The Migration of other Film-forming Substances

The method of Brattain and Becker (p. 351) has been used to show that migration of barium and caesium can occur. When barium (or caesium) was deposited on one side of the tungsten strip, which was then raised to 1000°K., the thermionic emissions from the front and back slowly became equal. The value of / = on the front was initially 0-80, while after the flashing at 1000° K., the flnal value of / on both back and front was 0 4. 

Roller (29) deposited caesium on a silver surface, to give a multimolecular layer, and then exposed the composite surface to the action of oxygen. Simultaneous observations of the photoelectric properties of the Cs-CsgO-Ag surface showed that as fast as caesium oxide was formed it was covered by a polyatomic layer by processes of readjustment by diffusion in 

Two other methods of obtaining information concerning surface migration are worthy of mention. The first is the method of the radioactive indicator. If polonium is deposited on a silver foil, at one end only, and the temperature is raised to 300 C., a creeping of the polonium along the silver could be noted, the velocity of which increased as the temperature was raised (52). No volume diffusion of polonium through the foil took place up to 500° C., an interesting commentary upon the relative ease with which surface and volume diffusion occur. 

The second method is one which may have some general applicability to amalgams. It consists in measuring the rate at which mercury will spread over metal surfaces. Spiers (53) found that a drop of mercury spreads over tin foil in circular or elliptical areas in which, when diffusion has ceased, there is a uniform mercury content (11 8 %Hg). There is thus a concentration discontinuity from 11 8 % to 0 % mercury at the edge of the area, and the edge may be easily observed. Alty and Clark (54) made quantitative measurements on rates of spreading, which they found to be sensitive to the pretreatment of the surface and the nature of the medium (water, oil, or air) in contact with it. The surface diffusion was much more rapid than the volume diffusion, for after a surface diffusion of several centimetres the mercury had penetrated into a tin block by a fraction of a millimetre only. 

The spreading of mercury up the surface of tin rods dipping into the mercury was a one-dimensional diffusion which was assumed to obey the following conditions  

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