14 PROCEEDINGS OF THE AMERICAN ACADEMY. 



If instead of using the value 0.000020 for /x, we make use of 0.000022, 

 which was the smallest value of the decrement actually measured, the 

 values of c and k are 



c — 0.1494 

 k = 0.0677 



and the fourth column gives the values of p calculated, in the way 

 described, from the equation with these values for the constants instead 

 of those used in the preceding case. This calculation is carried out to 

 call attention to the magnitude of the change produced by a slight 

 change in the value of the constant, /x, which is subject to some uncer- 

 tainty, as has been shown. It will be seen that it is only where the 

 decrement, I, is very small that the difference between the two results 

 is appreciable. The smallest value of p in the fourth column is nearer 

 to the corresponding value of p, measured by the McLeod gauge ; but 

 the measured value is subject to an inaccuracy about as great as the 

 difference between the measured and calculated values of p. 



The results given above make it highly probable that the measure- 

 ments of pressure by the McLeod gauge are reliable in the case of pure, 

 dry hydrogen for pressures as low as the smallest pressure recorded 

 in the table. 



It is to be observed that for pressures below, say, 0.01 mm. of 

 mercury the friction with which we have to do is largely external 

 friction, and this is proportional to the density of the gas and the mean 

 molecular speed. The friction, and, therefore, the decrement, corre- 

 sponding to a given pressure will be smaller for hydrogen than for, 

 say, oxygen, or mercury vapor. In the case of mercury vapor the 

 decrement at a given low pressure ought to be about ten times as great 

 as it is for hydrogen at the same pressure, since the molecular weight 

 of mercury is about one hundred times that of hydrogen, while the 

 mean molecular speed is about one-tenth as great as it is for hydrogen. 



To be sure it does not follow that the decrement of a mixture of 

 hydrogen and mercury vapor, in such proportions that the partial 

 pressures of the two are the same, is simply the sum of the two decre- 

 ments obtained when the gas and vapor are separate. If one accepts 

 the expression deduced by Meyer 10 for the external friction of a gas, 

 and applies the same method in considering external friction of mixtures 

 as he does in dealing with the internal friction of mixtures, he will be 

 able better to understand how the external friction of a mixture of 



10 Kinetic Theory of Gases, p. 210 (Eng. Trans.). 



