su 



Professor Sir James Dewar 



Apparatus for Studt of Rubber Diffusiox. 



For the purpose of extending Graham's investigation, membranes 

 jibout 0*01 mm. thick were used for demonstrating some aspects of 

 the diffusion of gases and vapours. The thickness after use was 

 determined by immersing the membrane in liquid air, mounted on its 

 brass-drum support, thereby changing it into a rigid body that could 

 be cut off at the low temperature and subsequently weighed ; knowing 

 the specific gravity and the area the thickness is found. 



Sheet Para rubber, about 1 mm. thick was gently warmed and 

 carefully pulled in all directions by the hands, before being stretched 

 on the brass support ; the outside edge being thinly coated with 



(a.) 



Fig. 16. 



melted rubber. It was found most convenient for one person to 

 stretch and hold the membrane down in position while a second 

 person carefully wrapped and tied on the securing thread. Much 

 thinner films could be obtained from the material employed by dentists, 

 giving three times the percolation shown by the stretched Para, but 

 they were often faulty through small leaks, and had to be abandoned. 

 For maintaining different conditions of temperature and pressure, 

 the mounted membranes were enclosed in boxes which were gas-tight. 

 The two types employed are shown in Fig. 16 — 



(^0 for pressures at or below one atmosphere ; 



{b) for pressures above one atmosphere. 

 In (a) the membrane is stretched across the open end of the shallow 

 metal drum A. Just inside this a copper gauze disc is fixed to 

 support the film against suction from within, communicated along 

 the lead tube B, which connects by a ground joint to the highly 



