Decay of Excited Radioactivity from Thorium, §c. 373 
Summary of Results. 
The results obtained from this series of experiments may 
be summed up as follows :-— 
(1) Different types of Rontgen rays produce different 
relative amounts of ionization in various gases. 
(2) One, at least, of the primary factors in causing this 
difference in the type of rays, which thus affect different gases 
differently, is the condition of the vacuum in the Rontgen- 
ray bulb which gives out the rays. 
(3) The ratio of the total ionization in oxygen and sulphur 
dioxide to that in air tended to become more nearly equal to 
the ratio of the densities of the gases as the rays became 
harder ; while in the comparison of hydrogen and carbon 
dioxide with air, the variation of the ratio for the relative 
ionization was in the opposite direction. In all cases, the 
general tendency appeared to be for the ionization in the 
denser gas to become more nearly equal to that in the iess 
dense gas as the rays became more penetrating. 
(4) This variation of the relative ionization in the different 
gases does not appear to have any connexion with the 
intensity of the radiation so long as the nature of the rays 
remains the same, but appears to be a direct result of the 
change in type of the rays. 
In conclusion I would express my thanks to Prof. Thomson, 
at whose suggestion this research was undertaken, for the 
interest shown and suggestions given during the course of 
these experiments. 
Cavendish Laboratory, Cambridge, 
June 15, 1904. 
XXXVI. The Decay of the Kacited Radioactivity from Thorium, 
Radium, and Actinium. By Miss H. Brooks, M.A.* 
may AEN a negatively charged metal plate has been ex- 
posed in the presence of the emanation from thorium 
for two or three days, the radiation from the activity excited 
on the plate steadily decays after removal according to an 
exponential law, and falls to half its original value in about 
eleven hours. If, however, a plate is exposed to the emana- 
tion for only a short interval of time, it was found by Prof. 
Rutherford+ that the radiation from a body thus excited 
increases at first after removal, reaches a maximum in the 
course of a few hours, and then decays at the same rate as 
the radiation from a plate exposed for a long interval. 
* Communicated by Prof. Rutherford, F.R.S. 
t Phil. Mag. Jan. 1908. 
Phil. Mag. 8. 6. Vol. 8. No. 45. Sept. 1904. 2C 
