98 Bolticood— Radio- Activity of Thorium Salts. 



the precipitation of thorium as hydroxide from a solution of 

 the nitrate is effective in separating thorium from mesothorium, 

 since this was the only treatment to which the thorium in oxide 

 No. 2 had been subjected. 



Over two years ago I had made some experiments with the 

 object of obtaining a more definite knowledge of the chemical 

 behavior of thorium-X. At that time a considerable number 

 of thorium-X residues had been prepared by the well-known 

 method of precipitating the thorium as hydroxide from a solu- 

 tion of the nitrate, filtering off and washing the hydroxide, 

 evaporating the filtrate to dryness, and gently igniting the resi- 

 due to remove the ammonium salts present. The amount of 

 thorium nitrate used in some of these experiments was as much 

 as a hundred grams and the volume of the filtrate was often 

 more than two liters. In these experiments it was always 

 found that the residue, after the removal of ammonium salts, 

 contained very appreciable amounts of thorium. This thorium 

 was finally removed from the residue by a second treatment 

 with ammonia in a solution of small volume. A number of 

 such thorium-free residues had been prepared and measure- 

 ments of their activity had shown a steady fall for a period of 

 about 30 days. After 30 days the residues still retained a defi- 

 nitely measurable activity, which was comparatively small, how- 

 ever, and amounted to a leak of only a few divisions per 

 minute in the electroscope. This residual activity, which was 

 observed further in some instances for a period of about one 

 week, appeared to be fairly constant during that period, after 

 which its progress was not further followed. It was attributed 

 at the time to the presence of a little radium in the original 

 nitrate. 



Several of these old thorium-X residues have been preserved, 

 and as soon as the possibility that they might have originally 

 contained mesothorium suggested itself they were introduced 

 into the electroscope and their activities noted. The activity 

 of each of them was found to have risen enormously, until it 

 has now reached a value whieh is many times greater than the 

 minimum value to which it had originally fallen. These thor- 

 ium-free residues, weighing together not more than a few milli- 

 grams, have now an activity equal to that of several grams of 

 thorium oxide of normal activity and spontaneously evolve an 

 emanation which is identical with that of thorium, falling to 

 half-value in 54 seconds and producing the characteristic thor- 

 ium-active deposit. These residues therefore contain radio- 

 thorium (and its products) which has been formed by the 

 disintegration of the mesothorium originally present. If the 

 half-value period of mesothorium is about 7 years, as suggested 

 by Halm, then these residues must still contain mesothorium 

 and their activity will continue to increase for some time longer. 



