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



In spite of this, we incline to the view that the surface charge does vary 

 with variations in the state of activity of the living cell, for in a natural 

 mixed culture of Gonium, Vorticella, and Amoeba, the fact that different 

 cells of the same species migrated at different rates was very noticeable. The 

 observations were made in the water in which the cells had been living, 

 exposed to air, so as to leave the respiratory exchange normal. Eed blood 

 corpuscles are living cells, with very slight or no intrinsic chemical activity. 

 In correspondence with this, they were found to migrate in blood serum to 

 the anode at a remai'kably uniform rate. 



Contact Potential at the Free Surface of Water. — When finely powdered 

 graphite was sprinkled upon distilled water contained in the observation 

 cell already described, and the current, led through non-polarisable elec- 

 trodes, was not more than 0*002 ampere, the following phenomena were 

 noticed : — Of the graphite particles some broke through the surface of the 

 water and sank slowly, others floated unwetted ; the latter therefore served 

 as an index of the movements of the actual skin. Except near the upper 

 and lower surfaces the graphite particles migrated to the anode, just below the 

 free surface and just above the glass they migrated to the cathode. The 

 unwetted floating particles either did not migrate at all, or performed 

 relatively slow irregular movements, which were not reversed on reversing 

 *he direction of the current and were due to heating. The movements of 

 the particles contained within the water were dead beat, and reversed 

 with the current. We may take it (1) that the actual surface skin is not 

 propelled at all, or so slowly that the movement escapes detection in a period 

 of, say, five seconds, during which submerged particles immediately below 

 have hurried half across the field of view ; (2) that the layer of fluid 

 immediately below is driven by the field past this skin in the same direction 

 and with the same order of velocity as the water past the glass. If 

 additional proof of this were wanted, it is to be found in the fact that yeast 

 and red blood corpuscles move in opposite directions in the layer immediately 

 below the free surface, just as they do in the layer next to the glass, and 

 for the same reason, namely, because the more slowly migrating yeast cells 

 are unable to stem the current of water. 



The stationary layer is exceedingly thin. With oc. 4, ob. B, focussed 

 on the floating graphite, submerged particles showing rapid movement are 

 scarcely out of focus, and the spectacle produces a remarkable impression 

 of the presence of a tenacious skin which has sufficient rigidity to act as a 

 relatively fixed layer past which the subjacent water is being driven. 



The flow of water in electric endosmose is due to " relatively enormous 

 electric forces acting on the superficial film, and dragging the fluid (as it 



