| Radio-active substance. | Fraction of radiation transmitted by the sheet. | |
| Uranium ............... | ... | 0·18 |
| Uranium oxide, U2O5 ......... | ... | 0·20 |
| Uranate of ammonium ... ... ... ... | ... | 0·20 |
| Phosphate of uranium and copper | ... | 0·21 |
| Thorium oxide of thickness 0·25 m.m. | ... | 0·38 |
| "" 0·5" | ... | 0·47 |
| "" 3·0" | ... | 0·70 |
| "" 0·60" | ... | 0·70 |
| Thorium sulphate ... ... 0·25" | ... | 0·38 |
The characteristics of the thorium radiation have formed the subject of very complete publications. Mr. Owens has [d]emonstrated that a uniform current is only obtained after [s]ome time has elapsed, with an enclosed apparatus, and that the intensity of the current is greatly reduced under the influence of a current of air (which does not occur with the compounds of uranium). Mr. Rutherford has made similar experiments, and has explained them by the proposition that thorium and its compounds produce, besides the Becquerel rays, another emanation, composed of extremely minute particles, which remain radio-active for some time after their emission, and are capable of being swept along by a current of air.
The characteristics of the thorium radiation, which have reference to the thickness of the layer employed and to the action of air currents, have an intimate connection with the phenomenon of the radio-activity induced, and of its propagation from place to place. This phenomenon was observed for the first time with radium, and will be described later.
The radio-activity of thorium and uranium compounds appears as an atomic property. M. Becquerel has already observed that all uranium compounds are active, and had concluded that their activity was due to the presence of the element uranium; he also demonstrated that uranium was more active than its salts. I have investigated, from this point of view, the compounds of thorium and uranium, and have taken a great many measurements of their activity under different conditions. The result of all these determinations shows the radio-activity of these substances to be decidedly an atomic property. It seems to depend upon the presence of atoms of the two elements in question, and is not influenced by any change of physical state or chemical decomposition. The chemical combinations and mixtures containing uranium or thorium are active in pro-
