266 
resulting products are quite distinct bodies, though they escape 
detection by chemical methods on account of the minute amount 
of the substance under investigation. Their existence is proved 
first of all by electrical measurements which allow us to make the 
quantitative investigations of the rate of change of these products. 
On looking over the series of successive products arising from 
different radioactive bodies, striking similarity between the pro- 
ducts of thorium and actinium is at once manifest. Thorium pro- 
duces ThX,. ThX the emanation, this gives rise to the active de- 
posit which undergoes two further transformations, the first slow 
change being a rayless one, the other emitting all three kinds of 
rays. Actinium in like manner produces an emanation which is 
transformed into an active deposit which undergoes two further 
changes, the first being a slow rayless change and the other a ra- 
pid change. 
This analogy in the number and nature of the products pointed !) 
to the possibility that there existed between the actinium and its 
emanation an intermediate product which bore the same relation 
to actinium that ThX bears to thorium. In a letter to Nature 
(26 Jan. 1905) I gave the preliminary results of the investigation 
which proved the existence of this product. 
Taking into consideration the similarity of actinium and tho- 
rium, I applied to the actinium the same method which had been 
used by Rutherford and Soddy for the separation of ThX from 
thorium. 
The experiments were made with the emanating substance of 
Giesel, which according to numerous investigations?), has been 
found to contain the same radioactive constituent as the actinium 
of Debierne. The saturation current due to the « ray activity of 
the products under investigations was measured with a quadrant 
eleetrometer of sensibility 120 divisions per volt; the needle was 
kept at the standard potential of 300 volts. The activity was mea- 
sured by means of a sensitive eleetroseope. Four different sets of 
experiments were made which gave very concordant results. 
In each case 0:15 g of the emanating substance, of activity 
!) See: Rutherford: Bakerian Lecture: The Succession of Changes in Radio- 
active Bodies. Phil. Trans. of the Royal Soc. Ser A. 204 190 and 204. 
*) See f. i. Rutherford. Bakerian Lecture Loc. cit. p. 188. 
