THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JUNE 1899. 



XLVIII. Thickness of the Black Spot in Liquid Films, 

 By Edwin S. Johonnott, Jun.* 



Introduction. 



THE principal object of the measurement of the thickness of 

 the black spot on certain liquid films is to determine the 

 radius of molecular attraction, which is supposed to be equal 

 to half the thickness of the film. The first determination of 

 the thickness of the film was made by Plateau f, who gives it 

 as 120 x 10~ 6 millim. or 120 /jl/jl. The present generally 

 accepted value is that given by the experiments of Professors 

 Remold and Rtickert, viz., 12 fifi for the thickness. 



They employed two methods, the first consisting in measuring 

 the electrical resistance of cylindrical films, and the second in 

 determining the relative retardation of two beams of light, 

 one of which had been transmitted through a larger number 

 of films than the other. 



P. Drude§, by considering the effect of capillarity on the 

 reflexion and refraction of light, obtained a value of 17 fjt/m, for 

 the thickness. He also gives a good historical summary of 

 the work that has been done on the determination of the 

 radius of molecular attraction. Lord Ray lei gh || has also 

 shown that an olive-oil film 2 fifi thick will just quiet the 

 movements of camphor particles on a water surface. 



* Communicated by the Author. 



t Statique des Liquicles, vol. i. p. 210. 



X Phil. Trans. 1881, p. 447 ; 1883, p. 645 ; and 1893 (a), p. 505. 



§ Wied. Ann. vol. xliii. (1891), pp. 126-157 and 158-176. 



|| Proc. Roy. Soc. vol. xlvii. (1889-90), p. 364. 



Phil. Mag. S. 5. Vol. 47. No. 289. June 1899. 2 M 



