8 POPULAR LECTURES AND ADDRESSES. 



from the exceedingly small force of attraction which 

 we should calculate for it according to the 

 Newtonian law, on the supposition of perfect 

 uniformity of density in each of the attracting 

 bodies. Well known phenomena of bubbles, and 

 of watery films wetting solids, make it quite certain 

 that the molecular attraction does not become 

 sensible until the distance is much less than 

 250 micro-millimetres. From the consideration 

 of such phenomena Quincke J came to the con- 

 clusion that the molecular attraction does become 

 sensible at distances of about fifty micro-millimetres. 

 His conclusion is strikingly confirmed by the 

 very important discovery of Reinold and Riicker 2 

 that the black film, always formed before an un- 

 disturbed soap bubble breaks, has a uniform or 

 nearly uniform thickness of about eleven or twelve 

 micro-millimetres. The abrupt commencement 

 and the permanent stability of the black film, 

 demonstrate a proposition of fundamental im- 



- Ann. der Phys. und der Chem. Bd.cxxxvii. 1869. 

 2 Proc. Roy. Soc. June 21, 1877 ; and Trans. Roy. Soc. April 19, 

 1883. 



