1881.] On the Viscosity of Gases at High Exhaustions. 453 



The remarkable character of hydrogen is the uniformity of resist- 

 ance which it presents. It obeys Maxwell's law almost absolutely up 

 to an exhaustion of about 700 M, and then it commences to break 

 down. Up to this point the line of viscosity is almost vertical. It 

 then commences to curve over, and when the mean free path assumes 

 proportions comparable with the dimensions of the bulb, and approaches 

 infinity, the viscosity curve in like manner draws near the zero line. 



The repulsive force of radiation is higher in hydrogen than in any 

 other gas. It commences at as low an exhaustion as 14 millims., but 

 does not increase to any great extent till an exhaustion of 200 M 

 is attained ; it then rises rapidly to a maximum at between 40 and 

 60 M, after which it falls away to zero. The maximum repulsion 

 exerted by radiation in hydrogen is to that in air as 70 to 42*6. This 

 fact is now utilised in the construction of radiometers and similar 

 instruments when great sensitiveness is required. 



Taking the viscosity of air at 760 millims. as 0*1124, and hydrogen 

 as 0*0499, the proportion between them is 0*4439. 



The Spectrum of Hydrogen. 



The red line (\=6562), the green line (X= 4861), and the blue line 

 (\=4340) are seen at their brightest at a pressure of about 3 millims., 

 and after that exhaustion they begin to diminish in intensity. As ex- 

 haustion proceeds, a variation in visibility of the three lines is ob- 

 served. Thus , at 36 millims. the red line is seen brightly, the green 

 faintly, whilst the blue line cannot be detected. At 15 millims. the 

 blue line is seen, and the three keep visible till an exhaustion of 

 418 M is reached, when the blue line becomes difficult to see. At 38 M 

 only the red and green lines are visible, the red being very faint. It 

 is seen with increasing difficulty up to an exhaustion of 2 M, when it 

 can be seen no longer. The green line now remains visible up to an 

 exhaustion of 0*37 M, beyond which it has not been seen. 



It is worthy of remark that although, when working with pure 

 hydrogen, the green line is always the last to go, it is not the first to 

 appear when hydrogen is present as an impurity in other gases, Thus 

 when working with carbonic anhydride insufficiently purified, the red 

 hydrogen line is often seen, but never the green or the blue line. 



Influence of Aqueous Vapour on the Viscosity of Air. 



In the foregoing experiments many discrepancies were traced to the 

 presence of moisture in the gas. The influence of aqueous vapour 

 does not appear to be great when present in moderate amount in gas 

 of normal density, but at high exhaustions it introduces errors which 

 interfere with the uniformity of the results. A series of experiments 

 were accordingly undertaken to trace the special action of aqueous 

 vapour when mixed with air. 



