﻿of the a. Rays of Actinium. 247 



clear filtrate must be used. Great care has to be taken to 

 use pure chemicals, and it is always advisable to use as little 

 liquid and reagents as possible. 



The actinium X is evaporated on a flat platinum cover and 

 heated to drive off any remaining ammonia salts. As the 

 active deposit is regenerated very quickly, it is difficult to 

 obtain an ionization curve of actinium X alone, for there is 

 always some actinium B present. Since, as we shall see later, 

 the range of the a. particles of actinium X is higher than the 

 range of those of actinium B and of the emanation, the 

 presence of the latter products does not interfere with the 

 accurate determination of the maximum range of actinium X. 



In the earlier experiments, great difficulties were en- 

 countered in determining the real range of the a particles of 

 actinium X, on account of disturbances due to the emanation 

 even when a strong air-current was used. A small ionization 

 was observed even to a distance of more than 8 cms. ; and, 

 in consequence, it was at first thought that traces of thorium X 

 and, therefore, thorium C were present. Finally, however, 

 these effects were found to be due only to the presence of the 

 emanation. 



In order to obtain accurate results, it was found necessary 

 to cover the actinium X hermetically with a thin layer of 

 mica. Tbe mica plate was waxed down close to the bottom 

 of the dish, and the small amount of the active matter on the 

 sides of the dish was covered with tinfoil to absorb the a. rays, 

 but even in that case a small amount of emanation coming 

 from the walls escaped into the gas. 



A separate experiment showed that the mica screen was 

 equivalent in stopping power to 1*20 cm. of air, and in the 

 curves given in fig. 2 the distance is corrected for this 

 absorption of the screen. 



Fig. 2 (p. 24:8) shows the curves obtained for actinium X. 



Curve a represents the ionization curve of the freshly pre- 

 pared product. 



Curve b represents the ionization curve 15 days later. 



Curve c represents the ionization curve 42 days old. 



In the figure, the ionization due to the a rays begins at 

 about 6*55 cms. This number gives the average of several 

 experiments, and is probably correct within 1 mm. The 

 ionization curves, after different intervals, are given to show 

 that the curves obtained are really due to actinium X, which 

 Godlewski has shown decays to half value in 10 days. 



In consequence of the use of the mica screen, it is not 

 possible to obtain the shape of the ionization curve closer to 



