418 



Professor Sir James Bewar 



[June 5, 



Measures on the rates of leakage across such meml)ranes for a 

 given surface in a given time Avere carried out by observing the in- 

 crease of pressure in a McLeod gauge. The values are given in terms 

 of the number of cubic millimetres of gas which passes per minute 

 across an area of 1 square centimetre. Both air and hydrogen diffu- 

 sion was examined. At ordinary temperatures the figures obtained 

 were approximately 1'5 in air and 4*9 in hydrogen. At -80°C. 

 the corresponding values were 0-0009 and 0*0044, while at - 185° 



Fig. 3. 



the figures were 0*00012 and 0*00013 respectively. One membrane 

 was also tested while immersed in liquid hydrogen and was found 

 to be quite tight. 



An india-rubber vacuum vessel can be formed on a similar prin- 

 ciple. Fig. 3 shows two little balloons, one of glass B, the other of a 

 thin india-rubber membrane A, both within larger bulbs, which are 

 connected together, the space within being maintained exhausted 



