384 Prof. E. Rutherford and Mr. F. Soddy on 



Rate of production of ThX. — A quantity of thorium nitrate 

 solution that had been freed from ThX about a month before, 

 was again subjected to the same process. The activity of the 

 residue from the filtrate in an experiment in which 10 grams 

 of this nitrate had been employed was equivalent to 8'3 grams 

 of thorium oxide. This experiment was performed on the 

 same day as the one recorded above, in which 5 grains of 

 new nitrate had been employed, and it will be seen that there 

 is no difference in the activity of the filtrate in the two cases. 

 In one month the activity of the ThX in a thorium com- 

 pound again possesses its maximum value. 



If a period of 24 hours is allowed to elapse between the 

 successive precipitations, the activity of the ThX formed 

 during that time corresponds to about one-sixth of the maxi- 

 mum activity of the total thorium employed. In three hours 

 the activity of the amount produced is about one-thirtieth.. 

 The rate of production of ThX worked out from those figures 

 well agrees with the form of the curve obtained for the 

 recovery of activity of thorium, if the latter is taken to- 

 express the continuous production of ThX at a constant rate 

 and the diminution of the activity of the product in geo- 

 metrical progression with the time. 



By using the sensitive electrometer, the course of pro- 

 duction of ThX can be followed after extremely short in- 

 tervals. Working with 10 grams of thorium nitrate, the 

 amount produced in the minimum time taken to carry out the 

 successive precipitations is as much as can be conveniently 

 measured. If any interval is allowed to lapse the effect is 

 beyond the range of the instrument, unless the sensitiveness 

 is reduced to a fraction of its ordinary value by the intro- 

 duction of capacities into the system. Capacities of *01 and 

 *02 microfarad, which reduce the sensitiveness to less than 

 one two-hundredth of the normal, were frequently emploved 

 in dealing with these active residues. 



The process of the production of ThX is continuous, and 

 no alteration was observed in the amount produced in a given 

 time after repeated separations. In an experiment carried 

 out for another purpose (section IX.) after 23 successive 

 precipitations extending over 9 days, the amount formed 

 during the last interval was as far as could be judged no less 

 than what occurred at the beginning of the process. 



The phenomenon of radioactivity, by means of the electro- 

 meter as its measuring instrument, thus enables us to detect 

 and measure changes occurring in matter after a few minute- 

 interval, which have never yet been detected by the balance 

 or suspected of taking place. 



