196 • Professor Sir James Dewar [Jan. 19, 



Table 9.— Approximately 1 per cent, glycerine-soap ; pale 

 golden colour ; hanging from glass nozzles. 



Diameter ^ 6 | 8 



Internal pressure • 55 0-36 0-28 



Rate of gas transference 0-285 0-103 0-050 



The internal pressure was not determined for every bubble ; the 

 values given above were deduced from results obtained with typical 

 bubbles connected to an alcohol nearly-horizontal displacement mano- 

 meter. The instrument gave w4th 1 mm. of water pressure a dis- 

 placement of the order of 100 mm. of the alcohol column. 



A study of these results shows at once that there are obscure 

 factors in some cases, causing notable divergences, even when the 

 conditions are not greatly altered. Other experiments have shown 

 that small differences in "^composition have in many cases a large 

 effect, on the behaviour of these thin films. Table 4 (air bubbles) 

 shows that 4 per cent, of alcohol added to the soap solution increases 

 the rate of gas transference through an air bubble. 



The constancy in thickness of the " black stage " is undoubtedly 

 subject to variation in different solutions. Johonnott, using a large 

 number of small black films in a Michelson interferometer, found 

 that there were at least two values for the thickness of the black film 

 he measured, and that the additions of either glycerine or potassium 

 nitrate tended to give the thicker of the two.'^ In last year's 

 Discourse a description of five distinct grades of black was given, 

 obtained with soap solution containing over 30 per cent, of glycerine. 

 The film covered a thin glass frame in an exhausted glass vessel. 

 These grades were seen to be unstable, coalescing to the deepest or 

 thinnest black when a portion of soap solution was brought in contact 

 with the lower pai't of the glass frame. It is possible that a few 

 ])er cent, of alcohol, with its low viscosity and surface tension, might 

 result in a still thinner black stage. 



Gas Transference THROuan Bubbles at Pressures 



OTHER THAN ATMOSPHERIC. 



A bubble of about 10 cm. diameter, shown in an exhausted 

 flask (Fig. 2), in which the air pressure had been reduced to the order 

 of a fraction of a mm., was seen to contract visibly from the rapid 

 percolation of the air from within outwards. Under such con- 



* "Thickness of the Black Spot in Liquid Films." By Edwin S. 

 Johonnott, Jun., llyerston Physical Laboratory, University of Chicago. 

 Phil. iNIag., xlvii., p. 501. 



