HOGG. — FRICTION IN GASES AT LOW PRESSURES. 11 



It would naturally collect at E. We shall have evidence as to this 

 later. After some days of incessant work the expected result was at- 

 tained, as the character of the spectrum, obtained from the spectrum 

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

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

 then very slowly filled with hydrogen. The glass tube connecting G 

 and H was then sealed off so that there were left no stop-cock joints 

 to give trouble by leaking. 



The Dewar vessel was removed from G, but the one surrounding E 

 still remained. After the apparatus had cooled down to room temper- 

 ature the disk of the viscosity apparatus was raised and adjusted as 

 described in the former paper. 5 



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. 6 The method of pro- 

 cedure 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 pro- 

 portional 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 arrangement 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 arrangement the mercury vapor was removed, 

 but when observations on the decrement at different pressures were un- 

 dertaken 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 charcoal, yet it was 

 found impossible to obtain a steady condition. As the evaporation of 

 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 pres- 

 sure 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 



5 See pp. 133, 134. 6 See pp. 124, 125 of former paper. 



