and its Successive Products. 41 



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 volatilized. The initial 

 increase of activity of AcX is thus explained in the following 

 manner. 



When actinium is precipitated with ammonia, the active 

 deposit is left behind in the nitrate together with AcX. In 

 the moment, 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 

 deposit. Any of the latter, if still remaining in the precipi- 

 tate, would be driven off during the process of drying. In 

 consequence, when we start the measurements of the activity 

 of the precipitate itself, no excited activity is present. The 

 activity at once commences to increase, since a fresh amount 

 of AcX is produced which in turn gives rise to excited 

 activity. In consequence we do not observe the initial decay 

 in the recovery-curve of actinium as in the corresponding 

 curve for thorium. 



Disregarding these small peculiarities, the behaviour of the 

 product AcX, and of actinium deprived of AcX, is, as we 

 have seen, completely analogous to that of ThX and of 

 thorium deprived of ThX. There is, however, the following- 

 distinct difference. After removal of ThX, thorium always 

 has a certain amount of residual activity about 25 per cent, of 

 the 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 per cent.) value. 

 In the case of actinium, immediately after removal of AcX the 

 actinium is almost inactive, its activity being only 5 per cent, 

 of its maximum value. I tried experiments to see whether 

 this activity could not bo removed by means of successive 

 precipitations with ammonia, but although eight precipitations 

 were made in the course of seven hours the residual activity 

 always remained. Nevertheless, the smallness of the initial 

 amount of activity pointed to the probability that in reality 



* 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. 



