280 EXCITED RADIO-ACTIVITY [CH. 
conditions did not become steady until an interval of 10 minutes 
after introducing or withdrawing the emanation, and in that time, 
the first change in emanation X was nearly completed. 
The division of the heating effect amongst the radio-active 
products of radium is given in the following table. The activity 
of each product measured by the a rays is also given for com- 
parison : - 





| Percentage 
| . Nature of pravenonses he easy | 
Active products yays total activity folalineatne | 
measured by frect 
the rays Hae 
Radium a rays |: PDB 25 
| (freed from active products) 
Emanation | a rays jas 
| | 33 4] 
_Emanation X (first change) | a rays 15 J 
| 
i (second change) | Nola rays | 0 ) 
| | | 42 34 
5 (third change) | a, 8, and y rays | 42 ) 


The heating effect of the active products is approximately. 
proportional to their activity measured by the a rays. There can 
be very little doubt that the emanation supples an amount of the 
heating effect proportional to its activity. 
The decay curve of the activity of radium to a minimum of 
25 per cent. after removal of the emanation is approximately the 
same as the corresponding decay curve of the activity of radium 
measured by the 4 rays. 
There is no doubt that the heating effect of radium is a result 
of the succession of radio-active changes occurring in it. The 
heating effect accompanies the expulsion of a particles, and is 
approximately proportional to the number expelled. The time- 
variation of the heating effect of the radio-active products 1s 
the same as the time-variation of their activity measured by 
the a rays. 
182. Effect of variation of E.M.F. on amount of 
excited activity from thorium. It has been shown that the 
