[EVE] RADIUM EMANATION IN ATMOSPHERE 23 
will rise more freely from the soil when driven out by heavy rain, by 
rapidly melting snow, or during a quick fall of barometer. The active 
deposit attracted to a charged wire also varies within wide limits, and 
the greatest values are sixteen or seventeen times the least, the variation 
depending upon the presence or absence of smoke, dust and fog particles. 
Thus the amount of active deposit obtained in a cylinder on the campus 
of McGill University would not be constant. Again, the emanation 
collected in the present experiments was taken from the air at a level of 
the fourth story of the Chemistry Building, about 50 feet from the 
ground; and in still weather there may be more emanation close to the 
ground than at some height above it. 
But, in addition, as Professor Rutherford has pointed out, the 
potential gradient tends to drag the active deposit downwards to the 
earth. The potential gradient is of the order of 1 or 2 volts per cm. 
and will generate a velocity of the active deposit of about 1.3 cm. 
per second, or about two or three feet a minute. Hence, the active 
deposit will tend to concentrate near the earth’s surface. 
Since Strutt found that one gram of rock on the average con- 
tained 1.4X 1012? grams of radium, and we have seen that 1 cb. metre 
of air contains the emanation from about 35 10—1!? grams (or less,) 
we may conclude that 25 grams of rock would provide the emanation 
contained in 1 cb.m. of the atmosphere, provided all the emanation 
escaped from the rock. But Boltwood has shown that only five to ten 
per cent of the emanation escapes from a mineral so that by far 
the greater part of the emanation in rocks even near the surface of the 
earth must undergo transformation without passing into the air. 
But if there reaches the air but 5 per cent of the total emanation 
supplied by the rocks and soils extending to the depth of 50 or 100 cms., 
the supply would be sufficient to account for the emanation observed in 
the atmosphere extending to a height of 5 kilometres. Thus in the 
present state of our knowledge the radium in the earth near the surface, 
the emanation in the atmosphere, the resulting active deposit, and the 
penetrating radiation resulting from these, are all of the correct order 
so as to correlate the observed magnitudes. 
T am grateful to Professor Rutherford for suggesting to me this 
experiment, and for his advice and encouragement durirg a long and 
difficult piece of work. 
(Note added 10 June, 1907). 
The probable average value of the amount of emanation in the 
atmosphere appears to be per c.km. that which may be derived from .04 
grams of pure radium bromide; or per cb.m. of the atmosphere that derived 
from 24X10—* grams of radium. The work is not however completed. 
