206 J. A. POLLOCK. 



a liquid film, would be incompatible with durability, if the 

 film were not everywhere strictly horizontal. For consider 

 a small element, between two level planes, of a vertical 

 film; the forces due to surface tension at the upper level, 

 acting upwards, must, for the equilibrium of the element, 

 be greater than the similar forces at the lower level, acting 

 downwards, by the weight of the element of liquid con- 

 sidered. Hence, for equilibrium, the surface tension must 

 continuously diminish from higher to lower levels of the 

 film ; this condition could not be satisfied in the case of a 

 film of a pure liquid, and, as a matter of fact, pure liquids 

 do not yield a lasting froth. 



From the consideration of such an argument Marangoni 1 

 was led to suggest that, for the durability of a liquid film 

 to be ensured, there must exist, on its surfaces, a layer, 

 of what might now be called contamination, having the 

 property of diminishing the surface tension of the liquid to 

 an extent which becomes greater as the thickness of the 

 layer increases. 



With such a surface layer the existence of a film would 

 be automatically maintained ; for any motion of the liquid, 

 involving, if unchecked, the breaking of the film, would 

 introduce forces opposing the movement. In the first 

 instance, the initial movement of the liquid, from higher to 

 lower levels under the action of gravity, would reduce the 

 thickness of the layer at the top of the film while increas- 

 ing it below, thus introducing a diminishing gradation of 

 values of the surface tension, from above downwards, which 

 would continue to vary, with the motion of the liquid, 

 until the tension, at every point, acquired the magnitude 

 sufficient to prevent further movement. Any subsequent 

 disturbance of the film would, in the same way, introduce 

 forces opposed to its development. 



1 Marangoni, loc. cit. 





