122 Prof. E. Rutherford on Uranium Radiation and 



as a diffuse radiation is produced at the surface of a metal by 

 the passage of Rontgen-rays through it. There is not, how- 

 ever, sufficient evidence at present to decide the question. 



§ 7. Thorium Radiation. 



While the experiments on the complex nature of uranium 

 radiation were in progress, the discovery * that thorium 

 and its salts also emitted a radiation, which had general 

 properties similar to uranium radiation, was announced. A 

 few experiments were made to compare the types of radiation 

 emitted by uranium and thorium. 



The nitrate and the sulphate of thorium were used and gave 

 similar results, although the nitrate appeared to be the more 

 active of the two. The leakage effects due to these salts were 

 of quite the same order as those obtained for the uranium 

 compounds; but no satisfactory quantitative comparison can 

 be made between the uranium and thorium salts as the amount 

 of leak depends on the amount of surface and thickness of 

 the layer. 



It was found that thorium nitrate when first exposed to the 

 air on a platinum plate was not a steady source of radiation, 

 and for a time the rate of leak varied very capriciously, being 

 sometimes five times as great as at others. The salt was very 

 deliquescent, but after exposure of some hours to the atmo- 

 sphere the rate of leak became more constant and allowed of 

 rough comparative measurements. Thorium sulphate was 

 more constant than the nitrate. 



The absorption of the thorium radiation was tested in the 

 same way as for uranium radiation. The following table 

 gives some of the results. The aluminium foil was of the 

 same thickness ('0005 cm.) as that used in the uranium 

 experiments : — ■ 



Number of Layers 

 of Aluminium foil. 



Leak per minute in 

 scale-divisions. 







200 



4 



94 



8 



37 



12 



19 



17 



7-5 



* G. C. Schmidt, Wied. Annal. May 1898. 



