Deivars Method of producing High Vacua. 505 



§12. In all the experiments which have been described, 

 the relation between the pressure in the enclosed space and the 

 time since the commencement o£ the absorption is given by 

 the formula 



log (?-#>) = A- A*. 

 By differentiation 



c ^ = -\( p -p ) (iv.) 



But we have already seen th^t the volume of air absorbed by 

 the charcoal (which we may denote by x) 



V 



<i-5> 



where Y ; is the total volume of the exhausted space. 



So that the rate at which air is absorbed by the charcoal is 



dx_ _V'dp 

 dt ~ p' dt 



= — r(p-Po) 



=x(f-*), ...... (v.) 



where f is the amount of air absorbed when equilibrium is 

 established. 



§ 13. An experiment was also made with the same sample of 

 charcoal used in Experiments Nos. 1-5, in order to determine 

 the rate of absorption of air at constant (atmospheric) pressure. 

 For this purpose the bulb containing the charcoal was connected 

 through a T-piece with the stopcock of a burette, inverted over 

 water. During the preliminary cooling of the bulb, which 

 lasted three-quarters of a minute, the third limb of the T-piece 

 was left open to the external air. At the end of this time 

 the aperture was closed and the charcoal commenced to absorb 

 the air in the burette. Readings of the volume of air absorbed 

 were taken every 15 seconds, the pressure exerted by the 

 column of water in the burette being neglected. The observed 

 volumes were subtracted from the final reading obtained when 

 the absorption was complete^ and the resulting values (repre- 

 senting the volume of air still to be absorbed, before equilibrium 

 is established) were plotted on semi-logarithmic paper with 

 the corresponding times as abscissae. The points were found 

 to fall on a straight line, showing that log(f — x) is connected 

 with t by a linear equation. On differentiation we obtain as 

 before doe w „ N 



The value of X in this case is 0*380. 



