of Gibbs's Theory of Surface- Concentration. 493 



observed whether any motion of the particles takes place or 

 not. This was done experimentally as follows : — 



A simple Nernst form of U-tube, identical with that em- 

 ployed by Burton *, having a vertical capillary tnbe affixed 

 at the bend through which water slowly runs so as to form 

 a definite surface "of separation (in both limbs) between the 

 water and the oil-water emulsion, is fitted with platinum 

 electrodes dipping into the emulsion. A potential-difference 

 of 230 volts was maintained between the electrodes, and the 

 movement of the emulsion-water boundary down in one 

 limb and up in the other was read off on a scale 

 attached to the vessel. It was found that the particles 

 moved towards the anode, being therefore negatively charged. 

 The velocity of the particles was 4*3 X 10 -4 cm. /sec. under 

 a gradient of 1 volt/cm. 



Burton t has deduced an expression for the potential 

 existing between the particles of colloidal metals and the 

 water in which they are suspended, viz.: 

 v _ Air 7)v 

 V "II' 



where V is the p.d. required (in electrostatic units); 

 K is the specific inductive capacity of the 



medium; 

 7j is the viscosity of the medium; 



— is the velocity of the particles under (elec- 



-^ trostatic) unit potential gradient, 



X being the p.d. maintained between the poles. 



Applying this to the present case of oil suspended in 

 water, we obtain 



V = 0*00048 electrostatic unit, 



or 0*15 volt. 



There is thus quite a measurable potential existing between 

 the oil and water; and in considering the adsorption of 

 electrolytes we must not overlook this fact. It may be 

 mentioned, in passing, that the above value for the potential 

 is quite of the same order as that obtained by Burton for 

 various colloidal metals. 



Since the oil has been shown to be negatively charged, 

 one would not be surprised, speaking generally, that the 

 cation should be selectively adsorbed. 



* Burton, Phil. Mag. ii. p. 434 (1906). 

 f Burton, loc. cit. 



