300 RADIO-ACTIVE PROCESSES [ CH. 
191. Radio-activity of radium. Notwithstanding the 
enormous difference in their relative activities, the radio-activity 
of radium presents many close analogies to that of thorium. Both 
substances give rise to emanations which in turn produce “ excited 
activity” on bodies in their neighbourhood. Radium, however, 
does not give rise to any intermediate product between the 
element itself and the emanation it produces, or in other words 
there is no product in radium corresponding to Th X in thorium. 
Giesel first drew attention to the fact that a radium compound 
gradually increased in activity after preparation, and only reached 
a constant value after a month’s interval. If a radium compound 
is dissolved in water and boiled for some time, or a current of air 
drawn through the solution, on evaporation it is found that the 
activity has been diminished. The same result is observed if 
a solid radium compound. is heated in the open air. This loss 
of activity is due to the removal of the emanation by the process 
of solution or heating. Consider the case of a radium compound 
which has been kept for some time in solution in a shallow vessel, 
exposed to the open air, and then evaporated to dryness. The 
emanation which, in the state of solution, was removed as fast as 
it was formed, is now occluded, and, together with the emana- 
tion X which it produces, adds its radiations to that of the original 
radium. The activity will increase to a maximum value where the 
rate of production of fresh emanation balances the rate of change 
of that already produced. 
If now the compound is dissolved or heated, the emanation © 
escapes. Since the emanation X is not volatile and is insoluble 
in water, it is not removed by the process of solution or heating. 
Since, however, the exciting cause is removed, its activity will 
immediately begin to decay, and in the course of a few hours 
will have almost disappeared. The activity of the radium mea- 
sured by the rays is then found to be about 25 per cent. of its 
original value. This residual activity of radium, consisting entirely 
of a rays, is non-separable, and has not been further diminished by 
chemical or physical means. Rutherford and Soddy? examined the 
effect of aspiration for long mtervals through a radium chloride 
solution. After the first few hours the activity was found to be 
l Phil. Mag. April, 1903. 
