and its Successive Products. 39 



The activity o£ AcX immediately after removal was weight 

 for weight more than a hundred times as great as that of the 

 original actinium. Tn the various experiments the value of 

 the initial activity of AcX was proportional to the amount of 

 actinium used, but was by no means proportional to the total 

 weight of matter obtained from the nitrate. In some cases, 

 for example, only a few milligrams of the substance were 

 obtained, which exhibited as great activity as a few centi- 

 grams obtained in other cases. This shows clearly that in 

 the case of actinium, the substance obtained from the filtrate, 

 which we see and weigh, does not all consist of AcX. The 

 substance contains some impurities ; in the present case, 

 probably some of the rare earths. The amount of actinium 

 X actually present is so minute that it precludes the pos- 

 sibility of a direct chemical investigation of its properties. 



We see from Table I. that the activity of AcX increases in 

 the first day after removal to about 15 per cent, of its original 

 value, reaches a maximum, and then decays with the time 

 according to an exponential law, falling to half value in 

 10*2 days. This exponential law of decay is clearly seen 

 in fig. 2, where the ordinates represent the logarithms of 

 the activity of the product AcX, and the abscissae the time 

 after separation. On subtracting from the quantities given 

 in the Table I. the number 2' 70, which represents the activity 

 of the residue which did not decay with the time, the points 

 fall accurately on a straight line, as in the figure. This 

 non-decaying residue comes probably from the small amount 

 of actinium, which is not precipitated, and is therefore present 

 in the filtrate. 



The activity of actinium, from which actinium X was 

 removed, increased so that the recovery-curve was approxi- 

 mately complementary to the decay-curve of AcX. The 

 small difference between the experimental and the theoretical 

 curve, as expressed by the equation I*=l (1— e~ kt ), where 

 X has the same value as in the decay-curve, 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 

 removal (see fig. 1 A and fig. 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 ex- 

 plained by the different properties of the excited activity 



* See Rutherford, < Radioactivity/ pp. 180 & 295. 



