178 PHYSICAL SCIENCE 



then evaporated, and the recovery of the activity of 

 the solid crystals of salt was traced by measuring 

 at intervals the ionizing power. The results are 

 shown in Fig. 32, where, neglecting the residual 

 activity, the recovery curve of the activity of the 

 salt is compared with the curve of decay of 

 activity of the separated emanation. It will be 

 seen that the two curves are complementary to 

 each other ; the activity of the emanation falls 



€ 8 H) fZ 



LcLys 



Fig. 32. 



to half its initial value in a little less than four 

 days, and the purified radium salt recovers half 

 its final activity in the same time. If the activity 

 of the emanation at any instant be added to that 

 of the recovering radium, the result is equal to 

 the normal activity of the radium when fully 

 recovered. Thus the total activity of the residual 

 radium and its separated emanation, considered 

 together, remains constant throughout, though 

 resolved into constituent portions. This result 

 again illustrates the characteristic feature of radio- 



