OIL FILMS ON WATER AND MERCURY DEVAUX. 263 



such as I devised in 1888/ and which is shown full size in figure 1. A 

 little fragment of camphor is stuck with wax in a notch in its rear. 

 A little mast bearing a streamer is fixed in the middle. This little 

 boat, placed upon the water, moves rapidly and continuously so as to 

 be seen from all parts of a room (pi. 1). 



I used this device the 19th of April, 1912, in Paris before the So- 

 ciete de Physique. Placed at first upon water with a film of oil at 

 its maximum extension it traveled just as on pure water, leaving in 

 its rear a large wake; the talc was thrown out with a marked vibra- 

 tion whenever it came in contact with the cam^^hor, just as if the 

 camphor corresponded to the propeller of the boat. I diminished 

 the surface. At once the wake became smaller. The boat slowed up, 

 I made the surface yet smaller. The boat stopped. I increased the 

 surface, the boat again moved. 



We may thus, by the simple movement of a capillary barrier (a 

 strip of paper) , show to a whole audience the effect of 

 sudden and considerable changes which the surface ten- 

 sion of water undergoes when covered with a film of 

 oil of the critical thickness. It is a very simple experi- 

 ment and very effective. Therefore it is particularly 

 interesting to know what thickness the film of oil must 

 have at this remarkable phase. 



II. THE THICKNESS AT MAXIMUM EXTENSION. 



(1) Experimental 'measures. — Lord Rayleigh, in his fig i.— camphor 

 admirable experiments of 1890, tried to find what is the ^oat (natural 



. „ ., ii.il size) ;c, grain of 



mmmium quantity or oil necessary to stop the move- camphor at the 

 ment of the camphor- and found an extremely small stem: 7n,mast 

 value, a thickness of about 1.6 [x|i,. In 1891 he published ^™' '^ ' 

 the letter of Mile. Pockel, which we have just mentioned, and in the 

 following year •'■ showed the stopping of the movements of the cam- 

 phor by a greasy body is due, as the law discovered by Mile. Pockel 

 led him to see, to a sudden fall in the surface tension of water when 

 the grease layer has the right thickness. In 1899 he published a 

 curve showing the relation between the surface tension and the quan- 

 tity of oil * and showed that the proportion of oil w^hen the surface 



1 See La Nature, April, 1888. 



2 Proceedings of the Royal Society, 47, Mar. 27, 1890. A French translation of the 

 article will be found in Conferences et allocutions de Sir William Thomson (Lord Ray- 

 leigh), translated by Lugol (1893), p. 48. 



» rhilosophical Magazine, oS, p. .''.66, 1892. 



* Philosophical Magazine, 47 and 48, 1899. In obtaining this remarkable curve, Lord 

 Rayleigh appears to have supposed implicitly that the oil on the water always forms 

 a continuous and homogeneous film, even when its surface is much diminished ; for ex- 

 ample when he gets the quotient of the weight of oil by the surface occupied. This is 

 proper only when the diminishing of the surface is small, say in the ratio of 1 to 1.8. 

 Beyond this limit this process is in error, for the surface begins to assume a globular 

 form, finally becoming a veritable mass of foam. We will speak of this later on. 



