Gases at Loic Pressures. 377 



force exerted by a lamp on a mica vane, blackened on the 

 face which is turned towards the lamp, and the mean pressure 

 in the gas in which the vane is placed. The three-fold 

 purpose of the investigation was pointed out there, viz. : — 

 First, to investigate the relation between friction and 

 pressure where the pressures are so small that " slip " is 

 appreciable; second, to determine the relation of transpira- 

 tion force in the special form of apparatus described there*; 

 and, third, to make use, if possible, of these two relations 

 to test the validity of the McLeod gauge measurements of 

 pressure, and, if these measurements should prove unreliable, 

 to make use of one of the relations named above to measure 

 gas pressure. 



There has been much delay in carrying out the investi- 

 gation with the apparatus improved in the manner indicated 

 in the closing paragraphs of that paper, but now some results 

 have been obtained in so far as the friction problem is 

 concerned. 



As was pointed out in the paper mentioned, the investi- 

 gation was defective in two respects. It was found that, in 

 spite of the care which was taken to exclude mercury vapour 

 from the apparatus, some of this vapour was undoubtedly 

 present. This no doubt was due to the fact that the whole 

 apparatus had to be maintained at a high temperature for 

 long periods to insure drying, and thus the presence of the 

 least speck of liquid mercury would cause, when evaporation 

 took place, the diffusion of comparatively large quantities of 

 the vapour through the apparatus. Again, the logarithmic 

 decrement due to the friction in the suspending fibre was 

 not determined directly by experiment, and in the discussion 

 •of the results obtained its value was calculated. 



The details of the method since used to exclude the mercury 

 vapour and to determine the decrement due to friction in the 

 fibre will appear later. Meanwhile a summary of what has 

 been accomplished is given here. 



First, the decrement due to the friction in the suspending 

 fibre of the viscosity apparatus has been determined 

 experimentally. 



Second, mercury vapour has been excluded to such a 

 degree that, even when the whole apparatus, in which the 

 presence of the vapour would be objectionable, was kept at a 

 temperature of 150° C, the mercury lines were absent from 

 the spectrum of the gas enclosed. 



Third, the value of the decrement has been obtained for 



* See p. 129 of that paper. 



