ON ENERGY OF RONTGEN AND BECQUEREL RAYS, ETC. 
39 
should expect the ahsorption to depend chiefly on the density of the gas, and such is 
not the case. Rontgen and others have observed that the gas itself which has been 
acted on by the rays gives out a radiation which is able to light uj) a fluorescent 
screen. This radiation may be due either to the scattering of the rays, or to the 
radiation caused by the recombination of the ions. In either case it is probable that 
the radiation is of a type similar to the secondary radiation set up at the surface of 
metals when X rays impinge upon them. This secondary radiation is far more 
readily absorbed in gases than the primary radiation, and would be absorbed in jdi’o- 
ducing ions in the gas. The rate of discharge wmuld be increased, and provided all 
the scattered, or secondary, radiation were used up in j^roducing fresh ions between 
the electrodes, no correction for the amount of scattered radiation would be required. 
This of course proceeds on the assumption that the ions produced by the primary and 
secondary radiation are the same, and require the same amount of energy in each case 
to produce them. 
It is not practicable to measure directly the total maximum current through the 
gas, due to the passage of all the ions produced between charged electrodes, as the 
rays could jDass through several hundred metres of the gas before ajDjDroximately 
complete absorption took place. 
In practice the number of ions produced in a known small volume of the gas is 
determined, and also the coefficient of absorption of the rays by the gas. The total 
number of ions that would be produced, j)rovided all the rays were absorbed, can he 
directly calculated. 
An account will now be given of the experiments performed to measure the 
absorption of the rays in gases. 
Ahsorption of the Rays in Gases. 
The bulb employed gave out rays of great intensity and penetrating power, and 
the absorption of the rays in air was small. About 3 per cent, of the rays were 
absorbed in passing through a metre of air at atmospheric j^ressure and temperature. 
In order to measure the absorption, a delicate null method was employed. No direct 
method can be employed on account of the smallness of the absorption and the varia¬ 
tion of the intensity of the rays during the experiments. 
Fig. 5 shows the general arrangement of the apparatus. A similar method was 
employed by one of us^' on a previous occasion to measure the absorption of the rays 
in gases. 
Two long brass tubes, A, A', 118 centims. long and 3'4 centims. in diameter, were 
placed horizontally at a slight angle to each other, and in such a position with regard 
to the bulb that the axes of the two tubes met at a point on the surface of the 
platinum plate of the anticathode. The ends of the tubes were covered with 
E. Rutherford, ‘Phil. Mcag.,’ April, 1897. 
