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 are similar in all respects to cathode rays, i.e. the rays are composed of small particles projected at great velocities. The absence of the ordinary properties of transverse light waves is thus to be expected.

7. The rays from uranium are complex in character, and, in addition to the penetrating deviable rays, there is also given off a radiation very readily absorbed by passing through thin layers of metal foil, or by traversing a few centimetres of air. The photographic action due to these rays is very feeble in comparison with that of the penetrating rays, although the discharge of electrified bodies is mainly caused by them. Besides these two types of rays, some rays are emitted which are of an extremely penetrating character and are non-deviable by a magnetic field. These rays are difficult to detect photographically, but can readily be examined by the electric method.

8. The question naturally arose whether the property of spontaneously giving out penetrating radiations was confined to uranium and its compounds, or whether it was exhibited to any appreciable extent by other substances.

By the electrical method, with an electrometer of ordinary sensitiveness, any body which possesses an activity of the order of 1/100 of that of uranium can be detected. With an electroscope of special construction, such as has been designed by C. T. R. Wilson for his experiments on the natural ionization of air, a substance of activity 1/10000 and probably 1/100000 of that of uranium can be detected.

If an active body like uranium be mixed with an inactive body, the resulting activity in the mixture is generally considerably less than that due to the active substance alone. This is due to the absorption of the radiation by the inactive matter present. The amount of decrease largely depends on the thickness of the layer from which the activity is determined.

Mme Curie made a detailed examination by the electrical method of the great majority of known substances, including the very rare elements, to see if they possessed any activity. In cases where it was possible, several compounds of the elements were examined. With the exception of thorium and phosphorus, none