Substance emitted from Thorium Compounds. 7 



by the theory of ionization, that the current through the gas 

 is proportional to the intensity of the radiation emitted by the 

 radio-active particles. We therefore see that the intensity of 

 the radiation given out by the radio-active particles falls off 

 in a geometrical progression with the time. The result shows 

 that the intensity of the radiation has fallen to one-half its 

 value after an interval of about one minute. The rate of leak 

 due to the emanation was too small for measurement after an 

 interval of ten minutes. 



If the ionized gas had been produced from a uranium com- 

 pound, the duration of the conductivity, for voltages such as 

 were used, would only have been a fraction of a second. 



The rate of decay of intensity is independent of the electro- 

 motive force acting on the gas. This shows that the radio- 

 active particles are not destroyed by the electric field. The 

 current through the gas at any particular instant, after 

 stoppage of the flow of air, was found to be the same whether 

 the electromotive force had been acting the whole time or just 

 applied for the time of the test. 



The current through the gas in the cylinder depends on the 

 electromotive force in the same way as the current through a 

 gas made conducting by Iiontgen rays. The current at first 

 increases nearly in proportion to the electromotive force, but 

 soon reaches an approximate "saturation" value. 



The duration of the radio-activity was also tested by another 

 method. The paper vessel containing the thorium oxide was 

 placed inside a long brass cylinder over 200 cms. in length. 

 A slow current of air (with a velocity of about 2 cms. per 

 second along the tube) was passed over the thorium oxide along 

 the tube, and then between two insulated concentric cylinders. 

 The rate of leak between the two concentric cylinders (po- 

 tential-difference 270 volts) was observed when the air had 

 been passing sufficiently long to produce a steady state. The 

 rates of leak were observed for varying positions of the 

 thorium oxide along the tube. Knowing the velocity of 

 the current of air along the tube, the time taken to carry the 

 radio-active particles to the testing-apparatus could be deter- 

 mined. In this way it was found that the rate of decay w r as 

 about the same as determined by the first method, i. e., the 

 intensity fell to half its value in about one minute. 



In this apparatus experiments were also tried to see whether 

 the radio-active particles moved in an electric field. The expe- 

 riments on the effect of a current of air on the rate of discharge 

 naturally suggest that possibly one of the ions was so large 

 that it moved extremely slowly even in strong electric fields. 

 The results obtained showed that the particles did not move 



