Gases at Loiv Pressures. 379 



is discussed in detail. The first column o£ the table contains- 

 a series of values of the decrement, for hydrogen, each of 

 which corresponds to a definite pressure in the gas. The 

 various values of the pressure are given in the second column. 

 The first three of them were obtained from a manometer. 

 From two values of p, the corresponding values of I, and 

 the value of //., there are obtained two equations for the 

 determination of the constants k and c in the above equation. 

 These determined, it is clear that from any value of /, within 

 the range indicated above, the corresponding value of p can 

 be obtained from the formula by a simple calculation. The 

 numbers thus obtained for the various values of / are given, 

 in the third and fourth columns. 



It would, therefore, seem highly probable that so far as 

 hydrogen is concerned the McLeod gauge can be relied upon 

 for pressures as low as the lowest used, and which are re- 

 corded in Table I.; and that, in the case of hydrogen, the 

 measurement of friction can be used as a convenient and 

 accurate method ot measuring pressure, provided care is 

 taken to exclude mercury vapour. This matter will be 

 discussed at length later. 



The details of the method used to overcome the difficulties 

 named above follow. 



Measurement of Decrement due to the Friction 

 in the Fil>re. 



Referring to fig. I it will be seen that the tube C is inserted 

 in such a position that nothing can pass to the viscosity 

 apparatus from the McLeod gauge B, or from the pump, 

 which is connected to I), without passing through it. This 

 tube C, therefore, replaces the tubes of sulphur and silver 

 whose purpose was explained in the earlier paper. C is 

 filled with granular charcoal, and is so arranged that either 

 a cylindrical electrical boater or a long Dewar vessel can 

 enclose it. AVhen C had been placed in position and sealed 

 in place, the whole apparatus was exhausted through D by 

 means of the mechanical pump, and then dry air was allowed 

 to pass in through an opening placed near the pump. The 

 exhaustion was again performed and the admission of dry 

 air repeated. This exhaustion and admission of air were 

 carried out alternately many times for the purpose of re- 

 moving the comparatively large quantities o£ moisture which 

 had been formed in the vessel during the process of making 

 the various joints in the construction of the apparatus. When 

 it was certain that the whole apparatus had been made fairly 



