10 PROCEEDINGS OF THE AMERICAN ACADEMY. 



If the mixture is one of hydrogen and mercury vapor such that^i = p 2 , 

 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 

 vapor, 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 vapor absent. If pi = 1000/> 2 , the ratio is 

 about 1.05, and if jt?i = 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 vapor that the lowest pressure of 

 vapor obtainable by the method used is the pressure of mercury vapor 

 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 

 temperature 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, 0.000016 

 mm., the vapor pressure of mercury at the temperature of liquid 

 air, boiling under atmospheric pressure, would require to be as low as 

 0.000,000,016 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 vapor 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 vapor. The McLeod gauge, of course, takes no 

 cognizance of it, and in fact serves to introduce the vapor where it is 

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

 to know the pressure in the vessel, and yet keep the gas pure, it would 

 be desirable to have a gauge which would not introduce any impurity. 



If the inference made above as to the validity of the McLeod gauge 

 measurements on gas pressure is allowed, then we can say that reliance 

 may be placed upon the measurements of pressure from decrement 

 measurements in the apparatus used in this investigation. This method 

 need introduce no mercury vapor, but it takes account of all that is in 

 the vessel. Moreover, a discussion similar to that used for mercury 

 vapor will show that in the case of oxygen the decrement, correspond- 

 ing to a certain gas pressure, will be about four times as great as it is 

 in the case of hydrogen. In the case of oxygen, therefore, a pressure 

 of 0.00001 mm. should be measured with an accuracy of from five to 



