208 J. A. POLLOCK. 



The relation between the surface tension and the thick- 

 ness of the layer of castor oil, as shown in figure 1, exhibits 

 characteristics which are typical of oil-contaminated sur- 

 faces, the work of Lord Rayleigh, in this connection, 

 extending that of Miss Pockels, which is described in 

 "Nature," Vol. xliii. 1891. 



As may be seen from the diagram, Lord Rayleigh found 

 that the tension of a water surface is not affected by the 

 presence of oil until the quantity is more than sufficient 

 to cover the surface with a uniform layer about one ten 

 millionth of a centimetre thick. When the oil is present 

 in less than this critical quantity, it may be assumed 

 to exist on the surface in patches, with lanes of pure 

 water between them. It is considered by Lord Rayleigh 

 that the surface tension begins to fall, as represented at 

 A in the figure, when, with increasing quantities of oil, 

 the patches, as a result of their growth, join to form a 

 continuous layer over the whole surface. Lord Rayleigh 

 suggests that the layer at this stage is probably not more 

 than one molecule deep, and he has, by the determination 

 of its thickness, made a most important contribution to the 

 estimates of molecular magnitudes. 



Once the continuity of the layer of oil is established, the 

 surface tension decreases as the thickness of the layer 

 increases; the oil thus forms just such a pellicle as was 

 considered by Marangoni to be essential for the stability 

 of films. This result of the investigations is of consider- 

 able importance, for, with the knowledge of the persistency 

 of oil-contaminated bubbles, it at once changes Marangoni's 

 explanation of durability from a matter of inference to one 

 of fact. A further, most interesting, result appears from 

 Lord Rayleigh's work; as may be seen from the relation 

 shown in figure 1, the layer of castor oil which gives 

 durability to a bubble may have in many cases a maximum 

 thickness less than two ten millionths of a centimetre. 



