110 Rate of Decay of Thorium Emanation. 
curve. This gives the charge at any time due to the emanation 
together with the excited activity. 
Now, knowing the time when the emanation was blown in, 
the excited activity at any time can be calculated, assuming 
‘an approximate rate of decay for the emanation, and from 
these values the curve B showing the charge due to the excited 
activity can be drawn. 
If the ordinates of this curve be subtracted from the 
ordinates of the former, then the curve C drawn through the 
resulting points will show the charge due to the emanation 
alone. An asymptote was then drawn, and the logarithms 
of the ordinates measured from this to the curve were plotted 
and, from fig. 3, it will be seen that the points thus obtained 
Fig. 3. 
lie on a straight line. 
ee or aE 
iets 
40 80 | : 
20 160 200 240 280 320 360 400 
SECONDS. 
L0G, OF ACTIVITY. 
This proves the curve to be a true exponential, From 
this the time taken by the emanation to decay to half value 
was found to be 51:2 seconds. Other experiments gave very 
similar values, e.g. 50°8 and 51°5 seconds. The mean of 
these numbers gives 51:2 seconds as the time taken to decay 
to half value. 
In conclusion we wish to express our thanks to Sir William 
Ramsay and Prof. Trouton for the kind interest they have 
taken in the work, and especially are we indebted to Mr. A. 
W. Porter for his valuable advice and assistance which were 
always at our disposal. 
Physical Laboratory, 
University College, London, 
