271 
equation {— 1, (L—e*), where À has the same value as in the 
decay eurve, is probably due to a variation in the rate of escape 
of the excited activity, which is extremely volatile. In all cases 
the decay curve agreed more closely with the theoretical equation 
than the recovery curve. 
The initial increase of activity of AcX immediately after re- 
moval (see Fig. 1a and 2) is analogous to the similar increase of 
activity of ThX. The only difference is that the recovery curve 
of actinium does not show the same initial decay as found in the 
case of thorium!). This fact is explained by the different proper- 
ties of the exeited aetivity of actinium and thorium. The active 
deposit of actinium is soluble in ammonia and is volatile when 
heated?). The active deposit of thorium on the other hand, is not 
soluble in ammonia and is not so readily volatilised. The initial 
increase of activity of AcX is explained in the following manner. 
When actinium is preeipitated with ammonia, the active depo- 
sit is left behind in the filtrate together with AcX. In the mo- 
ment, however, when we heat, the volatile active deposit is driven 
off also. But as soon as AcX is separated, it at once produces 
the emanation which gives rise to the active deposit. The activity 
of the latter, at first, more than compensates for the decay of 
activity of AcX, which has a comparatively slow change, and in 
consequence the activity of AcX first of all increases. 
On the other hand, the actinium, when treated with ammonia 
was deprived not only of AcX but also of most of the active de- 
posit. Any of the latter if still remaining in the preeipitate, would 
be driven off during the process of drying. In consequence, when 
we start the measurements of the activity of the preeipitate itself, 
no exeited activity is present. The activity at once commences to 
inerease since a fresh amount of AcX is produced which in turn 
gives rise to exeited activity. In consequence we do not observe 
the initial decay in the recovery eurve of actinium as in the cor- 
responding eurve for thorium. 
Disregarding these small peculiarities, the behaviour of the 
') See Rutherford: Radioactivity p. 180 and 295. 
®) The more complete account of the physical and chemical properties of 
the active deposit of actinium will be published later as the experiments are not 
yet completed. 
