192 



PROFESSOR K. RUTHERFORD ON THE 



These conclusions are very strongly supported by observations of the decay 

 measured by the ft rays for a long exposure. The curve of decay is shown in fig. G 

 and fig. 1 1, curve I. 



too 



65" 



in Minutes 



Fig. 11. 



P. CURIE and DANNE made the important observation that the curve of decay C, 

 shown in fig. 7, for a long exposure, could be accurately expressed by an empirical 

 equation of the form 



where X 2 = 3'10 X 10~* (sec)" 1 and X 3 = 4*13 X 10~* (sec)" 1 , and a = 4'20 is a 

 numerical constant. 



I have found that within the. limit of experimental error this equation represents 

 the decay of excited activity of radium for a long exposure, measured by the /3 rays. 

 The equation expressing the decay of activity, measured by the rays, differs con- 

 siderably from this, especially in the early part of the curve. Several hours after 

 removal the activity decays according to an exponential law with the time, decreasing 

 to half value in 28 minutes. This fixes the value of X.j. The constant a and the 

 value of X. 2 are deduced from the experimental curve by trial. Now we have already 

 shown (section 14) that in the case of the active deposit from thorium, where there are 

 two changes of constants X 2 and X 3 , in which only the second change gives rise to a 

 radiation, the value of 



__ 

 *- " 



"2 .! 



for a long time of exposure (see equation 8). This is an equation of the same form 



