Ashman — Radio- Activity of Thorium. 69 



acid method and converted into the oxide by ignition. Oxide 

 " B " was from sample " B " of thorium nitrate described by 

 McCoy and Ross. It had been precipitated 40 times with 

 hydrogen peroxide and contained about one third of the 

 equilibrium amount of radiothorium. It was twice purified 

 by Neish's method, and finally by four precipitations with 

 nitrobenzoic acid, made at intervals of two hours, Samples 

 a C" and "D'' were oxides made from thorite from Arendal, 

 Norway. Nine grams of the finely powdered mineral were 

 digested for a number of hours with concentrated hydro- 

 chloric acid. The chlorides were dissolved in hot water and 

 filtered from the insoluble residue. The thorium was then 

 precipitated with a boiling solution of oxalic acid and washed 

 free from the excess of the acid. The precipitated oxalates 

 were decomposed by boiling them for ten minutes with fifteen 

 grams of potassium hydroxide in 30 cc of water. The thorium 

 hydroxide was washed free from alkali and dissolved in dilute 

 nitric acid. From this solution the thorium oxide was obtained 

 free from thorium-X and all subsequent products by four 

 precipitations with nitrobenzoic acid in the manner described. 



The thoria thus prepared was made into films, 7 cm in 

 diameter, as quickly as possible by Boltwood's method,* paint- 

 ing the paste onto a metal disc with a camel's-hair brush. 

 Owing to the rapidity with which the activity of the thoria 

 increased, the slower sedimentation process, f which yields 

 more uniform films, could not be used. However, the error 

 due tolack of uniformity was doubtless negligible since one 

 set of films, " Dj," made by the sedimentation process gave the 

 same maximum specific activity, 948, as did the painted films 

 from the same material ; see Table I. Activity measurements 

 were made in the manner described by McCoy and Ross,J and 

 with the same electroscope. § From five to eight films were 

 made from each sample ; the films were made thin, 0*2 to 7 mg. 

 per square cm., in order to avoid errors due to the evolved 

 emanation in the case of material containing the equilibrium 

 amount of Th-X.| 



In order to find the activity of the thorium oxide at the 

 time zero, i. e., at the moment when thorium-X and its prod- 

 ucts were completely removed from it, the activity of each 

 film was measured at intervals of a few hours during the first 

 day after its preparation. The increase of activity with time 

 was plotted and the activity at time zero found by extending 

 the curve back to the ordinate for zero time as illustrated in 



* This Journal, xxv, 269, 1908. 



+ McCoy. J. Amer. Chem. Soc, xxvi, 391, 1905. 



% Log. cit. §This Journal, xxvi, 521, 1908. 



I McCoy and Ross; this Journal, xxi, 438, 1906. 



