222 Messrs. W. B. Hardy and H. W. Harvey. [June 21, 



electrodes, the observed effects would be produced by an unequal loss of heat 

 at the two ends of the trough, due to the disposition of the materials, to 

 differences in their specific heat, or to asymmetrical conductivity of heat. 

 The result would be an unequal rise of temperature in the two halves of the 

 chamber, and consequent differences in surface tension. If this explanation 

 be correct, though the direction of the movement of submerged particles is 

 independent of the direction of the current, the velocity past the observer 

 should vary. This was found to be the case. 



An analysis of the movements of the floating particles based on this 

 hypothesis shows that in stronger fields the surface skin itself is dragged 

 along. The following is an example : — Field approximately 35 volts per 

 centimetre. Movement of floating particles always towards the right, but 

 by reversal of current the velocity towards the cathode was 2 * towards 

 anode 10. The drift due to heating therefore was 6, and the migration 

 4 divisions per second, and the latter was towards the anode. The surface 

 film therefore was negative to the subjacent water. 



Appendix, July 26, 1911. 

 The Electrification of Surface Films. 

 By W. B. Hardy, F.K.S. 

 The observations recorded in the preceding paper upon the endosmotic drift 

 of the water in contact with a surface film involving foreign matter throw 

 some light upon the range of molecular attraction. Under the conditions of 

 the experiments, and for the short periods during which the current was on, 

 it may be taken that there was no sensible hydrostatic pressure established 

 due to change of level between the two ends of the trough. Under these 

 conditions, if u be the velocity of the water past the anchored surface film, 

 we have 



d<b 1 , 



ax 7 



where a is the electric density, and 7 is a coefficient of sliding friction of 

 water over the film. 



The surface film acts as a thin sheet past which the fluid can flow, just as 

 when the thickness of a soap film exceeds the range of molecular forces the 

 interior mass may flow past the surface films which act as fixed boundary 

 walls. 



Whatever view be taken of the physical significance of the coefficient 7 it 

 must be related in some simple way to the forces of attraction of the water 



* Measured in divisions of the micrometer scale. 



t H. Lamb, ' British Association Eeport,' 1887, p. 495. 



