G. F. Barher — Radioactivity of Thorium Minerals. 165 



factory impressions were obtained from all of them. Of the 

 two monazites, that from Brazil gave perhaps a trifle the 

 darker stain ; that specimen containing 90 per cent of monazite 

 and about 6 per cent of thoria, while the North Carolina sand 

 contained about QQ per cent of monazite and 4 per cent 

 of thoria. For purposes of comparison a piece -of metallic 

 uranium was exposed under the same conditions upon the same 

 plate.^ The darkening effect was very considerably greater 

 than tliat produced by the thoria. 



In view of the assertions of Hofmann and Zerban already 

 stated, to the effect, first, that Brazilian, monazite sand con- 

 tains no uranium, and second, that in consequence of this, the 

 thoria obtained from this source is not radioactive, the above 

 described experiments were carefully rejDeated. The results 

 obtained were practically tlie same. The plate was darkened, 

 during an exposure of 48 hours, by all the six substances placed 

 upon it ; by the two monazite sands from Brazil and from 

 J^orth Carolina, by the two thorium oxides obtained from 

 these, by the ammonium-thorium nitrate and by the tliorium 

 oxalate. The oxalate stain was perhaps a little darker than 

 the nitrate and tlie Brazilian stain a trifle darker than the 

 JN^orth Carolina one. Apparently, therefore, the second of 

 Hofmann and Zerban's conclusions, to wit : that thoria pre- 

 pared from Brazilian monazite sand is " absolutely inactive," 

 can hardly be accepted as final. 



The claim, liowever, that the radioactivity of thorium com- 

 pounds is not a primary but a secondary induced activity due 

 to the presence of uranium in the minerals in which it occurs, 

 needs to be considered here. The monazite sands are complex 

 and variable in composition, and may in isolated cases con- 

 tain even uranium. The specimens used in the experiments 

 above described therefore were submitted to several competent 

 chemists, all of whom reported that the}' were entirely free 

 from this element. The main conclusion would therefore 

 seem legitimate, that radioactive thorium compounds may be 

 prepared from minerals which do not contain uranium. If so, 

 the conclusion can hardly be evaded that thorium is a primary 

 radioactive substance. 



The composite character of the rays emitted by radioactive 

 substances appears to have been observed about the same time 

 by Becquerel,f by tlie Curies:}: and by Giesel.g According 

 to Becquerel|| these rays are of three sorts : (1) those deviable 



* Given me in 1899 in Moissan's Laboratory, by the courtesy of Professor 

 Lebeau. 



f Comptes Eendus, cxxix, 1205 ; cxxx, 206, 372 ; cxxxii, 371. 

 X Comptes Eendus, cxxix, 996 ; cxxx, 73, 76, 647 ; cxxxii, 133. 

 § Ann. Phys. Chem., II, Ixix, 834, 1899. 

 I Proc. Roy. Inst., xvii, 1, March 1902. 



