and the Formation of Liquid Films. 107 



being so arranged that the framework could be raised out o£ 

 the solution without removing a glass shade which covered the 

 whole apparatus. 



Concent ration. 



1 



IT. 



III. 



•2d normal, 



151 --es. 



127 - es. 



■07 



183 





•02 



19-3 



173 



007 



209 





•002 



21-0 



19 8 



In the above table the numbers in column II. are the times 

 after the formation of the film at which the green of the Gth 

 order (thickness 1100 fift) had reached a point 2 cms. from 

 the top of the frame : those in column III. the times at which 

 the blue of the 2nd order (250 /z//,) had reached a point *3 cm. 

 from the top ; they are each the mean of about ten measure- 

 ments, which throughout nowhere differed among themselves 

 by more than 1 second. The table shows that the time 

 required for a given degree of thinning increases in a marked 

 way with the dilution of the solution : and we may conse- 

 quently infer that circulation over the surface is an appreciable 

 factor in producing the thinning. 



The circulation must, however, cease when the whole 

 of the oleate has been removed from the interior of the 

 film. The surface excess will then everywhere take up a 

 stead// value of such a magnitude as will enable the tensions 

 to remain in statical equilibrium throughout the film; this 

 point, however, does not appear to be reached — unless perhaps 

 in very dilute solutions — before the black spot becomes 

 formed. 



Black Films. — Many observers have endeavoured to trace 

 the effect on the surface tension of the approach to molecular 

 magnitudes which films make as they thin ; but the results 

 have always shown that there is no appreciable difference in 

 the tensions of thick and of the thinnest black films. Any 

 other result would indeed be difficult to understand from the 

 point of view of the equilibrium of the film introduced by 

 Marangoni : but it is to be observed that it by no means 

 indicates that a thickness of twice the radius of molecular 

 action has not been reached and passed. If a film of pure 

 water could be obtained with a thickness less than twice the 

 radius of molecular action, its tension would d'onbtless be less 

 than that of a thick water-film ; but the tension of a soap-film 



