522 
HON. E. J. STRUTT OX THE COXDUCTIVITY 
al^out 150 millinis. This type of radiation is, therefore, more rapidly absorbed by 
the air than any of the kinds which have already been discussed. The rate of leak 
at ])ressnres so low was rather too small for convenient measurement. A somewhat 
different metliod of comparing the gases was used, which avoided this difficulty. 
The rate of leak was taken in air, at some convenient pressure, usually about 
250 millins., and afterwards at a slightly greater pressure, })erhaps 260 millims. The 
gas for comparison having been introduced into the vessel, its pressure was adjusted 
till the rate of leak was intermediate between those given by air at the two 
slightly different pressures. The pressure at which the air would have given the 
exact rate of leak observed in the other gas was determined by interpolation, on 
the sufficiently accurate assumption that the rate of leak could be represented 
between these narrow limits as a linear function of the pressure. 
In this way, then, were determined the pressures at which the two gases gave 
exactly tlie same rate of leak. 
If now we assume that the absorption of the radiation by a gas is proportional 
to the saturation conductivity produced in the gas, it follows that if two gases are 
adjusted to give ecpial conductivity, the radiation is absorbed to an equal extent 
in each. Consequently the correction foi' absorption of the radiation is eliminated, 
and the relative conductivities are inversely proportional to the observed pressures. 
The assumption that the absorption by a gas is proportional to the conductivity 
])roduced in it, is justified by PiUTHEEFOEd’s experiments (‘Phil. Mag.,’ Jan., 1899, 
p. 137). He found that if practically the whole of the radiation was absorbed, al} 
the gases tried gave nearly the same rate of leak under a saturating electromotive 
force. This implies the truth of the relation in question. The next table gives the 
values found for uranium radiation, with the different gases. 
Gas. 
Relative conductivity (air = 1). 
Separate determinations. 
Mean. 
Hydrogen .... 
•208 -209 -211 -222 
•213 
Sulphur dioxide . . 
2-11 2-06 2-08 2-0.3 
2-08 
Methyl iodide . . . 
3-48 3-41 3-69 3-62 
3 - 5.5 
§ 7. Summary of Results. 
The following table collects the final results of the experiments described in this 
paper. It includes also the results obtained by J. J. Thomson (‘ Proc. Camb. Phil. 
Soc.,’vol. 10, p. 9) and Peeetn (‘Piayons Cathodiques et Payons de Pontgen’) for 
relative conductivities under Pontgen rays, and by MacLennan, for cathode rays. 
It is convenient to quote these here for the sake of comparison. 
