638 Prof. E. Rutherford on Slow 
the intense radiations ; for if such were the case, it would be 
expected that the activity produced on the different plates 
would vary not only in quantity, but also in quality. This 
result is confirmed by the observation that the active matter 
can be removed from a platinum plate by solution in 
sulphuric acid, and has other distinctive chemical and physical 
properties. 
The variation of the residual activity with time will first 
be considered. A platinum plate was exposed in the presence 
of the radium emanation for seven days. The amount of 
emanation initially present was equal to that obtained from 
about 83 milligrams of pure radium bromide. The plate 
immediately after removal gave a saturation-current, measured 
between parallel plates by a galvanometer, of 13x 107" 
ampere. For some hours after removal the activity decayed 
according to an exponential law with the time, falling to half 
value in 28 minutes. Three days after removal the active 
plate gave a saturation-current, measured by an electrometer, 
of 5x 10-" ampere ; 27. e., 1/300,000 of the initial activity. 
The activity was observed to increase steadily with the time. 
rad/at/orn 
iad 
/rntens/t 
3 
Time 7 Days eS 
The results are shown in fig. 1, where the time is reckoned 
from the middle of the time of exposure to the emanation. 
The curve is a straight line passing through the origin. 
The activity increases uniformly with the time for the 
