1908] on the Nadir of Temperature and Allied ProUems. 



417 



The direct value of the thermal evolution for hydrogen, about 

 85°, measured in the liquid air calorimeter described in last year's 

 discourse, was found to be 4*6 x 426. 



I showed in the last discourse the high exhaustions obtainable 

 by the use of charcoal at low temperatures. The condensing action 

 is still effective against considerable dynamic heats. 



The following experiments illustrate this : 



A very thin india-rubber membrane is stretched across the open 

 end of a piece of glass tube 1 inch wide (being securely fixed by the 

 use of melted rubber and thread to the outside wall of the tube) and 

 connected to a discharsre tube and a bulb containinor charcoal which 



^Si3Sl/>-^- 



y^ 



"^^^^sf 



^ 



Fig. 2. 



can be shut off by a stopcock (see A, B and C, Fig. 2). The appar- 

 atus is exhausted down to a few mm. pressure, thus forcing the rubber 

 to be drawn out to a spherical shape, and the end of the tube is 

 immersed in liquid air. A discharge is passed and the charcoal, 

 already immersed in liquid air, is brought into play to increase the 

 vacuum. The rate of diffusion through the membrane is so greatly 

 diminished, that a vacuum is very soon produced by the charcoal 

 which is so high that no discharge will pass. If the liquid air is 

 taken away from A, a continuous discharge takes place in B owing 

 to rapid diffusion through the rubber film at the ordinary tempera- 

 ture. 



YoL. XIX. (No. 102) 2 E 



