384 Mr. J. L. Hogg on Friction in 



temperature of about 150° C. served to promote the evapo- 

 ration of the mercury, which in all probability adhered to 

 the inner glass surfaces. Comparatively large quantities 

 of pure dry hydrogen were allowed to pass into the vessel, 

 and were then taken out. Each exhaustion would assuredly 

 sweep out some vapour if it was present. It would naturally 

 collect at E. We shall have evidence as to this later. After 

 some days of incessant work the expected result was attained, 

 as the character of the spectrum, obtained from the spectrum 

 tube S, showed. Even when the temperature of the viscosity 

 apparatus was 150° C. the mercury lines were absent. The 

 apparatus was then very slowly filled with hydrogen. The 

 glass tube connecting Gr and H was then sealed off so that 

 there were left no stopcock joints to give trouble by 

 leaking. 



The Dewar vessel was now removed from G, but the one 

 surrounding E still remained. After the apparatus had 

 cooled down to room temperature the disk of the viscosity 

 apparatus was raised and adjusted as described in the former 

 paper*. 



Method of Experiment. 



The investigation of the relation of friction to pressure- 

 consists in measuring, for a given density of the gas, the 

 logarithmic decrement of the suspended disk, which is made 

 to oscillate as a torsion pendulum between the two fixed 

 plates of the apparatus f. The method of procedure was to 

 measure the gas pressure in the apparatus by means of a 

 manometer when the pressure was large, and by the McLeod 

 gauge when it was small, and then to set the disk of the 

 apparatus swinging and measure the decrement. Since the 

 latter can be shown to be proportional to the resistance 

 experienced by the disk, one gets data for the determination 

 of the relation between friction and pressure. 



It may be of interest to state here that in the first arrange- 

 ment of the apparatus for the determination of the above 

 relation, instead of the simple bent tube E, a tube containing 

 charcoal, similar to the tube C, was used. With this arrange- 

 ment the mercury vapour was removed, but when observations 

 on the decrement at different pressures were undertaken a 

 difficulty presented itself. Although all of the tube contain- 

 ing the charcoal was immersed in the liquid air, the surface 

 of which was always several inches above the top of the 



* See pp. 133, 134. 



t Pp. 124, 125 of former paper. 



