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 of the other substances possessed an activity even of the order of ¹⁄₁₀₀ of uranium.
The ionization of the gas by phosphorus does not, however, seem to be due to a penetrating radiation like that found in the case of uranium, but rather to a chemical action taking place at its surface. The compounds of phosphorus do not show any activity, and in this respect differ from uranium and the other active bodies.
Le Bon[[11]] has also observed that quinine sulphate, if heated and then allowed to cool, possesses for a short time the property of discharging both positively and negatively electrified bodies. It is necessary, however, to draw a sharp line of distinction between phenomena of this kind and those exhibited by the naturally radio-active bodies. While both, under special conditions, possess the property of ionizing the gas, the laws controlling the phenomena are quite distinct in the two cases. For example, only one compound of quinine shows the property, and that compound only when it has been subjected to a preliminary heating. The action of phosphorus depends on the nature of the gas, and varies with temperature. On the other hand, the activity of the naturally radio-active bodies is spontaneous and permanent. It is exhibited by all compounds, and is not, as far as is yet known, altered by change in the chemical or physical conditions.
9. The discharging and photographic action alone cannot be taken as a criterion as to whether a substance is radio-active or not. It is necessary in addition to examine the radiations, and to test whether the actions take place through appreciable thicknesses of all kinds of matter opaque to ordinary light. For example, a body giving out short waves of ultra-violet light can be made to behave in many respects like a radio-active body. As Lenard[[12]] has shown, short waves of ultra-violet light will ionize the gas in their path, and will be absorbed rapidly in the gas. They will produce strong photographic action, and may pass through some substances opaque to ordinary light. The similarity to a radio-active body is thus fairly complete as regards these properties. On the other hand, the emission of these light waves, unlike that of the radiations from an active body, will depend largely on the molecular state of the compound, or on temperature and other physical conditions. But the great point of distinction lies in the nature of the radiations from the bodies in question. In one case the radiations behave as transverse waves, obeying the usual laws of light waves, while in the case of a naturally active body, they consist for the most part of a continuous flight of material particles projected from the substance with great velocity. Before any substance can be called “radio-active” in the sense in which the term is used to describe the properties of the natural radio-active elements, it is thus necessary to make a close examination of its radiation; for it is unadvisable to extend the use of the term “radio-active” to substances which do not possess the characteristic radiating properties of the radio-active elements which we have described, and the active products which can be obtained from them. Some of the pseudo-active bodies will however be considered later in [chapter IX].
10. Thorium. In the course of an examination of a large number of substances, Schmidt[[13]] found that thorium, its compounds, and the minerals containing thorium, possessed properties similar to those of uranium. The same discovery was made independently by Mme Curie[[14]]. The rays from thorium compounds, like those from uranium, possess the properties of discharging electrified bodies and acting on a photographic plate. Under the same conditions the discharging action of the rays is about equal in amount to that of uranium, but the photographic effect is distinctly weaker.
The radiations from thorium are more complicated than those from uranium. It was early observed by several experimenters that the radiation from thorium compounds, especially the oxide, when tested by the electrified method, was very variable and uncertain. A detailed investigation of the radiations from thorium under various conditions was made by Owens[[15]]. He showed that thorium oxide, especially in thick layers, was able to produce conductivity in the gas when covered with a large thickness of paper, and that the amount of this conductivity could be greatly varied by blowing a current of air over the gas. In the course of an examination[[16]] of this action of the air current, the writer showed that thorium compounds gave out a material emanation made up of very small particles themselves radio-active. The emanation behaves like a radio-active gas; it diffuses rapidly through porous substances like paper, and is carried away by a current of air. The evidence of the existence of the emanation and its properties, is considered in detail later in [chapter VIII]. In addition to giving out an emanation, thorium behaves like uranium in emitting three types of radiation, each of which is similar in properties to the corresponding radiation from uranium.
11. Radio-active minerals. Mme Curie has examined the radio-activity of a large number of minerals containing uranium and thorium. The electrical method was used, and the current measured between two parallel plates 8 cms. in diameter and 3 cms. apart, when one plate was covered with a uniform layer of the active matter. The following numbers give the order of the saturation current obtained in amperes.
| Pitchblende from Johanngeorgenstadt | 8·3 × 10-11 |
| „ Joachimsthal | 7·0 „ |
| „ Pzibran | 6·5 „ |
| „ Cornwall | 1·6 „ |
| Cleveite | 1·4 „ |
| Chalcolite | 5·2 „ |
| Autunite | 2·7 „ |
| Thorite | from 0·3 to 1·4 „ |
| Orangite | 2·0 „ |
| Monazite | 0·5 „ |
| Xenotine | 0·03 „ |
| Aeschynite | 0·7 „ |
| Fergusonite | 0·4 „ |
| Samarskite | 1·1 „ |
| Niobite | 0·3 „ |
| Carnotite | 6·2 „ |
Some activity is to be expected in these minerals, since they all contain either thorium or uranium or a mixture of both. An examination of the action of the uranium compounds with the same apparatus and under the same conditions led to the following results:
| Uranium (containing a little carbon) | 2·3 × 10-11 amperes |
| Black oxide of uranium | 2·6 „ |
| Green „ „ | 1·8 „ |
| Acid uranic hydrate | 0·6 „ |
| Uranate of sodium | 1·2 „ |
| Uranate of potassium | 1·2 „ |
| Uranate of ammonia | 1·3 „ |
| Uranous sulphate | 0·7 „ |
| Sulphate of uranium and potassium | 0·7 „ |
| Acetate | 0·7 „ |
| Phosphate of copper and uranium | 0·9 „ |
| Oxysulphide of uranium | 1·2 „ |