G. F. Barlter — Radioactivity of Thorium Minerals. 167 



parations which did not affect a sensitive photographic plate 

 noticeably after an exposure of seventy hours." He prefers, 

 however, the electric method, and says : " I have prepared a 

 large number of thorium preparations, several hundred, and 

 have not yet succeeded in securing a thorium compound or 

 derivative which did not possess some radioactivity by the latter 

 method." Upon testing the " very pure " specimen received, 

 by an exposure of 96 hours, a satisfactory darkening of the pho- 

 tographic film resulted ; though much less intense than the stain 

 given by the thoria previously employed. Were it possible to 

 separate by fractionation or otherwise a sample of thoria into 

 two portions, one of wliicli had only a photographic and the 

 other only an electric action, it might suggest that the con- 

 stituent of the thoria wliich emits the /3 rays may be separable 

 from the one emitting the a rays. Inasmuch as all the thoria 

 preparations examined by Baskerville were obtained from mon- 

 azite sand, coming either from N^orth Carolina or Brazil, and 

 inasmuch as none of these preparations failed to show a radio- 

 activity either photographic or electric, the conviction is 

 strengthened that thorium does not owe its activity to an 

 induction from uranium existing in the minerals from which it 

 was obtained. 



The fact that inactive substances acquire under the influence 

 of radium a temporary activity was first observed by the 

 Curies,* who called the effect " la radioactivite induite." On 

 exposing a disc of zinc for example, S^''^ in diameter, opposite 

 a surface of radium 4""" in diameter and S""" distant, an activ- 

 ity 2000 times that of uranium w^as obtained ; practically the 

 same results being obtained with nickel, brass, bismuth, 

 aluminum and lead. Solutions of radium salts appear to pro- 

 duce the phenomenon with more intensity than the solid salts. 

 Gases even may in this way be made radioactive. Water dis- 

 tilled from radium solutions is radioactive. When barium 

 sulphate is precipitated from a solution of barium chloride by 

 sulphuric acid in presence of a salt of uranium, it is found to 

 be radioactive. Debierne,f proceeding in this way and using 

 actinium in place of uranium, obtained a barium salt of great 

 activity, which could be concentrated by fractionation, the 

 most active fractions being 1000 times as active as uranium. 

 The result w^as a radioactive barium, carrying its properties 

 into ah its salts and distinguished from radium mainly by its 

 spectrum and by the gradual loss of its properties. 



The discovery in 1899, almost simultaneously by the Curies;}: 

 and by Ilutherford,§ of the emission by both radium and 



* Comptes Rendus, cxxix, 714, 1899 ; cxxx, 1013, 1900. 

 \ Comptes Eendus, cxxxi, 333, 1900. 

 X Eapports au Congres Intern, de Physique, iii, 79, 1900. 

 § Phil. Mag., V, xlix, 2, 1900 ; Yl, v, 95, 1903. 



