Pressure- Gauges for the Highest Vacua. 95 



clined to think that a trace had been left in the hydrogen 

 which both the Viscometer Gauge and the Transpiration 

 Gauge have succeeded in detecting. The first question that 

 naturally arises is, why should we not have similar evidence of 

 the presence of water-vapour in the case of other gases ? the 

 answer to which is that, just as Graham showed experimentally 

 in his study of the viscosity of mixed gases that the viscosity 

 of hydrogen is specially sensitive to the presence of other 

 gases, a partial theory of the phenomenon being given in 

 " The Viscosity of Mixed Gases " (Phil. Mag. [_5] xl.), so it 

 is probable that a small amount of impurity largely affects 

 the slipping of hydrogen while only slightly affecting that 

 of the other gases, the difference between the case of hydrogen 

 and those of the other gases being due to two causes, first, 

 the small attraction of the solid for the hydrogen molecules, 

 whereby they are less condensed at the surface than mole- 

 cules of other gases, so that a given amount of vapour of 

 water in the gas makes a larger impurity in the surface 

 hydrogen than in the surface layers of other gases, and, 

 secondly, the small mass of the hydrogen molecule. Thus we 

 can give the following general sketch of the effect of the 

 presence of water-vapour in a gas on the surface phenomena 

 which we are treating of at present. Water-vapour appears 

 almost to dissolve in solid glass, forming a union which also 

 retains some free water-vapour at the surface ; thus a vibra- 

 ting solid surface would practically carry such a layer of 

 water-vapour as part of itself, and thus the layer would in- 

 troduce error into the slipping of the gas, the amount of 

 which for the ordinary gases is small, but becomes important 

 with hydrogen for the reasons already stated. Now if the 

 water- vapour is mostly gathered at the surface it cannot 

 affect the general viscosity of the gas, but only the slipping 

 and the deflecting force. In the case of hydrogen the effect 

 of the water-vapour seems to be to shorten the mean free 

 path by an amount which is nearly constant, and, moreover, 

 in the case of slipping alters the constant a in the relation 

 f=a\ to 1/3 of its usual value, so efficient is the layer of 

 massive water molecules in sweeping the light hydrogen 

 molecules with it. The curious result that the shortening of 

 the free path of hydrogen near a solid surface by the water- 

 vapour should be independent or nearly independent of the 

 pressure invites inquiry, but we can hardly delay any longer 

 here on this hydrogen episode. 



Resuming the study of the pressure-gauges, we will now 

 calculate the pressures in Crookes's highest hydrogen vacua 



12 ° 



