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product AcX, and of actinium deprived of AcX, is, as we have 
seen, completely analogous to that of ThX and of thorium depri- 
ved of ThX. There is however the following distinct difference. 
After removal of ThX, thorium always has a certain amount of 
residual activity about 25°/, of maximum value. A similar effect is 
observed in the case of radium, where the ,de-emanated“ radium 
has always a non separable activity of about the same (25°/,) va- 
lue. In the case of actinium, immediately after removal of AcX, 
the actinium is almost inactive, its activity being only 5°/, of its 
maximum value. I tried experiments to see if this activity could 
not be removed by means of successive precipitations with ammo- 
nia, but although 8 pricipitations were made in the course of 7 
hours the residual activity always remained. Nevertheless, the 
smallness of the initial amount of activity pointed to the probabi- 
lity that in reality the actinium itself is not active and that the 
residual activity observed is due to a small quantity of AcX, 
which is left behind. The interval between the last preeipitation 
and the first measurement was always one hour or more, but this 
alone would not account for the observed current. It seems very 
probable that at the moment of the removal of AcX, if the sepa- 
ration were complete, actinium would be entirely devoid of acti- 
vity. From the point of view of the theory of radioactive changes, 
this shows that the change of actinium into AcX is not accompanied 
by either «a, 8 or y rays or, in other words, is a „rayless“ change. 
By means of an electroscope. it was found that actinium X 
gave out all three kinds of rays a, 8 and y. Now the products 
of exeited activity are very quickly formed owing to the very 
rapid change of the emanation. The activity of these products are 
consequently measured together with AcX. It was separately pro- 
ved that the active deposit gave out B rays!). Taking into consi- 
deration the analogy with thorium and even with radium, we 
should expeet that the measured ß activity of AcX arises not from 
AcX itself, but from the exeited activity resulting from it. There 
is however strong evidence that in the case of actinium the 8 and 
1) It was found that the exeited activity of actinium, measured by f rays, 
after a long exposure decayed according to an exponential law with the time, 
falling to half value in 36 minutes. The complete account of these investigation 
will be given in another place. 
