22 Prof. Rutherford and Miss Brooks : Comparison of 



If N is the rate of production of ions immediately after 

 removal, it follows that 



The decay of excited radioactivity is thus a function of 

 the time of exposure, and in some cases may vary very 

 greatly for different values of t. 



The decay-curve for a long time of exposure may be quite 

 different in general shape from that of the individual particle 

 which gives rise to excited radioactivity. 



This is seen to be the case for radium (c) (compare figs. 7 

 and 8). It is, however, much more marked in the case of 

 excited activity from thoria, where the radioactivity for a 

 few minutes' exposure increases after removal to three or four 

 times its value in the course of a few hours, and then decays 

 at the normal rate. For a long exposure the radioactivity on 

 removal at once begins to decay. 



The decay-curve of the individual particle on which, we 

 have based the predetermination of the curves of decay and 

 rise of excited radioactivity, is obviously very nearly the 

 same as the decay-curve for a body which has been exposed 

 for a short interval in the presence of an emanation. The 

 only condition to be fulfilled is that the time of exposure is 

 to be so short that the radiation from the individual particle 

 does not appreciably vary. 



This condition can readily be fulfilled in the case of thorium- 

 excited radiation ; but in the case of radium the initial rapid 

 rate of decay introduces experimental difficulties. 



As a rule it takes a minute or two to remove the excited 

 conductor from the vessel containing the emanation, and to 

 place it in position in the testing apparatus. During this 

 time the radioactivity has appreciably diminished. 



Fig. 8 shows approximately the decay-curve of a radioactive 

 particle for radium (c) . The plate was only exposed a short 

 time and then immediately tested. 



Some experiments have been made on the rise of excited 

 radioactivity with time in the case of radium (c). A curve 

 for thoria is given in a previous paper. From the general 

 shape of the decay-curve we should expect the radioactivity 

 to increase rapidly for about the first ten minutes and then 

 more slowly afterwards. This was experimentally found to 



