426 Prof. McLennan on Induced Radioactivity 



window, immediately caused an increase in the amount of 

 radioactivity excited in the exposed disks. It was evident, 

 therefore, that the electrical machine in action served as a 

 means of removing from the atmosphere surrounding it the 

 constituent upon whose presence the excited radioactivity 

 depended. 



In seeking for an explanation of the diminished radio- 

 activity excited at the Falls, the experiments just described 

 suggest the presence of an agency having an effect similar to 

 that of the electrical machine on the air in the room. 



The negative electrification of the wire when in the spray- 

 area pointed to the presence of this agency in the spray itself. 

 The time at the writer's disposal did not permit of a direct 

 investigation into the charge carried by the spray, but the 

 negative electrification of the wire, which it will be remem- 

 bered was always present and always increased as the wire 

 was extended into the heavier spray, seemed to find its only 

 explanation in a similar electrification of the spray. This 

 being so, it follows at once that the vast quantity of the spray 

 produced by the Falls would act as a huge negatively-charged 

 body in attracting to it from the surrounding atmosphere 

 the constituent responsible for the excitation of induced 

 radioactivity. 



In this way a satisfactory explanation is afforded of the 

 relatively small amount of induced radioactivity excited at 

 the foot of the Falls. This explanation requires the spray 

 itself to be radioactive, but when the enormous volume of 

 the spray and the very limited amount of induced radio- 

 activity observed in the locality are taken into account, 

 together with the known decay of induced radioactivity, it 

 would appear reasonable to conclude that experiments con- 

 ducted with very much larger quantities of spray than those 

 in the writer's tests would be necessary to obtain observable 

 results. 



From Lenard's observations one would have expected a 

 positive electrification to be developed on the exposed wire by 

 the splashing of the spray, but the opposite was found. Some 

 experiments on dropping water through ionized gases recently 

 made by Schmauss*, give some aid in explaining this anomaly. 

 He found that when water was dropped through air ionized 

 by Rontgen rays, it acquired a negative charge from the gas, 

 and he explained this fact by a reference to the experiments 

 of Rutherford f and of Zeleny |, in which it was shown that 



* Schmauss, Wied. Ann. [9] i. pp. 224-237 (1902). 

 f Rutherford, Phil. Mag. xliii. p. 241 (1897). 

 \ Zeleny, Phil. Mag. xlvi. p. 120 (1898). 



