It is seen that there is a very striking similarity between the 
number and nature of the changes for actinium and thorium. But 
the periods of decay, the radioactive, chemical and physical pro- 
perties of the products of actinium all point conclusively to the 
fact that we have in actinium a distinct chemical element. 
Whilst writing this paper the December number of the „Jahr- 
buch der Radioactivität und Elektronik“ was received containing 
a paper by Giesel on emanium !). In this paper Dr. Giesel gives 
an account of his investigation, in which he finds that it is possible 
to separate from emanium by precipitation with ammonia a small 
amount of very strongly active substance. The method of separa- 
tion employed by him was then identical with the method I have 
used. I cannot, however, compare quantitatively my results with 
his, inasmuch as Dr. Giesel does not publish any measurements. 
A short account will now be given of some experiments which 
are still in progress on the nature of the ß and y rays of acti- 
nium. 
The 5 rays of actinium are completely distinct in their charac- 
ter from the 8 rays emitted by the other radio-elements, inasmuch 
as they are completely homogenous. This fact was established by 
the measurements of absorption of 8 rays in passing through solid 
bodies. The activity measured by f rays decreased strietly accor- 
ding to an exponential law with the thickness of matter traversed. 
The equation 1—1,e-" where d is the thickness was applicable 
even in the case when I was less than 1°/, of its original value. 
The 8 rays from actinium differ also from the 8 rays of other 
radioactive elements in the absolute value of the absorption con- 
stant À which is about 25 times as great with actinium as with 
uranium. Thus the 8 rays of actinium have less than half the pe- 
netrating power of those emitted by any other radio-element. 
The existence of the y rays from actinium was also distinetly 
proved. The absorption measurements showed that the y rays of 
actinium are fairly homogenous and their penetrating power was 
only about one quarter of that observed for the y rays from ra- 
dium. A more complete account of these investigations will be 
published later on. 
1) Giesel: Untersuchungen über das Emanium (Actinium) Jahrbuch f. Ra- 
dioakt. VI. pp. 375—358. 
