38 
PEOFESSOR E. RUTHEEFORD AND ME. E. K. McCLUXG 
In order to determine the number of ion.s, n, it is necessary to measure the maxi¬ 
mum current that can be produced between two electrodes when all the ions produced 
by the rays in the gas reach the charged electrodes before there is any appreciable 
loss of their number due to recombination. If i is the maximum or saturation 
current through the gas, then 
i = ne, 
where e is the charge on an ion. 
The value of e has been determined by J. J. Thomson,^ and is equal to 6'5 10~^° 
electro-static unit. 
From (1) and (2) 
nW = JH, 
therefore 
In order to determine W it is thus nece.s.sary to determine the value of H and i. 
The considerations on which the method is based are ; 
(1.) When the X rays are absorbed by a solid substance, the greater pro¬ 
portion of the energy is given up to the substance in the form of heat. 
(2.) The energy of the rays absorbed in passing through a given volume of 
the gas is used up in ^^I’oducing ions. 
(1) has been considered earlier in the paper, and it has been sho^vn that we are 
probably justified in assuming that a very large proportion of the energy due to rays 
absorbed in a substance like jdatinum is transformed into heat. A small proportion of 
the total energy is used ujd in setting up secondary rays at the point of incidence of 
the rays on a solid conductor and also at the point of emergence. 
In regard to (2), one of theauthorst has previously shown that the absorption of the 
rays in a gas is roughly jDrojDortional to the intensity of the ionization in the gas. 
Gases and vapours, which are made good conductors by the rays, also strongly absorb 
them. The absorption of the rays in the gas has no direct connection with the 
molecular weight or density of the gas. For example, in hydrochloric acid gas the 
rays are far more readily absorbed than in carbonic acid, a gas of greater density. 
Perrix has shovai that the ionization of a gas is approximately proportional to the 
pressure. This result has been confirmed by us, and the authors have also found 
that the absorption of the rays varies directly as the pressure, i.c., as the ionization of 
the gas. These results point to the conclusion that the absorption of the rays in a 
gas is closely connected witli the number of ions produced. It is possible that there 
is a certain anioimt of scattering of the rays in passing through a gas, but if the 
apparent absorption of the rays were due in any great measure to scattering, Ave 
* ‘Phil. Mag.,’ Dec., 1898. 
t Rutuerfokd, ‘Phil. Mag.,’ April, 1897. 
