Gases at Low Pressures. 383 



similar to C of fig. 1. It can be surrounded by a heater or 

 a Dewar vessel as circumstances may require. A connecting 

 tube leads from G to a point on the tube H, which connects 

 E to the viscosity apparatus A (fig. 1). I leads to the pump 

 •and. McLeod gauge. Anything which proceeds from the 

 pump or McLeod gauge towards the viscosity apparatus 

 must pass through E. Moreover, the gas entering from the 

 generator will, with the given arrangement, retard the dif- 

 fusion of mercury vapour from the pump and gauge towards 

 the viscosity apparatus. If there is no vapour entering with 

 the gas there can be none entering the viscosity apparatus 

 without passing through E, and, since throughout the ex- 

 periment this tube was kept surrounded by the liquid air, 

 the pressure of the vapour due to diffusion from the mercury 

 in the pump or gauge could never exceed the vapour pressure 

 of mercury at the temperature of the liquid air. The tube C, 

 when surrounded with the liquid air, was sufficient safeguard 

 against the entrance of water vapour with the gas. 



The method of removing all water vapour and mercury 

 vapour already in the apparatus beyond the tube E was that 

 •of repeated exhaustion and filling with the gas to be ex- 

 perimented with, the whole apparatus meanwhile being kept 

 at a high temperature *. 



At the first exhaustion, when the pressure had been re- 

 duced to a few centimetres of mercury, the tube G was 

 surrounded by the electric heater, and the heat was applied 

 to the oven in which the viscosity apparatus is placed. 

 Practically the whole apparatus, except the gas generator, 

 was kept hot while the pumping proceeded. After a fair 

 vacuum was reached the pomp was stopped and the hydrogen 

 from the generator was allowed to enter very slowly, passing 

 first over phosphoric pentoxide, and then over spongy plati- 

 num, heated in a combustion tube, before entering the tube 

 'G-. This filling process was followed by another exhaustion 

 under the same conditions. After the apparatus had been 

 exhausted and filled a number or* times in this way, when it 

 seemed certain that the apparatus and the pores of the char- 

 coal were filled with fairly pure hydrogen, the heater was 

 removed from G, and the vessel containing the liquid air 

 substituted for it. The same process of alternately exhausting 

 and filling was continued, great care being taken in filling 

 to allow the hydrogen to pass very slowly so that the drying- 

 process might be complete. Keeping the apparatus at a 



* The suspended disk was, of course, lowered before this operation 

 began. 



