380) RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
of Strutt. McLennan’s results however point to the conclusion 
that the ionization was mainly due to an emanation emitted from 
the metal. Since the air was rapidly removed, a proportionate 
amount of the emanation would be removed also, and it might 
thus be expected that the current would vary directly as the 
pressure. If this is the case the current through the gas at low 
pressures should increase again to a maximum if time is allowed 
for a fresh emanation to form. 
H. L. Cooke, using an electroscopic method, obtained results 
very similar to those given by Strutt. Cooke observed that a pene- 
trating radiation was given out from brick. When a brass vessel 
containing the gold-leaf system was surrounded by brick, the 
discharge of the electroscope was increased by 40 to 50 per cent. 
This radiation was of about the same penetrating power as the 
rays from radio-active substances. The rays were completely 
absorbed by surrounding the electroscope by a sheet of lead 2 mis. 
in thickness. This result is in agreement with the observation 
of Elster and Geitel, already mentioned, that radio-active matter 
was present in clay freshly dug up from the earth. 
Cooke also observed that the ionization of the air in a brass 
electroscope could be reduced to about one-third of its usual 
value if the interior surface of the brass was carefully cleaned. By 
removing the interior surface of the brass he was able to reduce 
the ionization of the enclosed air from 30 to 10 ions per c.c. per 
second. This is an important observation, and indicates that a 
large proportion of the radio-activity observed in ordinary matter’ 
is due to a deposit of radio-active matter on its surface. It has 
already been shown (sections 173 and 188) that bodies which 
have been exposed in the presence of the radium emanation 
retain a residual activity which decays extremely slowly. There 
ean be no doubt that the radium emanation is present in the 
atmosphere, and the exposed surface of matter, in consequence, 
will become coated with an invisible film of radio-active matter, 
deposited from the atmosphere. On account of the slow decay of 
this activity it is probable that the activity of matter exposed in 
the open air would steadily imcrease for a long interval. Metals, 
even if they are originally inactive, would thus acquire a fairly 
permanent activity, but it should be possible to get rid of this 

. 
