174 



I'KOFF.SSOR K. IMTHKKFOKD ON TilK 



1ZO 



100 



c 

 o 



4 so 



or 



t36o 



20 



40 



I2O I6O ZOO 



Time in Minutes. 



240 



28o 



320 



Fig. 1. 



completely on the supposition that there are two successive changes occurring in the 

 matter deposited by the emanation, viz. : 



(1) A ' rayless ' change, in which the matter undergoes a transformation 



according to the same law as the radioactive changes, but which is not 

 accompanied by the appearance of either a or y8 rays. 



(2) A second change which gives rise to all three types of rays. 



It will be shown later (section 13) that the matter in the two changes is half trans- 

 formed in 55 minutes and 11 hours, respectively; but without further physical data it 

 is not possible to deduce directly from the experimental curves whether the first change 

 has the period 55 minutes, or 11 hours. It is supposed that the matter A, deposited 

 from the emanation, changes into the matter B, and this in turn changes into C. The 

 change from A to B is not accompanied by any observable radiation, while the change 

 from B to C is accompanied by all three types of rays. The matter C is either 

 inactive or active to such a slight extent that its radioactivity cannot be detected. 



The theory of these numerous changes will be considered in detail, later, together 

 with the corresponding changes in radium and actinium. 



6. Decay of the Excited Activity of Radium. Miss BROOKS and the writer* 

 showed that the curves of decay of the excited activity of radium were very 

 complicated in character and varied greatly with the time of exposure. It was later 

 pointed out that this was probably due to a triple change in the deposited matter. 



* 'Phil. Mag.,' July, 1902. 



