1015] on Electrons and Heat 451 



distribution of the normal velocity component for the emitted elec- 

 trons is that given by Maxwell, it is necessary (and sufficient) that 

 the currents i\ and i,, which flow against potentials Y^ and Y., 

 respectively, should satisfy the equation 



where R is the constant in the equation pv = RT of a perfect gas, 

 and Q is the quantity of electricity which liberates half a cubic centi- 

 meter of hydrogen at 0° C. and 760 mms. in a water voltameter. 

 The requirements of this formula are found to be fully satisfied by 

 the results of the experiments. Thus the logarithms of the ratios of 

 the currents are found to be accurately proportional to the differences 

 in the corresponding opposing potentials at a given temperature. 

 Again, since Q is a Avell-known physical constant and the value of T 

 was estimated during the experiments, we can use the experimental 

 data to obtain a value of the gas constant R. Eight experiments, 

 made under conditions as varied as possible, when treated in this 

 way gave values of R which varied between the extreme limits 

 3 '08 X 10^ and 4*4:6 x 10^ ergs per c.c. per deg. 0. These values 

 exhibit a rather wide variation which, however, is believed to be 

 fortuitous, so that the mean value should be much more accurate. 

 The mean of the eight values gives R = 3*72 x 10-^, whereas the 

 number given by the gas equation is R = 3*711 x 10^ in the same 

 units. 



The fact that the value of the gas constant can be deduced in 

 this way from purely electrical measurements must be regarded as a 

 remarkable confirmation of the general position. The results of 

 these experiments, and others of a similar nature which I have not 

 time to describe, show not only that the velocities of the electrons 

 are distributed about the average value in accordance with Maxwell's 

 law, but also that the emitted electrons are kinetically identical with 

 the molecules of a hypothetical gas of equal molecular weight at the 

 temperature of the hot metal. The experiments referred to formed, 

 so far as I am aware, the first direct experimental demonstration of 

 the truth of Maxwell's law of distribution of velocities, and, although 

 many of the consequences of this law have been made visible by 

 the beautiful experiments of Perrin on the Brownian movement, I 

 believe that they still furnish the most direct experimental verification 

 of its truth. 



Quite recently a number of experimenters have called in question 

 the general position which I have taken as to the nature of the 

 process of electron emission from hot bodies, and have asserted that 

 this effect is caused by chemical action between the hot solid and 

 traces of contaminants, usually supposed to be gaseous, which have 

 access to it. Whilst T feel that the value of the evidence in favour 



