658 



Professor Sir James Dewar 



[JaB. 22, 



Diffusion through Indiarubber. 



A widely different example of a diffusing medium is afforded by 

 indiarubber stretched into thin membranes. To enable them to 

 stand the necessary difference of pressure on the two sides they are 

 supported on discs of copper gauze mounted in a shallow metal drum 

 of about 4 cm. diameter ; the drum A, Plate 0, is closed at one end 

 by a brass plate pierced to connect to a lead tube E, of about 2 mm. 

 bore and 8 mm. diameter. Connection is made to the manometer by 

 a ground coned joint B ; a brass cone fitting this joint is soldered on 

 the end of the lead tube. The indiarubber membrane is stretched 



Plate 9. 



over the open end of the drum and rests on the copper gauze disc 

 inside, which is raised a few millimetres above the brass plate 

 forming the closed end of the drum. Tightly wound thread is used 

 to bind on the membrane, a little melted indiarubber as luting being 

 first smeared thinly on the brass. To enable different gases to be 

 used, and their temperature and pressure to be varied, as applied 

 to the indiarul)ber membrane, a gas-tight metal box is fitted on (this 

 is not shown in the diagram). 



For demonstrating diffusion through the membrane a manometer 

 is employed of the same pattern as that shown in Plate 6 ; the index 

 tube is charged with very high boiling-point mineral oil instead of 

 sulphuric acid. In order to dry or purify the diffusing gas a plain 

 U-tube C (Plate D) is sealed to the coned joint connecting to the 

 mounted membrane. Beyond the U-tube is a two-way stopcock D, 

 whose upper limb is sealed to the manometer, while the lower 



