1890.] 



on Foam. 



89 



Another body which we may use for this purpose is camphor. If 

 we spread some camphor scrapings on a surface of pure water, they 

 will, if the surface is quite clean, enter into vigorous movement, as 

 you now see. This is because the dissolved camphor diminishes the 

 surface tension of the water. But if I now contaminate the water 

 with the least possible quantity of grease, the movements of the 

 camphor will be stopped. I merely put my finger in, and you observe 

 the effect. There is not much poetry about that ! A very slight film, 

 perfectly invisible by ordinary means, is sufficient so to contaminate 

 the water that the effect of the dissolved camphor is no longer visible. 



I was very desirous to ascertain, if possible, the actual thickness 

 of oil necessary to produce this effect, because all data relating to 

 molecules are, in the present state of science, of great interest. From 

 what I have already said, you may imagine that the quantity of oil 

 required is very small, and that its determination may be difficult. 

 In my experiments,^ I used the surface of water contained in a largo 

 sponge bath three feet in diameter. By this extension of the surface, 

 I was able to bring the quantity of oil required within the range of a 

 sensitive balance. In Diagram 2, I have given a number of results 



DIAGRAM 2. 



A Sample of Oil somewhat Decolokised by Exposure. 



Jan. 28 

 Jan. 28 



0-63 



1-06 



Barely perceptible. 

 Just enough. 



obtained at various dates, showing the quantity of oil required to pro- 

 duce the effects recorded in the fourth column. Knowing the weight of 

 the oil deposit, and the area of the water surface upon which it was 

 uniformly spread, it was easy to calculate the thickness of the film. 



I'roc. Roy. Soc, March 1890. 



