130 THE ROYAL SOCIETY OF CANADA 
lead screen was used at each range in order to determine the amount 
due to secondary radiation from ground or air. The secondary radia- 
tion, about 45 per cent. of the total, was in all cases deducted. It will 
be noted that À decreases as the range increases, showing that the softer 
rays are first eliminated. The result is well known when aluminium or 
other screens are used at short ranges. It is possible to detect without 
difficulty the Rontgen rays at 140 metres from the source, but accurate 
measurements become difficult. 
With a thin-walled electroscope experiments were made to find the 
radiation (1) in free air, or nearly so, (2) inside an aluminium vessel 
0-6 mm. thick, and the ratio was 40 : 31. In an iron or zinc electro- 
scope 0:6 mm. thick the ionization is about 15 per cent of that in free 
air. With this correction the value of K was calculated for free air as 
3.1 x 10", and of N as 6-6 x 10%. These results of course hold merely 
for the given bulb under the existing conditions, which were as efficient 
as possible. 
