1906.] 



on Studies on Charcoal and Liquid Air. 



439 



Metallic Vacuum Vessels. 



With the aid of charcoal, it is easy to make useful vacuum vessels of 

 metal instead of glass. The metallic vessels resemble the ordinary 

 glass silvered vessels and are of from 2 to 10 litres capacity (see Fig. 4). 

 The envelopes may be made of brass, copper, nickel, or tinned iron, 

 with necks made of a bad conducting alloy. The vacuum between 

 the walls of these vessels was maintained by enclosing some charcoal 

 in a small globular space. A, constituting part of the inner vessel 

 that is filled with liquid air. The necks may be covered with 

 silvered glass vacuum cylinders w^hich act as stoppers, and at the 

 same time utilise the cold of the slowly evaporating liquid. The 

 efficiency of the best metallic flasks is equal to that of the chemically 

 silvered glass vacuum vessels, now generally used in low tempera- 

 ture investigation. A^essels of this type will be of use in industrial 

 cryogenic operations and for the storage and safe transit of liquid 

 air. 



Diffusion of Gases into Charcoal at Low Temperatures. 



Although, as regards general absorption, charcoal under corre- 

 sponding conditions behaves in a similar manner to all gases, never- 

 theless it possesses a certain selective power, w^hen in presence of a 

 mixture of gases. K quantity of charcoal which has been heated, 

 exhausted, and then saturated with ordinary air at — 185° C, must have 

 gases occluded in its pores of the average composition of the air. Let, 

 however, a stream of air at the same low temperature pass slowly and 

 continuously over it for some hours, and it will be found that at first 

 the issuing gas is almost pure nitrogen, while the occluded gas reaches 

 a new and definite composition. If now the gas absorbed by the 



