Gases at Low Pressures. 385 



charcoal, yet it was found impossible to obtain a steady con- 

 dition. As the evaporation o£ the liquid air proceeded, 

 sufficient gas was given off from the charcoal to produce a 

 large increase in pressure ; as much as thirty per cent, was 

 observed. When a fresh supply of the liquid air was added 

 the pressure diminished again. The difficulty became more 

 serious as the pressure at which the observations were made 

 became smaller. 



The phenomenon was probably due to the fact that the 

 fresh supply of liquid air was richer in nitrogen than it was 

 after the process of boiling had proceeded for some hours. 

 The nitrogen is the more volatile, and so the boiling will 

 proceed more vigorously just after a fresh supply of air has 

 been added than at any other time. Consequently the tem- 

 perature of the boiling liquid will be lower at first than it is 

 later, and the charcoal will thus absorb better at each addition 

 of liquid air to the Dewar vessel. The charcoal is necessary 

 for the phenomenon, for when the tube E was substituted 

 for the tube containing the charcoal the effect disappeared 

 or became inappreciable. 



It was suggested earlier in the paper that there would be 

 adduced evidence to show that the mercury driven out from 

 the apparatus collected in the tube E. After the measure- 

 ment of pressure and decrement had proceeded down to the 

 least value given in the Table, the supply of liquid air in the 

 Dewar vessel in which E was placed was allowed to disappear 

 gradually. As the evaporation proceeded it was found that 

 the decrement increased much more rapidly than the pressure, 

 as indicated by the McLeod gauge, showing that vapour was 

 finding its way into the apparatus. 



Results, 



In the first and second columns of Table I. are contained 

 the corresponding values of the decrement and pressure. 

 Not all the numbers given in these columns w r ere obtained 

 by actual measurement. Only those which are marked with 

 an asterisk were obtained in this way. The others were 

 obtained as follows : — A curve was plotted using the values 

 of the pressure which were measured as abscissas and the 

 corresponding values of the decrement as ordinates. This 

 curve was drawn on such a scale that the value of the decre- 

 ment, corresponding to any arbitrarily chosen pressure, could 

 be obtained from the curve as accurately as it could be 

 measured by the apparatus. The unmarked numbers in the 

 first two columns were obtained by choosing arbitrarily a 



Phil. Mag. S. 6. Vol. 19. No. 111. March 1910. 2 



