rate owing to the gain of one 

 electron in the case of particles of visible dimensions in water is below the limit of 

 accuracy of the present measurements. 



The general opinion at present appears to be that the charge is conferred by the 

 combination of the particle with an ion, although the nature of this combination is 

 uncertain. 



' Winslow, C.-E. A., Falk, I. S., and Caulfeild, M. F.: /. General Physiol., 6, 197. 1924; Winslow, 

 C.-E. A., and Shaughnessy, H. J.: ibid., p. 697. 1924; Winslow, C.-E. A., and Fleeson, E. H.: ibid.. 

 8, iQS. 1925- 



'von Hevesy, G.: Kolloid-Ztschr., 21, 129. 1917. 



3 Beans, H. T., and Eastlack, H. E.: /. Am. Chem. Soc, 37, 2667. 1915. 



•'McBain, J. W.: Colloid. Symp., 4, i. 1925. 



