Substance emitted from Thorium Compounds. 5 



A thick layer of thorium oxide was enclosed in a narrow 

 rectangular paper vessel A (fig. ]) ? ma de up of two thick- 

 nesses^ of foolscap-paper. The paper cut off the regular 

 radiation almost entirely, but allowed the emanation to pass 

 through. The thorium thus enclosed was placed inside a 



Fig. 1. 



B 



long metal tube B, One end of the tube was connected to a 

 large insulated cylindrical vessel C, which had a number of 

 small holes in the end for the passage of air. Inside C was 

 fixed an insulated electrode, D, connected with one pair of 

 quadrants of a Thomson electrometer. The cylinder, C, was 

 connected to one terminal of a battery of 100 volts, the other 

 terminal of which was connected to earth. 



A slow current of air from an aspirator or gasometer, which 

 had been freed from dust by its passage through a plug of 

 cotton- wool, was passed through the apparatus. The current 

 of air, in its passage by the thorium oxide, carried away the 

 radio-active particles with it, and these were gradually con- 

 veyed into the large cylinder C. The electrometer-needle 

 showed no sign of movement until the radio-active particles 

 were carried into C. In consequence of the ionization of the 

 gas in the cylinder by the radio-active particles, a current 

 passed between the electrodes G and D. The value of the cur- 

 rent was the same whether C was connected with the positive 

 or negative pole of the battery. When the current of air had 

 been Bowing for some minutes, the current between C and D 

 reached a constant value. The flow of air was then stopped, 

 and the rate of leak between and D observed at regular 

 intervals. It was found that the current between C and D 

 persisted for over ten minutes. 



The following is a series of observations. 



