CHAPTER I. RADIO-ACTIVE SUBSTANCES.

10. Thorium. In the course of an examination of a large number of substances, Schmidt¹³ 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 Curie¹⁴. 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 Owens¹⁵. 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 examination¹⁶ 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.