Heating Eqfect of the Radium Emanation. 211 
Within the limit of experimental error the sum total of the 
heating effect of the radium, together with that of the 
emanation-tube, was, over the whole course of the experiment, 
always equal to that of the original radium. Measurements 
of radioactivity showed that about 6 per cent. of the ema- 
nation in the above experiment was not released from the 
radium by the heating. We may conclude from these results, 
that about 75 per cent. of the heating effect observed from 
radium is not directly due to the radium, but to the emanation 
and emanation X,which it produces from itself. There isa close 
connexion between the variation of the radioactivity of the 
radium and its rate of heat emission. After separation of the 
emanation, the activity of the radium falls to a minimum of 
about 25 per cent. in the course of a few hours, and then gra- 
dually increases again, At the same time, the activity due to 
the emanation (observed in a closed vesse]) increases with 
the time, on account of the excited activity produced by the 
emanation on the walls of the vessel. The curves of recovery 
of the heating effect of radium, and the gradual decrease of 
the heating effect of the emanation, are almost exactly the 
same as the corresponding curves for the recovery of activity 
of the radium and the loss of activity with time of the 
separated emanation. The rate of heat emission of the ema- 
nation, like its activity, falls to about half value in about 
four days. Half of the lost heating effect, as well as half of 
the lost activity of the radium, is spontaneously recovered 
during the same interval. The curve of diminution of the 
heating effect of the emanation-tube is thus expressed by the 
equation 
Q, 
Qo 
where @; is the rate of heat emission at any time ¢ and Q, 
the maximum rate. 
The rate of heat emission Q: of the radium, at any time ¢ 
after reaching its minimum value, is expressed by the equation 
=e, 
where Q, is the maximum rate and A the same constant as 
before. The same numerical constants as well as the same 
equation are obtained for the recovery curves of activity of 
de-emanated radium, measured by the a rays (see Rutherford 
and Soddy, Phil. Mag. April 1903). 
P2 
