t] RADIO-ACTIVE SUBSTANCES df 
important property of discharging both positively and negatively 
electrified bodies. These results were confirmed and extended by 
Lord Kelvin, Smolan and Beattie. The writer made a detailed 
comparison” of the nature of the discharge produced by uranium 
with that produced by Réntgen rays, and showed that the dis- 
charging property of uranium is due to the production of charged 
ions by the radiation throughout the volume of the gas. The 
property has been made the basis of a qualitative and quantitative 
examination of the radiations from all radio-active bodies, and is 
discussed in detail in chapter I. 
The radiations from uranium are thus analogous, as regards 
their photographic and electrical actions, to Rontgen rays, but, 
compared with the rays from an ordinary X ray tube, these 
actions are extremely feeble. While with Rontgen rays a strong 
impression is produced on a photographic plate in a few minutes 
or even seconds, several days’ exposure to the uranium rays 1s 
required to produce a well-marked action, even though the uranium 
compound, enveloped in black paper, is placed close to the plate. 
The discharging action, while very easily measurable by suitable 
methods, is also small compared with that produced by X rays 
from an ordinary tube. 
6. The rays from uranium show no evidence of direct re- 
flection, refraction, or polarization®. While there is no direct 
reflection of the rays, there is apparently a diffuse reflection set 
up where the rays strike a solid obstacle. This is in reality due 
to a secondary radiation set up when the primary rays impinge 
upon matter. The presence of this secondary radiation at first 
gave rise to the erroneous view that the rays could be reflected 
and refracted like ordinary light. The absence of reflection, re- 
fraction, or polarization in the penetrating rays from uranium 
necessarily follows in the light of our present knowledge of the 
rays. It is now known that the uranium rays, mainly responsible 
for the photographic action, are deviable by a magnetic field, and 
are similar in all respects to cathode rays, 1.e. the rays are composed 
1 Nature, 56, 1897; Phil. Mag. 43, p. 418, 1897; 45, p. 277, 1898. 
2 Phil. Mag. Jan. 1899. 
3 Rutherford, Phil. Mag. Jan. 1899. 
