Boltwood — Radio- Activity of Thorium Salts. 95 



and the washed hydroxide had been reconverted into nitrate. 

 In order to again obtain the solid salt the solution of the 

 nitrate had been evaporated to dryness under conditions iden- 

 tical with- those under which a considerable number of other 

 salts have been prepared and which give a salt containing 

 about forty-eight per cent of thorium oxide. The number 

 given by Dadourian as expressing the specific activity of the 

 thoria in this salt is therefore undoubtedly too high and the 

 correct value would be about 9'0. The reason for this low 

 value will appear later in this paper. 



The specimen of Welsbach salt examined by the writer con- 

 sisted of a kilogram of thorium nitrate made from North 

 Carolina monazite and had been received about fifteen months 

 before the time of my experiments. 



The three Welsbach salts examined by Dadourian and the 

 writer were therefore at least four years old, three and one- 

 half years old and one and one-third years old, respectively. 

 The oldest salt contained about forty per cent of the radio- 

 thorium in equilibrium with the thorium present and the 

 youngest salt must have contained at least thirty per cenf of 

 its equilibrium amount of radiothorium. The difference in 

 ages of these two salts was about three years and they had 

 both been prepared from the same mineral by the same pro- 

 cess. If radiothorium was a product formed directly from 

 thorium it was obvious that its period of decay (recovery) 

 could not be less than half-value in about six years and might 

 be somewhat longer. 



In April, 1906, the belief that the above data might have 

 an important bearing in indicating the rate of disintegration 

 of radiothorium was privately communicated to Dr. Otto 

 Hahn, the discoverer of radiothorium. Dr. Hahn replied that 

 the data supplied were difficult to reconcile with the results of 

 his own experiments, made directly with a preparation of radio- 

 thorium, which seemed to show a half-value period of about 

 two years for this substance. He made, however, at the same 

 time the interesting suggestion that the lack of agreement 

 could be explained if a rayless product having a slow rate of 

 change intervened between thorium and radiothorium. 



The rate of disintegration of radiothorium has since been 

 determined by Blanc,* who finds that the half-value period is 

 737 days, and the existence of a product intermediate between 

 thorium and radiothorium has recently been demonstrated by 

 Hahnf in a very convincing manner. 



Hahn made a careful examination of a series of samples of 

 the intermediate chemical products obtained in the technical 



*Rend. della R. Accad. d. Lincei, xvi, 291, 1907. 

 fBerichte d. chem. Ges., xl, 1462, 1907. 



