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 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 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. 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.

i Pitchblende from Johanngeorgenstadt  8·3 × 10^{-11} "          Joachimsthal         7·0   " "          Pzibran              6·5   " "          Cornwall             1·6   " Clevite                              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