PASSAGE OF GASES THROUGH MOIST FILMS. 85 



the moisture of the film of the bubble, and will then evaporate 

 again inside the bubble ; whilst the gas contained inside the 

 bubble will also dissolve in the moisture, and Avill similarly 

 evaporate into the external atmosphere; so that a continual 

 passage of the original gas from within outwards, and of atmo- 

 spheric air from the outside to within, will be brought about. 

 Tims a soap bubble (made of soap solution of such a kind as to 

 give a bubble permanent for some hours Chapter XVIII.), and 

 tilled with coal gas, will be at first light enough to ascend in the 

 air like a balloon, and capable of carrying up with it a small 

 attached car or weight (Expt. 307) ; but after some time so much 

 air will have passed in, and so much of the lighter gas will have 

 passed out, that the bubble and car will no longer be light enough 

 to float in the air, and will sink again instead of tending to 

 ascend. 



Expt. 80. Passage of Gases through Moist Membranes. 

 The following experiment affords another illustration of the same 

 kind of action. Obtain a small bladder, immerse it in water till 

 thoroughly moist and softened, and then blow it up so as to fill it 

 not quite full of air, using a pair of bellows for the purpose, and not 

 the lungs. Tie the neck up tightly, and then suspend the bladder 

 in a large jar of carbon dioxide (see Expt. 100); this gas will 

 dissolve in the moisture of the membrane, and evaporate again 

 into the air inside more rapidly than the air will pass outwards 

 by the reverse action; accordingly the bladder will gradually 

 become more and more distended, and if nearly full at first, 

 and free from punctures, may ultimately burst by the increased 

 pressure inside. 



Certain solids possess the power of exerting an action of this 

 kind on certain liquids (as the bladder in Expt. 40, where alcohol 

 and water are thus partially separated), and are consequently 

 capable of being used for the purpose of separating mixed fluids. 

 Some solids exert an analogous action on gases, and will conse- 

 quently separate these gases from mixtures of them with other 

 gases ; thus, india-rubber possesses the property of absorbing and 

 parting with oxygen in this way, and is consequently sometimes 

 employed to separate oxygen from nitrogen in the air (vide 

 Chapter VII.). 



Supersaturated Solutions of Gases. 



When aerated water is drawn from a siphon or gasogene, 

 or poured out from a bottle, a vigorous effervescence takes 

 place at first, but soon subsides, so that no further bubbling 



