6 
NOTE ON THE IRON-TURNINGS CELL. 
gaseous solutions is due to the appearance of bubbles in the 
neif/hhourhood of points and edges, which from their verj^ nature 
are signs of a rough and cavernous region of the surface. 
It is probable, however, that in the case of the electrolytic 
evolution of gas the points of the kathode, or at least the edges 
of the hollows, play an important part in the adjustment of the 
thickness of the polarizing layer. 
For a layer of gas of uniform thickness over an uneven 
surface, such as that represented (on an exaggerated scale) in 
vertical section in fig. 4, 
Fi^. 4. 
A 
cannot be in stable equilibrium owing to the variation of 
curvature of the surface. The capillary pressure on the gas 
is greatest at a projection A, and least at a depression B, and 
consequentlj^ there will be a flow of gas from A to B. The 
thickness at A will diminish, while that at B will increase. 
Now, the flow of electricity will be generally greatest towards a 
projecting point, such as A, and we may conceive that the 
hydrogen is evolved more rapidly at A than at B, but is at once 
forced down from A to B by the capillary pressure and there 
forms a bubble which finally escapes. If the layer of gas be 
extremely thin its flow will be impeded by friction with the 
liquid and with the solid, to a greater extent than when thicker,- 
and if the thickness is less than the radius of molecular action 
