1886.] On Thickness and Surface-tension in Liquid Films. 443 



maximum or minimum ordinate which lies halfway between the 

 rings), and from the sensitiveness, in the calculation of which the 

 assumption is made that the generating curve of the film passes 

 through the edges of the upper and lower cups. 



The result of comparing the values of cZT/T obtained by the two 

 methods was to show that only a small part of the difference of form 

 of two films could be ascribed to a slipping of the film over the 

 liquid attachments to the solid supports. 



The method of measuring three ordinates was applied to a great 

 number of films, and gave results from 05 to 1 per cent, lower than 

 those obtained from the sensitiveness. In the case, however, of two 

 films which were both flooded, and presumably in the same state, the 

 two methods gave results, the differences between which were both 

 more irregular and less in amount, averaging not more than 0'2 per 

 cent. It is therefore probable that when the bulging or contraction 

 of a film becomes considerable there is a little slipping, though not 

 more than enough to account for a small part of the total change of 

 form observed. 



Phenomena similar to those described are explained by van der 

 Mensbrugghe by the consideration that when the surface of a film is 

 being continually renewed, it is cooled, and its surface-tension is in 

 consequence increased. This explanation is shown to be inadequate. 

 A colourless film being certainly 250 times thicker than a black one, 

 the increase of surface-tension due to cooling would be only 0'0016 

 per cent., whereas in some of the experiments there is evidence of a 

 difference of 9 per cent. 



The cause of the phenomenon cannot at present be assigned with 

 certainty. Perhaps many causes are at work. Experiments designed 

 to test the effects of oxygen and carbonic acid on the result, though 

 not conclusive, indicate that the phenomenon under discussion is 

 affected by the nature of the atmosphere in which the films are formed. 

 Reasons are given for the conclusion that it is merely an instance of 

 the difficulty which many observers have found in preserving a liquid 

 surface pure. 



On the assumption that the rapid change in the surface-tension of 

 a newly formed film is not due to its thinning, but to a disturbing 

 cause, attempts were then made to eliminate this cause, or reduce it 

 so as to compare films of very different thicknesses. 



Two methods of attacking the problem were carried out. In the 

 first the procedure was as follows : — The diameters of two cylindrical 

 films were measured when they were in the same state, an electric 

 current was then passed up one of them in order to thicken ir, and 

 after a sufficient length of time had elapsed for the direct effect due 

 to the disturbance produced by the current to pass off, the diameters 

 were again measured. By this means it was possible to compare two 



