164 G. F. Barker — Radioactivity of Thorium Minerals. 



or by the electroscope. They then rubbed one ^rain of this 

 thoria with hve sjranis of a specially purified uranoso-uranic 

 oxide, heated the mass to 400° for four hours and allowed the 

 mixture to stand for fourteen days. It was then dissolved in 

 nitric acid, the thoria precipitated as oxalate, washed and 

 ignited. On testing it, it was found to be strongly active, dis- 

 charging the electroscope in Im. 5s., even after four weeks. 



In the course of some experiments on the photographic 

 action of radioactive substances made in December, 1899, 1 

 obtained an intense darkening of the plate after an exposm-e 

 of 8 hours, with uranyl chloride, nranium chloride, metallic 

 nranium and uraninite, the darkening being in the order 

 named. A few weeks later, using uraninite as the radioactive 

 source, with an exposure of 16 hours, I observed that the 

 penetrating power of the rays it emitted, through metal plates 

 •0026 inch in thickness, were in the order aluminum, tin, cop- 

 per, silver, lead, platinum. Similar experiments with de Haen's 

 radium salts, brought from Europe in September of that year, 

 showed much more powerful action, an exposure of fifteen 

 minutes giving a distinct effect upon the plate. 



During the past winter these experiments have been resumed, 

 with especial reference to the compounds of thorium and the 

 minerals containing it. To compare the intensity of these 

 radioactive minerals, an exposure of 48 hours was made with 

 uraninite (from Bohemia and from Saxony), gum mite (North 

 Carolina), autunite (Limoges), euxenite (JN'orway), thorite (Nor- 

 way), samarskite (North Carolina), and orangite (Norway). 

 While the intensity of the darkening was in general in the 

 order above given, the effect of the first three minerals far 

 exceeded any of the others. Moreover the penetrating power 

 of the emitted rays was examined at the same time by placing 

 each of the minerals upon a thin brass stencil plate, having its 

 initial cut through it. The rays from gum mite seemed to 

 penetrate the brass the most readily ; then followed the uranin- 

 ite, the autunite, the euxenite, the samarskite and the thorite. 



The investigations were then continued with especial refer- 

 ence to the mineral monazite, the chief source of the thoria 

 now so largely used in this country and Europe, for the manu- 

 facture of the Welsbach gas mantles'. By the courtesy of Mr. 

 M. C. Whitaker, of the Chemical Department of the Welsbach 

 Company, I was furnished with authentic samples of Brazilian 

 monazite sand, of monazite sand from North Carolina and of 

 pure thoria from each of these ; this thoria having been 

 obtained in the course of analysis. I also obtained a sample 

 of pure ammonium-thorium nitrate and one of thorium oxalate. 

 On exposing these six substances on a sensitive photographic 

 plate thoroughly protected from the light, for 48 hours, satis- 



