OIL FILMS ON WATER AND MEECUKY DEVAUX. 271 



have not been struck long since with the resemblance between these 

 " holes " in the oil films and the black spots of soap bubbles. The 

 mode of sudden appearance, the circular form, size, and progressive 

 enlargement are very similar, and each hole is really occupied by an 

 oil film whose thickness is comparable with that of the black spot of 

 the soap bubble. 



The holes in the oil film are, it is true, always more numerous, and 

 further, they finally become surrounded Avith droplets and then flow 

 together (pis. 3 and 4). In reality, soap bubbles often show several 

 simultaneous black spots, especially just before rupture. Further, 

 and which is of special interest, Herbert Stansfield ^ has called atten- 

 tion to black spots in soap bubbles accompanied by collars of disks 

 and granules which correspond to what occurs with oil films, only, 

 since the soap bubbles are never horizontal, gravity necessarily pulls 

 the thick portions away from where they appear. The confluence of 

 the spots is not then peculiar to oil films. 



The phenomena in the two cases are the same, the differences aris- 

 ing from the changed conditions under which the films are formed, 

 an independent and two-faced skin in the case of a soap bubble, a 

 bkin adherent to and supported by water in the case of the oil film. 

 Accordingly, the study of the evolution of oil films throws light upon 

 the final stages through which a soap bubble goes when it does not 

 break. It becomes reduced to a black, very thin film, dotted with 

 thick portions, either circular disks or droplets. 



Further, similar, very large, black spots have been obtained in the 

 films of soap bubbles by Eeynold and Eucker*' in their beautiful 

 researches made between 1877 and 1893. Upon these films they 

 determine the thickness of the black spots which were all found 

 sensibly equal and equal to about 12 [;.[/.. Johannot ^ later showed that 

 films could exist having a thickness one-half as great, or 6 [i\},. 



We can now compare the thicknesses of oil and soap bubble films. 

 In both instances we have black films formed from much thicker 

 ones. 



Black films of oil with a maximum thickness of 2 to 3 •jl[x. 



Black films of soap bubbles, maximum thickness of 6 to 12 [xpi. 



These thicknesses are of the same order. Oil films are certainly 

 always at least one-half as thin as the thinnest soap-bubble films. 

 This important difference must be due to the fact that in the case 

 of oil films on water there is only one free surface. 



1 Proceedings of the Royal Society, 1906, p. 311. 



- A. W. Reynold and A. W. Rucker, Proc. Roy. Soc. of London, 1877 ; Phil. Trans, ditto, 

 pt. 2, 1881, 1883; Phil. Mag., vol. 19, 1885; Phil. Trans. Roy. Soc. of London, II, 1880; 

 Wied. Ann., vol. 44, 1891 ; Phil. Trans. Roy. Soc. of London, vol. 184, 1895. 



3 Johannot, Phil. Mag., vol. 47, 1899. 



