2 Prof. E. Rutherford on a Radio-active 



from thorium is of a more penetrating character than the 

 radiation from uranium. Attention was also directed to the 

 inconstancy of thorium as a source of radiation. Owens* 

 has investigated in more detail the radiation from thorium 

 compounds. He has shown that the radiations from the 

 different compounds are of the same kind, and, with the 

 exception of thorium oxide in thick layers, approximately 

 homogeneous in character. 



The intensity of thorium radiation, when examined by 

 means of the electrical discharge produced, is found to be 

 very variable ; and this inconstancy is due to slow currents 

 of air produced in an open room. When the apparatus is 

 placed in a closed vessel, to do away with air-currents, the 

 intensity is found to be practically constant. The sensi- 

 tiveness of thorium oxide to slight currents of air is very 

 remarkable. The movement of the air caused by the opening 

 or closing of a door at the end of the room opposite to where 

 the apparatus is placed, is often sufficient to considerably 

 diminish the rate of discharge. In this respect thorium com- 

 pounds differ from those of uranium, which are not appre- 

 ciably affected by slight currents of air. Another anomaly 

 that thorium compounds exhibit is the ease with which the 

 radiation apparently passes through paper. The following 

 table is an example ot the way the rate of leak between two 

 parallel plates, one of which is covered with a thick layer of 

 thorium oxide, varies with the number of layers of ordinary 

 foolscap-paper placed over the radio-active substance. 



Table I. 

 Thickness of each Layer of Paper = # 008 cm. 50 volts 



between 



plates. 



Number of Layers of 

 Paper. 



! 

 Rate of Discbarge. 







1 



1 



■74 



2 



•74 



5 



•72 



10 



•67 



20 



■35 



Phil. Mag. Oct, 1899, p. *jC0. 



