PASSAGE OF GASES THROUGH POROUS FILMS. 



153 



Diffusion through India-rubber. 



a glass bottle, such a thin tissue be tightly tied, and the bottle placed 

 in an atmosphere of carbonic acid gas, movement at once takes place, a 

 little air flowing out of the bottle into the carbonic acid, and so large a 

 Fig. 69. quantity of the acid passing the opposite way 



that the India-rubber soon swells outward^ 

 and eventually caps the bottle like a dome, as 

 in Fig. 69, at b. Or, if the conditions be re- 

 versed, the bottle being filled with carbonic 

 acid, and then exposed to the atmosphere, the 

 India-rubber will be depressed, as at #, and 

 stretch so as almost fo sink to the bottom. 

 Such experiments therefore prove that, even 

 though barriers of a very close texture should 

 intervene, gases will pass through them, and 

 with so much force, as Dr. Mitchell showed, that many inches of mercury 

 may be lifted, nor does the movement cease until the gases on both sides 

 of the membrane have the same composition. 



Other substances having a close texture may be thus 'readily permea- 

 ted. I found that a little bladder of shellac, blown on the Experiments 

 end of a glass tube, permitted the passage of the vapor aris- ^anTii^' 



ing f rom water of ammonia. The "ids. 

 instantaneousness of these motions is, how- 

 ever, most -beautifully illustrated by employ- 

 ing soap-bubbles, the liquid nature of which 

 excludes the idea of pores in the strict accepta- 

 tion of that term. If a bottle, a a, Fig. 70, be 

 rinsed out with amponia, and then, by means 

 of a piece of glass tube, b b, a soap-bubble, <?, 

 be blown therein, the air from the bubble be- 

 ing immediately drawn into the mouth with- 

 out a moment's delay, the strong taste of the 

 ammonia is perceived. Or if a rod, dipped in 

 hydrochloric acid, be presented to the projecting end of the glass tube, 

 copious white fumes arise. This therefore shows that vapors will pass 

 through barriers having no proper pores, the transit taking place instan- 

 taneously. 



Soap films enable us to demonstrate the endosmosis of gases in a very 

 advantageous manner, owing to their cohesiveness and thinness. If the 

 finger be dipped in soap-water, and then rapidly passed over the mouth 

 of an empty bottle, so as to leave a horizontal film attached across, on 

 exposing the bottle to carbonic acid gas, the horizontally of the film is 

 immediately disturbed, and it soon swells up into an almost spherical 

 dome. Or if the bottle be filled with carbonic acid, and then exposed 



Instantaneous passage of gases 

 through films. 



