Active Deposit of Thorium in Various Solvents. 331 



measured by observing the rate of leak of an electroscope. 

 It was at once placed in the solvent for an observed time, 

 carefully dried by placing between filter-paper, and its 

 activity again measured. It was now set aside for a few 

 hours to enable the transformation products to get into 

 equilibrium, after which the activity was again measured. 

 From this third reading could be calculated the equilibrium 

 activity of the plate at the instant it was taken out of the 

 solvent. This was necessary in order to ascertain which of 

 the substances had been dissolved in the greater quantity. 

 Thus if thorium B had been dissolved more than thorium C *, 

 the quantities of each remaining would not be in equilibrium, 

 and the activity of the plate would be greater than that 

 calculated from the third reading when the two substances 

 had adjusted themselves. If the reverse of this occurred it 

 would indicate that the thorium C had dissolved more than 

 the thorium B. (This of course is due to the fact that the 

 activity of the plate arises from the thorium C only.) Thus 

 when thorium B is dissolved from the plate, the loss does not 

 immediately affect the activity of the plate, and can only be 

 detected by calculating backward from some value of the 

 activity when the two have reached equilibrium. The loss 

 of thorium C is noticed directly from the reading immediately 

 after treatment with the solvent. 



It was found that water, solutions of salts, hydroxides, 

 and acids dissolved both products, but thorium B was always 

 dissolved in a relatively larger amount than thorium C. This 

 was most noticeable in the case of short treatments, as would 

 be expected from the rates of decay of the products. The 

 results show that the solubility in water (boiling) is small, 

 about 35 per cent, being dissolved in 15 minutes' treatment. 

 With an hour's treatment about 70 per cent, had dissolved; 

 but after about 1^ hours the rate of solution was very little 

 greater than the rate of decay of thorium B. A solution of 

 potassium iodide (boiling) was found to dissolve the active 

 deposit at more than twice the rate of the water. Dilute 

 acids (HC1, HN0 3 ) had a much more rapid rate of solution 

 even at ordinary temperatures. In the case of organic 

 solvents the rate o£ solution was found to be about the same 

 as in the case of water, but the thorium C dissolved more 

 easily than thorium B. In fact, thorium B was absolutely 

 insoluble in carbon bisulphide and methylene iodide, whilst 

 thorium C was soluble to the extent of 20 per cent, in 



* In these experiments I had no means of differentiating 1 between 

 thorium Ci, C 2 , and D ; hence I have used the term thorium C in the paper 

 to represent all the products after the B. The nomenclature is the 

 amended one of Rutherford and Geiger, Phil. Mag. xxii.p. 621 (1911). 



