the Cause and Nature of Radioactivity. 



381 



minimum of about 25 per cent., and the above result holds 

 even more accurately if the recovery is assumed to start from 

 this constant minimum, as, indeed, it has been shown to do 

 under suitable conditions (section IX., fig. 4). 



This is brought out by fig. 3, which represents the recovery 



Fiar. 3. 



T//iie/nO-7</s~ 



20 



24- 



22 



curve of thorium in which the percentage amounts of activity 

 recovered, reckoned from this 25 per cent, minimum, are 

 plotted as ordinate-. In the same figure the decay curve 

 after the second day is shown on the same scale. 



The activity of ThX decreases very approximately in a 

 geometrical progression with the time, i. e. if I represent the 

 initial activity and 1^ the activity after time t, 



(1) 



where X is a constant and e the base of natural logarithms. 



The experimental curve obtained with the hydroxide for 

 the rate of rise of its activity from a minimum to a maximum 

 value will therefore be approximately [expressed by the 

 equation 



i-'=l-<r 



(2) 



where I represents the amount of activity recovered when 

 the maximum is reached, and I, the activity recovered after 

 time t, \ being the same constant as before. 



