86 ROYAL SOCIETY OF CANADA 



of air enclosed in an aluminium receiver when the apparatus was set 

 up in a rock salt mine. Later still, Cooke^ was able to reduce the 

 ionization 12 per cent by immersing his cylinder in a reservoir of water 

 to a depth of several feet. The greatest decrease, however, was that 

 observed by McLennan and Burton,^ who cut off 37 per cent of the 

 conductivity of the air enclosed in a galvanized iron cylinder, by sur- 

 rounding it wit^ a layer of water drawn from Lake Ontario, 60 cms. 

 thick. This last result, taken m conjunction with an obseiTation by 

 McLennan that Lake Ontario water contains no appreciable radioactive 

 impurities, gave rise to the idea, that possibly a large body of water, such 

 as the lake itself, might furnish an efficient screen for the earth's 

 radiation. 



The object of the present investigation, therefore, was to observe 

 the conductivity of the air confined in metallic vessels possessing little 

 if any radioactive impurity and from the results to determine, if pos- 

 sible, what portion of the ionization was due to an intrinsic activity 

 in the metal. 



In carrying out the investigation, the conductivity of air confined 

 in vessels of lead, zinc, and aluminium was measured at a number of 

 points in the neighbourhood of Toronto, both on land and over the 

 water of Lake Ontario, and it was found that while a uniformly low 

 and steady value was obtained for the conductivity over the water at 

 all depths beyond a few metres, values varying over a wide range were 

 obtained for the ionization, in measurements made on land at different 

 places and on different soils. 



The lowest values for " q,'' the number of ions generated per cc. 

 per sec. in air confined in the metallic cylinders, were obtained in 

 measurements on the surface of the lake and on the top of large masses 

 of sand on the lake shore. With a lead receiver, under these circum- 

 stances, the value 8.6 ions per cc. per sec. was obtained for " q," and 

 with zinc and aluminium cylinders, under the same concîitions, the 

 values 6.00 and 6.55 respectively. These values, it will be seen from 

 Table I, are considerably below those hitherto recorded for the con- 

 ductivity obtained under any circumstances of air contained in closed 

 metallic receivers. 



'H. L. Cooke, Phil. Mag., 1903. 

 'McLennan and Burton, Phys. Rev., 3, 1903; Burton, Phys. Rev., 3, 1904. 



