644 Pref. E. Rutherford on Slow 
under identical experimental conditions) the corresponding 
value of Q.= 2 where A, is the constant of change of D. 
ry 
Since by hypothesis Q,=Q., 
My 
a ae 
. 2. oe . . ‘ . 
The ratio 2 is determined experimentally, and since } for 
ey 
the emanation is known, ), is determined. 
The details of the experiments by which the ratio 7,/2; was 
determined need not be given here. It was deduced on the 
above assumption, that half of the matter of radium D should 
be transformed in forty years. In a similar way, the total 
number of @ particles expelled from radium C during the 
time radium D was being deposited must equal the number 
of @ particles expelled from radium E during its life, 
supposing that there is only one change which gives rise to 
arays. Assuming for the moment that the @ ray activity, 
observed for the active deposit ten months old, decayed from 
that time according to an exponential law, it was calculated 
that the period of the change could not be longer than 
eighty years. If the a ray change has a period short 
compared with the 8 ray change, the a ray activity will 
finally decay at the same rate as the @ ray activity. These 
two computations will agree, if it is supposed that the a 
ray activity increases to twice the value observed after an 
interval of ten months, and then decays with the time 
according to the period of the B ray change. This would 
fix the period of the a ray change at about one year. When 
the a ray activity reaches its maximum value, it is to be 
: . : a 
expected, on this view, that the ratio B should be the same 
as for the product radium C. This is, however, somewhat 
at variance with experiment ; for the ten months old deposit 
: a ° . 
has about the same ratio — as radium C, while on the above 
B 
computation the ratio 2 should only be one half of that 
value. ‘This difference may possibly be due to radium H 
undergoing a further change, giving rise to @ rays, which 
has not so far been detected. In these calculations, it has 
been assumed that the a and B particles given out in these 
slow changes produce the same ionization as the corresponding 
particles trom radium C. There is no doubt, however, that 
