McCoy and Ross — Thorium Compounds. 435 



Good films of thorium dioxide may be made, as in the case 

 of uranium compounds, using alcohol as the suspending liquid. 

 Critical consideration leads to the conclusion that the emana- 

 tion produced by a thorium compound must be distributed into 

 three parts ; (1) retained in the granules of the substance ; (2) 

 emanated by the granules and absorbed by the film ; (3) ema- 

 nated by the granules and evolved by the film. These three 

 portions of the emanation will be referred to as the retained, 

 the absorbed, and the evolved emanation. Together the 

 absorbed and the evolved constitute the whole of the emanated 

 emanation. 



The effect of the evolution of the emanation on the observed 

 activity of the film can be determined theoretically. In the 

 case of a film which has been freely exposed to the open air 

 for at least several hours, the activity due to the film alone is 

 diminished by reason of the loss of emanation. This diminu- 

 tion is due to a deficiency, both of the emanation and of -thor- 

 ium B, the active product of the emanation. The activity of 

 ThB is approximately equal to that of the equilibrium quan- 

 tity of emanation* and therefore the loss of activity is equal to 

 twice that due to the emanation which has escaped. The 

 activity of a film placed in a small closed space, such as an 

 electroscope case, increases with time and reaches a maximum 

 in about five minutes. The increase of activity is due to the 

 accumulation of the evolved emanation. The activity of this 

 evolved emanation is exactly twice as great as if it had 

 remained in the very thin film, because there is now no absorp- 

 tion of the radiation by the metallic plate, on which the film 

 is deposited. It follows from these considerations that the 

 actual activity of the evolved emanation exactly compensates 

 for the deficiency of activity of the solid film. Therefore the 

 observed activity of a very thin film of a thorium compound, 

 is equal to the true activity. This relation holds very closely 

 for films of strongly ignited thorium dioxide up to about '005 g 

 per sq. cm. The thorium dioxide, sample A, used in the 

 preliminary measurements was made by heating and finally 

 igniting strongly, in the blast, thorium nitrate made by C. A. 

 F. Kahlbaum. This sample was said to contain ammonia only 

 as an impurity. Its purity otherwise and freedom from non- 

 volatile impurities was established by the analysis of a solu- 

 tion made by dissolving about 4 g in 250 cc of water. 25 cc 

 evaporated to dryness and ignited gave O1750 g of residue ; 

 portions of 25 cc diluted and precipitated with ammonia gave 

 upon ignition "1746 and *1748 g . Finally portions of 25 cc gave 

 by Neish's methodf -1751 and -1754 g of pure Th0 2 . \The 



* Rutherford, Radioactivity, p. 307, 1904. 

 fj. Amer. Chem. Soc, xxvi, 780, 1904. 



