XI] AND OF ORDINARY MATERIALS 379 
There are thus marked differences in the leakage observed for 
different materials and also considerable differences in different 
samples of the same metal. For example, one specimen of platinum 
caused nearly twice the leakage of another sample from a different 
stock. 
McLennan and Burton, on the other hand, measured by means 
of a sensitive electrometer the ionization current produced in the 
air in a closed iron cylinder 25 cms. in diameter and 130 cms. in 
length, in which an insulated central electrode was placed. The 
open cylinder was first exposed for some time at the open window 
of the laboratory. It was then removed, the top and bottom 
closed, and the saturation current through the gas determined as 
soon as possible. In all cases it was observed that the current 
diminished for two or three hours to a minimum and then very 
slowly increased again. In one experiment, for example, the initial 
current observed corresponded to 30 on an arbitrary scale. In the 
course of four hours the current fell to a minimum of 6°6, and 
44 hours later had risen to a practical maximum of 24. The 
initial decrease observed is probably due to a radio-activity of 
the enclosed air or walls of the vessel, which decayed rapidly 
with the time. The decay of the excited activity produced on 
the interior surface of the cylinder when exposed to the air was 
probably responsible for a part of the decrease observed. McLennan 
ascribes the increase of current with time to a radio-active ema- 
nation which is given off from the cylinder, and ionizes the enclosed 
air. On placing linings of lead, tin, and zinc in the iron cylinder, 
considerable differences were observed both of the minimum current 
and also of the final maximum. Lead gave about twice the cur- 
rent due to zinc, while tin gave an intermediate value. These 
results are similar in character to those obtained by Strutt. 
McLennan and Burton also investigated the effect of dimi- 
nution of pressure on the current. The cylinder was filled with 
air to a pressure of 7 atmospheres, and allowed to stand until 
the current reached a constant value. The air was then allowed 
to escape and the pressure reduced to 44 mms. of mercury. The 
current was found to vary approximately as the pressure over the 
whole range. These results are not in agreement with the results 
of Patterson already described, nor with some later experiments 
