434 McCoy and Moss — Thorium Comjiounds. 



thorium dioxide, ThO a , resulting from the analysis of thorium 

 minerals, increases with time and reaches a maximum about 

 thirty days after extraction ; at the end of this time the activity 

 due to the thorium and products is practically equal to that of 

 an equal mass of thorium contained in a mineral. 



Since it has been shown* that radio-thorium, Rt, extracted 

 from a thorium mineral, gives ThX, etc., it would seem almost 

 certain that the following series represents the successive pro- 

 ducts of thorium, if, as the present work clearly shows, Rt is 

 the product of Th. 



Th -» Rt -» ThX -> Em -> ThA -> ThB. 



Rutherford and Soddyf have shown how ThX and subsequent 

 products may readily be removed, and Schlundt and Moore;}; 

 have confirmed and extended these observations. It is an easy 

 task to reduce, by the methods of these investigators as well as 

 by a number of other new methods which we have found 

 equally effective, the activity of thorium to a small fraction of 

 the original value ; but within a month practically the whole of 

 the activity is regained. These processes do not appreciably 

 remove radio-thorium, the separation of which from thorium is 

 a difficult matter, and one which we have not yet completely 

 accomplished. 



The determination of the radio-activity of thorium com- 

 pounds is not as simple as in the case of uranium compounds, 

 because the former produce a gaseous emanation, the activity 

 of which is considerable compared with that of the solid film. 

 One of us has shown§ that the total radio-activity of a uranium 

 compound may be determined in two ways. Each of these 

 two ways involves the quantitative measurement of the activ- 

 ity of thin films of the substance, of known weight, varying 

 from about 1 to 20 ms per sq. cm. The thickest of such films 

 of all uranium compounds show maximum activity. The 

 total activity of unit weight of a uranium compound is twice 

 the activity due to one surface of an infinitely thin film of unit 

 weight. The activity of such an ideal film is found by graphi- 

 cal extrapolation, for zero weight, of the observed values of 

 the ratio of the weight, w, to the activity, a, of very thin films. 



The limiting value of this ratio is designated as ( — ) . The 



total activity of unit weight of the compound, Jc 1 — 2/1 — ) . 

 This method may be applied to the compounds of thorium. 



* Hahn, loc. cit. 



+ PM1. Mag. [6], iv, 370. and v, 69, 1902. 



i J. Phys. Chem., ix, 682, 1905. 



§ McCoy, J. Am. Chem. Soc, xxvii, 391, 1905. 



