166 G. F. Earlier — Radioactivity of Thorium Minerals, 



by a map^netic field, which are apparently identical with 

 cathode rays ; (2) those not so deviable, bnt very readily ab- 

 sorbed ; and (3) non-deviable rays, which like Rontgen rays 

 have great penetrating power. He states that the rays emitted 

 by uranium are substantially of the first kind, those from 

 •polonium are of the second kind, and those sent out by radium 

 and thorium consist of all three kinds. In subsequent investi- 

 gations, Rutherford,* in connection first with Grierf and 

 afterward with Soddy,:}: made this classification more rigid. 

 Radium rays were divided into three distinct tyj^es ; a rays, 

 easily absorbed by thin layers of matter, and producing the 

 greater part of the ionizing effect ; /3 rays, consisting of nega- 

 tively charged particles moving with high velocity and similar 

 in all respects to cathode rays ; and 7 rays, non-deviable in a 

 magnetic field and of a very penetrating character. The 

 energy radiated in the form of a rays is about 1000 times 

 greater than that emitted as /3 rays. During the present year 

 Rutherford§ has succeeded in obtaining the deviation of the 

 a rays both in the magnetic and the electric fields, and has 

 thereby proved these rays to consist of j)ositively charged 

 material particles having a much greater mass and a lower 

 velocity than those constituting the /3 rays. In fact the mass 

 of those particles is of the same order as that of the hydrogen 

 atom and they travel with a speed only about one- tenth that of 

 light. Moreover, it has been shown || that the a rays con- 

 tribute by far the greater part of the electrical effect, while 

 the ^ rays produce practically all of the j^hotographic effect. 

 Thus uranium when freed from UrX, though inactive to the 

 photographic plate, possesses a nearly normal activity when 

 examined by the electric method. 



In his investigations upon thorium, Baskerville says :^ " The 

 oxide (sp. gr. 9*25) obtained from the insoluble citrate affects 

 the sensitive plate in the dark after an exposure of seventy- 

 two hours but slightly, while the oxides of higher specific 

 gravity are quite active. A number of plates have been 

 exposed using oxides obtained through the research, monazite 

 sand from which the thorium salts were prepared, uranium 

 nitrate, acetate, uraninite and blanks for comparison. The 

 radioactivity increased with increase in specific gravity." 

 Hearing that he had finally obtained a thoria so pure that it 

 did not affect the photographic plate, I wrote him asking 

 for the loan of a small sample to compare w^ith my own speci- 

 mens. In the letter accompanying the specimen which he 

 was good enough to send me, he says : " I have secured pre- 



* Phil. Mag. VI, V, 177, 1908. \ Phil. Mag. VI, iv, 315, 1902. 



X Phil. Mag. VI, V, 445, 1903. i< Eutherford, 1. c. 



II Phil. Mag. VI, V, 441, 1903. H J. Am. Chem. Soc, xxiii, 761, 1901. 



