1911.] Note on Surface Electric Charges of Living Cells. 



225 



external electric field being taken as the same in all cases, they are : (1) the 

 electric density at the interface, (2) the coefficient of sliding friction (7), 

 and (3) variations in the attraction of water for different substances. 



Taking these in the order mentioned, so far as I know it the literature 

 of electric endosmose without exception supports the view that the electric 

 density on surfaces in contact with water varies within narrow limits. 

 The velocity of a submerged visible particle is independent of size and 

 shape, and varies directly with the electric density on the particle. It was 

 easy in our experiments to see chance fragments, motes of dust, and living 

 cells, travelling with velocities which agreed to within 1 or 2 per cent. 

 The evidence, therefore, is in favour of the view that the electric density 

 at the film-water interface did not differ much from that at the glass- 

 water face. 



By hypothesis 7 and <£ (f), the coefficient of sliding friction and the 

 intermolecular force, are dependent variables. If the thickness of the 

 matter on each side of the interface exceeds the range of molecular 

 attraction, ft varies directly as $ (/), where (/) refers only to the molecular 

 attraction across the interface. 



Here, again, there is evidence that ft does not vary. Putting the 

 external electric field at unity, the velocity of a particle is given by the 

 equation 



V = -<rjft* 



that is, in particles of 1 /x diameter and upwards, the velocity is inde- 

 pendent of size and shape. But if #(/) and therefore ft were different 

 for different substances, the velocity should depend upon the nature of the 

 particle. 



Instead of this being the case we find protein masses, metals, and motes 

 of dust in water, all moving in unit field witli velocities of from 10 to 

 20xl0~ 5 cm./sec, and the variations within this range can be traced to 

 the influence of the chemical nature of the particle upon the polarisation of 

 the interface. 



We are thus driven to the conclusion that the adhesion of the film to the 

 water practically reaches its maximum when the thickness is still much 

 less than the accepted value for the range of the molecular forces. 



In the case of a small sphere at a potential different from the water 

 urged along by an electric field, the hypothesis which has been adopted 

 would make 7 sensibly constant until the diameter of the sphere fell to 

 about 300 fifi, when the pressure at the interface would be about 90 per cent. 



* Lamb, loc. cit., p. 502. 



