mr. j. c, Mclennan on electrical conductivity in gases 
pressures, has shown that for any particular gas the coefficient of absorption varies 
directly as the pressure. In the case of air, taking I to denote the intensity of the 
rays issuing from the window of the discharge tube, and Ie - ^ their intensity at a 
distance x from the window, he found for X the values given in Table IX. 
Table IX. 
Air pressure. 
Coefficient of absorption. 
miUims. 
760 
3-43 
331 
1-51 
165 
•661 
83-7 
•396 
40’5 
•235 
19-3 
•117 
io-o 
•0400 
2-7 
•0166 
•78 
•00416 
These numbers, it will be seen, amply support Lenard’s conclusion. Similar 
tables, given by him for a number of gases, all exhibit the same relation between the 
values of X and the corresponding pressures of the gas. 
Now, if the values of the coefficient of absorption are taken to represent the rela¬ 
tive ionizations produced in a gas, at a point where the pressure is varied but the 
intensity of the rays kept constant, it follows from Lenard’s numbers that the 
ionization in any particular gas would vary directly as the pressure to which it was 
subjected. 
This result, which follows as a deduction from the preceding experiments, has also 
been found experimentally by Perrin* to characterise the ionization produced by 
Rontgen rays. It is true that with Rontgen rays a number of experimenters have 
found quite different relations to hold between the ionization and the pressure ; but 
in most cases they have vitiated their results either through omitting to use satu¬ 
rating electromotive forces, or through neglecting to arrange their experiments so as 
to eliminate the metal effect observed by Perrin. 
With uranium radiation also, Rutherford f has found the ionization to be propor¬ 
tional to the pressure of the gas traversed. 
The direct experimental verification of a law of this kind is always accompanied by 
a serious difficulty. The law has reference to the action of rays whose intensity is 
constant throughout the region ionized. With rays that are easily absorbed by 
gases at ordinary pressures, this condition can be realised either by the use of very 
thin layers of gas or by investigating the ionizations at very' low pressures. Owing 
* ‘Comptes Rendus,’ vol. 123, p. 87S. 
t ‘Phil. Mag.,’ January, 1899, p. 136. 
