THE TRANSFORMATIONS OF ELEMENTS 531 



disintegration products of thorium X ; these are also radio- 

 active, and produce their own electrical effect. They were 

 precipitated with the thorium hydroxide. The initial part of 

 the recovery carve is due to their activity rapidly becoming 

 smaller as they themselves disintegrate — more rapidly than is 

 counteracted by the gradual reproduction of thorium X. The 

 initial part of the decay curve is due to their reproduction 

 from thorium X, when their activity gives an added effect to the 

 activity of thorium X itself. In both cases a time is reached 

 when in any given interval, say a minute, as much of these 

 substances is produced as decays in that same time. These 



10 15 



Time in Days. 



Fig. 2 a. 



10 15 



Time in Days. 



Fig. 2B. 



20 



products are then said to be in radioactive equilibrium, and 

 no longer affect the nature of the curve. The times in 

 question are represented by the minimum and maximum on 

 the respective curves, and it will be noticed that these exactly 

 correspond. If the true curves are then continued back till 

 they meet the axis, and the maximum values in both cases 

 again taken as 100, the corrected values are shown in fig. 2, b 

 {Radioactivity ', p. 222). The decay curve is logarithmic, that is 

 to say, if the logarithms of the activity values are plotted 

 against time (instead of the true activity values), the result 

 is always a straight line. This is shown in the figure. Such 



