1917] on Soap Bubbles of Long Duration 197 



ditions the small internal excess pressure of about 0-2 mm. water, 

 necessary to maintain a 10 cm. bubble, is a very large proportion, 

 say about 10 per cent., of the total air pressure in the flask. There 

 is, therefore, a proportionately small resistance to its percolation 

 through a film sufficiently thin. As the air pressure in the vessel is 

 increased, the time for complete contraction becomes greater, Itecause 

 the internal excess pressure is a relatively smaller proportion of the 

 total opposing pressure ; thus when the containing vessel is at 

 atmospheric pressure, the 10 cm. bubble is distended by only 

 Tf^V^th of this total pressure. There is therefore a proportionately 

 small force urging the contained air to pass out compared with that 

 where the vessel wa^ under a pressure of only a fraction of a mm. 

 The result is naturally that the complete contraction takes weeks 

 instead of minutes. In the same way if a bubble be formed in a 

 strong glass vessel charged to, say, 10 atmospheres, then even after six 

 months, during the major part of the time being black, no sensible 

 diminution in diameter was detected. 



The measures of contraction at very low pressures were hindered 

 by the difficulty in securing uniformity of thickness in the film. The 

 black bubbles when obtained are extremely fugitive, as they contract 

 so quickly. Then also the ammonium oleate solutions, which are 

 so well adapted for obtaining black bubbles, do not remain of the 

 same composition under low air pressures, due to the removal of the 

 volatile easily-dissociated ammonia. 



The results of some of the attempts with ammonium oleates are 

 given in Fig. 8, showing the contraction at pressures of 1 mm. and 

 5 mm. Hg. respectively, with approximately the same bubble thick- 

 ness. The curves in Fig. 9 show the relative rates of contraction 

 with potassium olcMte at three different thicknesses of bubble, all at 

 the same pressure of just over 1 mm. Hg. 



The first of these two diagrams show's that at less than 1 mm. 

 pressure of air in the vessel, a 14 cm. bubble composed of 3 per cent, 

 ammonium oleate in 30 per cent, glycerine, of intense green colora- 

 tion, contracts completely in about 17 minutes ; whereas if the air 

 pressure be raised to 5 mm., the contraction of a similar bubble has 

 only gone from 14 cm. to 11 J cm. in the same time, and to 10 cm. 

 in 25 minutes. The second diagram shows that a bubble 10 cm. in 

 diameter, composed of 5 per cent, potassium oleate, 50 per cent, 

 glycerine, and almost too thick to show colour, takes 47 hours to 

 contract to 6 cm. ; but when a bubble of similar size and composition 

 is thinned to intense green, the same amount of contraction occurs 

 in 9 hours : when further, the black stage is secured, the time for 

 complete collapse from 3 cm. diameter is only 25 minutes. The 

 green bubble, from its graph, would have taken from 2 to 3 hours 

 for the same contraction. Such preliminary results may serve at 

 least to point the way to a more complete inquiry. 



