[ MCLENNAN-MURRAY] RESIDUAL IONISATION IN AIR 



107 



conductivity of air was measured at three stations, the one being 

 on the ice on Toronto bay, where the water was about 20 feet deep, 

 the second in the Library of the Physics Building and the third in a 

 brick house free from radio-active contamination on land at a point 



Fig. 2. 



about 2. miles from the shore of Lake Ontario. Three difïerent types 

 of measurements were made. In the one the air was confined in a 

 vessel of clean zinc of about 30 litres capacity, in the second it was 

 enclosed in an air-tight zinc Wolff electrometer of about two litres 

 capacity and in the third it was confined in a vessel of ice whose ca- 

 pacity was about 30 litres. Three distinct ice vessels were used two 

 being made from ordinary tap water drawn from Lake Ontario and the 

 other from water distilled in a large still in the Chemical Laboratory 

 of the University of Toronto. In making these ice vessels a number 

 of galvanized iron trays were made which were annular in form. 

 The water was poured into these and the trays were placed out in the 

 open until the water was frozen solid. 



The ice was then taken out of these trays, carefully scraped and a 

 cylinder was built up by placing these ice rings one upon the other and 

 frozen together. The solidified ice cylinder was then placed on a thick 

 plate of ice and frozen to it and finally the top of the cylinder was closed 

 with a second plate of ice. Before the ice cylinders were used all chinks and 

 cracks were carefully filled with water when the temperature was about 



1 McLennan and McLeod, loc. cit. 



