1] RADIO-ACTIVE SUBSTANCES 25 
to actinium, the two emanations should have identical rates of 
decay. 
Baskervillet, working with thorium minerals, was able to obtain 
thorium less active to the photographic plate than ordinary 
thorium. He put forward the view that thorium was a mixture 
of two elements, one of which was active and the other inactive, 
These results were probably due to the separation of the active 
product Th. X from the thorium (see section 119). This process 
would temporarily greatly reduce the activity as tested by the 
photographic method. 
Until thorium is obtained permanently free from activity, the 
question whether the radio-activity is due to a small trace of very 
active matter, or to the thorium itself, must remain in doubt?. The 
fact that ordinary commercial thorium and the purest chemical 
preparation show equal activity supports the view that the effect 
is not due to a radio-active impurity, but to the element itself. 
If the activity of thorium is due to a small trace of active matter, 
this active substance is certaimly not radium or actinium or any 
other known material. 
Hofmann and Zerban*® obtained a substance from pitchblende 
similar in radio-active properties to thorium. The activity of this 
product did not diminish much in four months’ interval. The 
substance was probably the same as Debierne’s actinium. They 
also examined the thorium minerals broggerite and clevite, but 
obtained only some active residues the activity of which decreased 
rapidly with the time. 
23. Radio-active lead. Elster and Geitel* found that lead 
sulphate obtained from pitchblende was very active. They con- 
sidered that the activity was due to admixture with radium, and 
by suitable treatment the lead sulphate was obtained in an inactive 
state. 
1 Jour. Amer. Chem. Soc. 23, p. 761, 1901. 
2 In a recent paper (Ber, deutsch. chem. Ges. p. 3093, 1903) Hofmann and 
Zerban state that they have obtained a preparation of thorium from gadolinite 
which was almost inactive when tested by the electric method and conclude that. 
pure thorium is not radio-active. 
° Ber. deutsch. chem. Ges. p. 531, 1902. 
4 Wied. Annal. 69, p. 83, 1899. 
