291 
tes !). Prof. Rutherford kindly suggested to me, that I should make 
some investigations to explain these phenomena. 
3. The separation of UrX from uranium by means of fractional crystallization. 
The experiments were first made in order to find out the con- 
ditions under which this first rapid decay of the 8 activity of ura- 
nium is obtained. 
As in the experiments of Meyer and Schweidler, equal parts of 
uranium nitrate ?) and water were taken and this solution was shaken 
with an equal weigth of ethyl ether. The ether solution, was then 
carefully separated from the water solution, and both were evapo- 
rated to dryness. In the ease of water solution the evaporation was 
continued until even the water of erystallization was driven off. 
The @ and ß activity of both portions were then measured. 
The ß activity was measured by means of an electroscope °) 
of the type of ©. T. R. Wilson; the bottom of the eleetroscope 
was removed and replaced by aluminium foil 0:08 mm thick, which 
absorbed all the @ rays. 
The measurements showed that uranium nitrate from the aque- 
ous solution which contained the excess of UrX, derived from the 
ether solution, lost the corresponding excess of its 8 activity accord- 
ing to an exponential law with the time, falling to half value in 
22 days. The ether portion, which was at first almost completely 
inactive, when measured by ß rays. recovered its activity accord- 
ing to a complementary curve !). 
The only difference between my experiments and those of Meyer 
!) Bezüglich der Deutung dieses Verhaltens ist zunächst die Möglichkeit ge- 
geben, dass es sich um Änderungen der Aktivität selbst handelt, oder dass durch 
physikalische Zustandsänderurgen der Kristallplatten ihr Absorptionsvermögen 
beeinflusst wird. Eine definitive Entscheidung zu geben wäre verfrüht. Meyer 
und Schweidler, Loc. cit. p. 1075 (19). 
?) The uranium nitrate under the experiments was obtained from Merck in 
Darmstadt and was labelled „extra pure“. 
®) See Kntherford, Radioactivity p. 71 and Fig. 11. 
*) I omit the detailed numbers obtained in these measurements because the 
results are completely normal; and further during the time when these investi- 
gations were being made Meyer and Schweidler published a short paper (Wiener 
Sitzungsber., Dee. 1904) in which they showed that when a very small amount 
of water was present in the solution, the decay of activity of UrX was quite 
regular. 
