with Soap-bubbles. 413 



bubble will slowly rise or fall, according to the relative 

 quantities of gas and air. The diffusion through the film is 

 well shown in the next experiment. 



Exp. 5. — Blow a pair of bubbles, as shown in fig. 6, but 

 make the inner bubble only just light enough to rest against 

 the top of the outer one. Lower a bell-jar over all, and pass 

 a stream of gas into the bell-jar by means of a tube passing 

 through the top. As the air is gradually driven down, the 

 outer bubble begins to feel the want of buoyancy, and 

 gradually settles clown, as shown in fig. 9. After a short 

 time, the effect of the diffusion through both bubbles tending 

 to enrich the gas of the outer bubble is made evident by the 

 gentle descent of the inner bubble. If the jar is raised 

 quickly, and a little air is blown into the outer bubble, it is 

 possible to again cause the inner one to rise and float against 

 the top of the outer one as before. The bell-jar may be 

 lowered and the process repeated until the outer bubble is so 

 large that the ring is unable to support its weight when in an 

 atmosphere of coal-gas. 



The very rapid diffusion of a vapour which will mix with 

 the solution of which the film is made is easily shown. 



Exp. 6. — Into a large inverted bell-jar pour a small quantity 

 of ether, or to fill the jar with the vapour quickly wet a piece 

 of blotting-paper with ether and stand it on edge in the 

 jar. Remove the paper, then blow a bubble and drop it 

 into the jar. The bubble will rest on the ether vapour as 

 on carbonic anhydride, and while floating the most violent 

 agitation of the colours of the film will be seen. The bubble 

 does not remain floating long at the same level ; it gradually 

 sinks into denser and denser layers of vapour until it reaches 

 the bottom or breaks on the way. This gradual sinking is 

 due to the penetration into the bubble of the ether vapour, 

 as may be shown as follows : — The bubble may be taken out 

 of the vapour by means of a ring wetted with soap-solution 

 and carried to a flame, when instantly there is a blaze of ether 

 vapour a foot or more in diameter. That the flame is not due 

 to liquid ether condensed on the film is shown by exposing a 

 plane film to the vapour and carrying it to a light in the 

 same way, when no trace of flame will be seen. 



Exp. 7. — At the end of a wide tube, which has been 

 enlarged at the lower end, blow a large bubble and lower it 

 gently into the vapour of ether, holding the finger at the 

 mouth of the tube. After a few seconds it will be found 

 difficult to remove the bubble by means of the tube, because 

 its weight may have become sufficient to tear it away when 

 buoyed up by the air only. If it is removed successfully it 



