PROCEEDINGS OF SECTION B. 35& 



The few examples I have given are selected, as in some cases the 

 products obtained were out of the ordinary run, also the behaviour of 

 certain impurities totally different to what is generally stated in text- 

 books bearing on this subject. 



9.— THE RADIO-ACTIVITY OF THORIUM. 

 By G. J. GRA Y. B.E.. B.Sc. 



[With Plate.] 



The radio-activity of thorium has been much discussed during the 

 last couple of years. 



All the thorium salts of commerce are prepared from minerals from 

 three sources, or from admixtures of these ; and when several inves- 

 tigators claimed to have isolated more or less permanently inactive thoria 

 from other minerals, it seemed quite possible that the activity was due to 

 some accidental impurity The constancy of the association was paral- 

 leled by that of uranium and thorium in the well-known localities. The 

 isolation of radio-thorium by (o) Hahn heightened the interest. It fol- 

 lowed thorium closely in properties, hence might easily account for the 

 activity of the latter ; whilst if it were a product of thorium, its resem- 

 blance in chemical properties to it pointed to sufficient difference in 

 atomic weight to involve several uninvestigated changes. 



The question seemed important enough for investigation. The 

 method chosen consisted in the determination of the ratio of the mass of 

 thorium to the mass of radio-thorium present in minerals from several 

 different localities. The minerals are old enough for the various non- 

 volatile products of the radio-chain to have reached equilibrium values. 

 When in equilibrium the ratio of the masses of any two such products 

 will be a constant. 



If radio-thorium be a product of thorium, then the ratio of radio- 

 thorium mass to thorium mass will be a constant in all thoriferous 

 minerals, since no volatile product intervenes between the thorium and 

 radio-thorium, so far as is known. 



The thorium w^as determined as ThO^ analytically as detailed. The 

 radio-thorium was determined from measurements of the activity of the 

 " thorium emanation." Its total activity is proportional to the mass of 

 radio-thorium when in equilibrium, and it reaches its equilibrium value 

 in a very short time. The mineral in solution was placed in a horizontal 

 glass tube, which it partly filled, and a constant current of air was drawn 

 over the solution. The active gas was carried into a testing vessel and 

 its activity measured. After a certain time the amount of emanation 

 formed and carried over per second became constant, and for solutions of 

 the same order of thorium concentration this was proportional to the 

 emanation activity value. The work was started some 18 months ago, 

 but, owing to delay in obtaining specimens and pressure of teaching work, 

 the results are only ready now. 



Recently results on the same question have been published. 



(b) Boltwood obtains the correct total activity by McCoy's method. 

 He estimates the uranium present analytically, and subtracts the activity 



(a) Hahn: Proc. Roy. Soc, Lend., A., Lxxvi., 115, (1905). 

 (b) Boltwood: American Journal of Science, xxi., 126, p. 415. 



