G. F. Barker — Badioactivitij of TTiorium Minerals. 163 



containing both uraninm and thorinm. Tiiey all were found 

 to be radioactive, though in very different degrees. Measured, 

 as above, in terms of current, xenotime gave O'OS.IO"" ampere, 

 niobite 0*3, fergusonite 0*4, monazite 0*5, aeschynite 0'7, 

 samarskite 1"1, thorite 1-4, cleveite 1"4:, orangeite 2-0, autunite 

 2*7, chalcolite 5*2, carnotite 6'2 and pitchblende (uraninite) 

 from 6'5 to 8*3.10"^^ ampere. The remarkable fact that four 

 of these minerals were more active than metallic uranium and 

 especially that the uraninite from Johanngeorgenstadt was 

 nearly four times as active, led, as is well known, to the dis- 

 covery in it of the three new radioactive substances, polonium, 

 radium and actinium. 



In February, 1902, Hof mann and Zerban published a paper 

 on Kadioactive Thorium^ obtained from certain minerals allied 

 to uraninite. In connection with Strauss, the former of these 

 chemists had previously prepared thoria from broggerite, 

 cleveite and samarskite, and had found it to be radioactive. 

 Hofmann and Zerban now observe that the activity of these 

 thoria preparations may be materially increased by fractional 

 precipitation with jDotassium sulj)hate or chromate, with hydro- 

 gen peroxide or with sodium thiosulphate, the most active 

 fractions being those most readily precipitated. By using 

 ammonium carbonate as the precipitant, on the other hand, 

 the most active material is found in. the most soluble fraction. 

 To their surprise, however, they found that the thoria thus 

 obtained, on keeping it for five months, even in closed vessels, 

 lost a large part of its radioactivity. Thus the most active 

 thoria, which in Jane blackened the photographic plate very 

 strongly even through glass, acted verj^ feebly in the following 

 ]^ovember. Electroscopic tests also confirmed this diminution 

 in radioactivity. A specimen of thoria from broggerite wdiich 

 in November discharged the electroscope in 1 minute required 

 3m. 45s. to do the same thing in January. A specimen from 

 cleveite, which in November took Im. 10s. to discharge the 

 electroscope, required in January 2m. 6s. And a preparation 

 from samarskite, which took Im. 30s. in November to effect 

 this discharge, did the same thing in January only after Im. 

 50s. 



These results lead the authors to assume (1) that the thoria 

 prepared from broggerite, cleveite and samarskite possesses, 

 not a primary but a secondary induced activity, and (2) that 

 this induced activity is due to the uranium always present in 

 the minerals referred to. In proof of this assumption, the 

 authors prepared thoria from Brazilian monazite sand, which 

 they assert contained no trace of uranium, and found it to be 

 absolutely inactive, whether tested by the photographic plate 



* Ber. Berl. Chem. Ges., xxxv, 531, 1902. 



