428 Radioactivity Excited in Air at the Foot of Waterfalls. 



a light fall of fine snow began, which continued throughout 

 the night. An exposure was made from 3.30 to 5.30 p.m., 

 and also one from 9 to 11 p.m. The resulting value for the 

 former was 0*2, and for the latter 0'32. 



As the numbers just given indicate, the falling of snow on 

 both occasions was accompanied by an immediate drop in the 

 excited radioactivity. It will also be noticed that while the 

 ground was covered with snow from the first fall the dimin- 

 ished activity continued. The snow on both occasions was 

 collected as it fell, melted and evaporated, and the residue 

 found to be highly radioactive. Snow which fell in the 

 first storm was tested after it had lain two days on the 

 ground and was found still to possess about 1 per cent, of its 

 original activity. 



These results, it will be seen, lend support to the theory 

 that the constituent of the atmosphere to which the excitation 

 of radioactivity is due is an emanation from the earth's sur- 

 face. It would also seem from the values found for the 

 induced radioactivity that this emanation is cut off to a con- 

 siderable extent when a wide area of the earth's surface is 

 thickly covered with snow. 



The limited number of observations made hardly justifies a 

 definite conclusion as to the manner in which rain and snow 

 become radioactive ; but it is possible that water- vapour in 

 moving through the air, whether in the form of clouds or 

 otherwise, gains a negative charge in the same manner as the 

 spray at the Falls, and consequently becomes radioactive. 

 This view is supported by the results of some experiments 

 recently made by the writer. Water-vapour was condensed 

 from the atmosphere upon the surface of a number of cylin- 

 ders containing a freezing-mixture of ice and salt. The 

 water thus obtained was evaporated in a metallic tray 

 and the residue tested for radioactivity. Such tests were 

 made daily for a period of two weeks. In most of them no 

 radioactivity was observed, but in several slight traces were 

 noticeable, and on four occasions the radioactivity excited 

 was very marked. The experiments were all conducted with 

 equal care, and no cause was apparent for the different results 

 of the observations. 



The thanks of the writer are due to Mr. J. S. Plaskett, 

 B.A., for assistance in conducting the experiments, which at 

 Niagara were attended with considerable difficulty, and to 

 Mr. J. C. Rothery, Superintendent of the Niagara Falls' Park 

 and River Railway, for facilities afforded in transporting the 

 apparatus. 



Physical Laboratory, University of Toronto. 

 December 8th, 1902". 



