Boltwood — Thorium Minerals and Salts. 423 



thoria is greatly reduced by heating the material to a white 

 heat. The oxides used in the present research were all heated 

 in a platinum crucible to the highest heat of the blast lamp. 

 Quantities several hundred times the weights of the films 

 employed were tested for emanation by placing them in a 

 closed vessel for 24 hours with a plate charged negatively to 

 about 400 volts at a distance of about one centimeter above 

 them. The plate was then tested in the electroscope, but no 

 evidence of an active deposit was obtained. The oxides were 

 therefore essentially non-emanating. 



Discussion of Mesidts. 



The results obtained from the measurements, of the activity 

 of the separated thorium oxides indicate a number of interest- 

 ing facts with regard to the activity of thorium. The oxides 

 obtained from the commercial thorium salts (Nos. 1 to 4) are 

 uniformly about half as active as the oxides separated from, 

 or contained in, the natural thorium minerals. Since the 

 source of the commercial thorium salts is monazite sand, and 

 since it is shown that the thorium in this mineral and in the 

 salts prepared from it by certain described analytical methods 

 are of normal activity, it is obvious that the chemical treat- 

 ment to which the commercial salts are subjected results in 

 the separation of about one-half of their radio-active constitu- 

 ents. If it is assumed that the activity of thorium salts is 

 due to the presence of radio-thorium and its disintegration 

 products, then it must be assumed that in the salts of lower 

 activity about one-half of the total radio-thorium present in 

 the mineral has been separated from these salts. From the 

 data given by Halm* it would appear that in the method of 

 separation by which his radio-thorium was obtained not more 

 than at most two per cent of the total radio-thorium was 

 separated from the mineral. The commercial method of pre- 

 paring pure thorium salts is therefore much more efficient in 

 effecting the separation of the radio-thorium. \ 



The fact that the specific activity of the thorium in the 

 minerals was found to be constant is strongly in support of 

 the theory that radio-thorium is a disintegration product of 

 ordinary thorium. It would appear quite impossible to explain 

 the agreement by any other assumption. 



* Loc. cit. 



+ Several pounds of residues obtained from the treatment of monazite 

 with concentrated sulprmric acid were kindly supplied by the Welsbach 

 Company. These residues were examined for the presence of radium and 

 radio-thorium, but with practically negative results for both substances. 

 The amounts of these elements retained in the residues could not have been, 

 'at most, more than a fraction of a per cent of the amounts contained in the 

 original mineral. 



