Gases at Low Pressures, 389 



the nature of the gas it follows that the ratio of the ex- 

 ternal friction of the mixture to the external friction of 

 the gas whose partial pressure is y> l5 if it were in the vessel 

 alone, is 



^A\7j + 



Pi ™2 Jh 



'$xm 1 £l 1 N^^O 





m^ P2 



m x ' p 



If the mixture is one of hydrogen and mercury vapour 

 such that pi=p<i, the above ratio becomes about 14. This 

 means that if the pressure is measured by the McLeod gauge, 

 which takes no account of the mercury vapour, the friction 

 of the mixture would be about fourteen times as much as it 

 would be with the same hydrogen pressure as in this case, 

 but with the mercury vapour absent. If ^ = 1000 _p 2 the 

 ratio is about 1*05, and if £>x = 10,000 /> 2 it becomes about 

 1-005. 



It might be urged with regard to the method described 

 above for freeing the hydrogen from mercury vapour that 

 the lowest pressure of vapour obtainable by the method used 

 is the pressure of mercury vapour at the temperature of 

 liquid air boiling at atmospheric pressure. This pressure at 

 0° C. is about 0*0005 mm., but what it is at the lower tem- 

 perature mentioned can hardly even be conjectured. We 

 have simply to fall back upon the spectroscopic test. The 

 above discussion shows, however, that if this pressure is less 

 than 0*001 of the pressure of the hydrogen it will not very 

 seriously affect the results. If it is as low as 0*0001 of the 

 hydrogen pressure then the error in the observations will 

 easily be greater than any error introduced in this way. 

 Considering the lowest pressure reached, namely, 0*00001f>mm., 

 the vapour pressure of! mercury at the temperature of liquid 

 air, boiling under atmospheric pressure, would require to be 

 as low as 0*000,000,01C>, in order that the ratio of the partial 

 pressures should be 1 : 1000. 



This case serves to show how important it may be to 

 consider mercury vapour when we are dealing with these 

 very low pressures. It indicates that, in all high vacua 

 work where we are considering the properties of a particular 

 gas, it is important that great care should be taken to exclude 

 this vapour. The McLeod gauge, of course, takes no cogni- 

 zance of it, and in fact serves to introduce the vapour where 

 it is not wanted. In all cases where the vacuum is high 



