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Prof. A. W. Remold and A. W. Rucker. [May 27, 



films, one of which was nearly all black, while the other displayed a 

 little black and the colours of the first and second orders. Both films 

 were then allowed to thin, and assuming (in accordance with previous 

 observations of the authors) that that which was already black 

 remained in a constant state, any change of diameter which took 

 place as the coloured film became black, could be regarded as due to 

 changes in the thinning film. 



An objection to which this method is open requires discussion. 

 Most of the films observed were partly black and partly coloured. 

 If any difference of surface-tension existed between the different parts, 

 the films would not be cylinders or simple unduloids, but the black 

 and coloured parts would have different curvatures. Measurements 

 based on the sensitiveness would not therefore be trustworthy, if, 

 owing to this cause, appreciable changes took place in the forms of the 

 films, or in the pressures which they exerted upon the enclosed air. 

 It was therefore necessary to investigate the form of a film consisting of 

 two parts of different surface-tensions, assuming that it does not much 

 differ from a cylinder. As a result of the mathematical investigation, 

 a table was drawn up giving the ratio of the change of pressure due 

 to a change of surface-tension affecting a part of the surface, to that 

 produced by an extension of the change over the whole film. 



Next follows a detailed description of a number of experiments in 

 which two cylinders were balanced against each other, and the electric 

 current was made use of to influence their rate of thinning. The 

 theory applied to the results of these experiments gave the percentage 

 change of surface-tension due to change of thickness. 



In a second group of experiments a cylinder was balanced against 

 a sphere. As a spherical film thins more slowly than a cylinder, a 

 comparison between a thick film (sphere) and a black or partially 

 black film (cylinder) could be made without having recourse to an 

 electric current, and greater differences of thickness were obtained 

 than in the earlier observations. 



The differences of surface-tension measured in these observations 

 were very small. They never exceeded 1*5 per cent., and the black 

 films were sometimes more and sometimes less curved than the thicker 

 films with which they were compared. There was no evidence of any 

 regular change in the surface-tension as the thickness diminished, and 

 the average difference between the tension of the black and coloured 

 films as deduced from fifteen experiments was only 013 per cent. 



The general result of the inquiry therefore appears to be that 

 when the black part of a soap film forms in the normal way, spreading 

 slowly over the surface, no evidence of any change in surface-tension 

 dependent on the thickness of the film is furnished by a direct comparison 

 of the tensions of thin and thick films over a range of thickness extending 

 from 1350 to 12 millionths of a millimetre. 



