﻿86 Dr. 0. Halm on some Properties 



determine the range of the a. particles emitted from it. 

 This method of procedure, although somewhat laborious, 

 proved successful. 



Ionization Curve of Radiotlwrium itself, temporarily freed 

 from all its products. 



The radiothorium preparation used for this and the following 

 experiments was not very active at first, for it contained a 

 large amount of iron and other imparities. The activity was 

 certainly not more than a few thousand times that of uranium. 

 In order to obtain a product free from thorium X and con- 

 taining as little inactive substance as possible, the following 

 method was used. The acid solution was nearly neutralized 

 by means of ammonia. A small precipitate fell down, leaving 

 the greatest part of the thorium X and a large amount of 

 iron &c. in solution. After filtering, the precipitate was 

 dissolved in nitric acid, and the solution again treated in the 

 same way, namely, ammonia was added to such an extent 

 that only a small fraction of the former precipitate appeared, 

 the greater part of the former remaining again in solution. 

 This process of solution and precipitation was again repeated. 

 The writer has stated * that in this way a concentration of 

 the radiothorium may possibly be carried out, since the 

 radiothorium seems to be carried down to a larger extent in 

 the first part of an ammonia precipitate than in the latter 

 part. On the other hand, no trace of thorium X was likely 

 to be still contained in the very first precipitate, since 

 thorium X remains in the solution. The nearly invisible 

 precipitate on the filter was again dissolved in nitric acid and 

 the solution evaporated in a flat porcelain dish. The layer 

 obtained was very thin and showed comparatively strong 

 a-ray activity, The ionization-distance curve was taken 

 almost immediately afterwards. The result is shown in fig. 2, 

 curve a. The curve has the characteristic shape to be expected 

 for a single a-ray product. The ionization of the a rays begins 

 at about 3'9 centims., increases rapidly, reaches a maximum 

 and then decreases with the distance from the source. 

 Another curve was taken three hours afterwards, and was 

 practically the same. 



We therefore have to conclude, that the a particles of 

 radiothorium itself cease toj ionize the air at a distance of 

 about 3*9 centims. 



That the curve is really due to radiothorium is seen by its 

 later behaviour. Curve 2 b was obtained four days later 



* Jahrbuch fur Bad. u. Elektr. ii. 3, p 233. 



