HOGG. — FRICTION IN GASES AT LOW PRESSURES. 9 



make use of the value 0.000020 as the true value to which the decre- 

 ment will approach as the exhaustion is pushed higher and higher. It 

 will be seen from Table I that the calculations are carried through not 

 only with this value, but also with the actually measured value 

 0.000022. This is done simply to show what effect such a change in 

 the value of p has on the series of results obtained. 



It may be of interest to state that, at the stage of exhaustion when 

 ^ = 0.000022 was obtained, the McLeod gauge indicated a pressure 

 certainly less than 0.000001mm. It is, to be sure, of little value to give 

 the measurement of a pressure by the gauge where a column of mercury 

 a fraction of a millimeter high requires to be measured, and especially 

 is this true where the tube containing the mercury has been heated 

 and cooled repeatedly. The mercury has a habit of sticking to the 

 glass to such an extent that pressure measurements under the condi- 

 tions mentioned are surely not reliable. The value of the pressure 

 given above, then, only indicates the order of magnitude of the pres- 

 sure. Though the factor of the gauge used was 95813, yet it was 

 quite inadequate to measure the pressure of the gas in the vessel. 



Removal of Water Vapor and Mercury Vapor from the 

 Hydrogen in the Viscosity Apparatus. 



For this purpose it was necessary to make arrangements by which 

 no vapor should be carried into the apparatus with the entering gas, 

 and also all the vapor which was already in the apparatus might be 

 taken out. The following arrangement was finally adopted, Figure 2. 

 E is a U-tube of small bore, and bent so that it may enter the long 

 Dewar vessel already mentioned. For reasons which will appear later 

 it was found necessary for the remainder of the investigation to replace 

 the tube C, Figure 1, by this tube E. F is a tube leading from the 

 gas generator. It enters G, which is similar to C of Figure 1. It can 

 be surrounded by a heater or a Dewar vessel as circumstances may re- 

 quire. A connecting tube leads from G to a point on the tube H, 

 which connects E to the viscosity apparatus A, Figure 1. I leads to 

 the pump and McLeod gauge. Anything which proceeds from the 

 pump or McLeod gauge towards the viscosity apparatus must pass 

 through E. Moreover, the gas entering from the generator will, with 

 the given arrangement, retard the diffusion of mercury vapor from the 

 pump and gauge towards the viscosity apparatus. If there is no vapor 

 entering with the gas, there can be none entering the viscosity appara- 

 tus without passing through E, and, since throughout the experiment 

 this tube was kept surrounded by the liquid air, the pressure of the 



