McCoy and Ross — Thorium Compounds. 433 



Art. XXXIX. — The Relation hetween the Radio-activity and 

 the Composition of Thorium Compounds / by H. N. McCoy 

 and "W. H. Ross. 



The work here described was undertaken with two objects 

 in view. The first was to determine the relation between the 

 radio-activity of thorium minerals and their thorium content, 

 and the second to determine the intensity of radio-activity of 

 thorium, free from its transformation products. The first of 

 these problems was readily solved in a very decisive manner ; 

 but the complete separation from thorium of all of its products 

 has proved to be so difficult that we shall not longer delay 

 reporting on the results already obtained. 



One of us has shown* that the total activity of any pure 

 uranium compound or of any uranium mineral, free from 

 thorium, is, for each class, proportional to the uranium con- 

 tent of the substance ; but that minerals are 4" 15 times as 

 active as the pure compounds of equal uranium content. The 

 excess of activity of the ores is due to radium, etc. ; and the 

 above results seem to show that all of the radio-active bodies 

 found in thorium-free uranium minerals are transformation 

 products of uranium. 



The present study, in a similar manner, of compounds and 

 minerals containing thorium, with or without uranium, forms 

 the natural sequel to the work on uranium. The reports of 

 the interesting work of Ramsay, f Sackur^; and Hahn§ on the 

 isolation, in a crude state, of a very highly active body, called 

 radio-thorium, from a thorium ore, though they appeared 

 while the present work was in progress, only served to increase 

 the importance of this investigation. 



All of the thorium minerals which we have studied con- 

 tained uranium in measurable, though often very minute, 

 quantities. When the activity due to uranium and its products 

 had been allowed for, the remaining activity, due to the thor- 

 ium and its products in the mineral, was, in every case, propor- 

 tional to the thorium content of the sample. The activity due 

 to l g of thorium, together with its products, in a mineral, is 

 about 950, the unit of activity being that due to l sq cm of a 

 thick film of uranium oxide, U 3 8 .j| The activity of pure 



* McCoy, Ber. d. cheni. Ges., xxxvii, 2641, 1904; Phil. Mag. [6], ix, 176, 

 1906. 



f J. de Chimie Phys., iii, 617, 1905. 



^Ber. d. chem. Ges., xxxviii, 1756, 1905. 



§Hahn, ibid., xxxviii, 3371, 1905, Radioaktivitat, ii, 233, 1905. 



I The activity of Is of uranium in a pure compound is 791, while that of 

 the same quantity of uranium, plus its products, in a mineral, is 3280. 

 3280=415x791. 



