﻿88 Dr. 0. Halm on some Properties 



as of thorium C present, but as the range of the a particles 

 of the former is only 5 centims., its effect simply adds to that 

 of the thorium X, without showing any distinct break in 

 the curve. 



Fig. 2, curve c, shows the curve obtained from the same 

 product 21 days after preparation. The whole ionization 

 effect is now more than four times as strong as at the begin- 

 ning, the part due to thorium C and thorium X being far 

 more marked than in curve b. 



It will be seen that curve c in fig. 2 closely resembles 

 curve a in fig. 1. This is to be expected, since both represent 

 the ionization of radiothorium in equilibrium. 



It is difficult to obtain accurately the slope of the curve 

 close to the radiating source on account of the disturbances 

 due to the released emanation. For example, the final bend 

 of the curve c, fig. 2, could not be obtained with accuracy. 

 It was for this reason that the curves in some cases are not 

 continued closer to the radiating source. The disturbances 

 due to the emanation are naturally most marked in strongly 

 emanating substances, and increase rapidly as the ionization- 

 chamber is brought close to the active matter. Since, how- 

 ever, it is the object of this paper to obtain the maximum 

 range of the a, particles from each particular product, it is of 

 no great importance to determine the slope of the curve close 

 to the active material. 



Ionization Curve of Thorium X. 



The thorium X used for the investigation of the ionization 

 curve was obtained in the usual way by precipitating a 

 thorium, or, in this case, a radiothorium solution with ammonia, 

 the filtrate containing a large amount of the thorium X. In 

 the case of radiothorium one does not at once succeed in 

 obtaining a filtrate completely free from the radiothorium 

 itself. The thorium X generally contains a small quantity 

 of radiothorium, and its activity consequently does not decay 

 to zero. If there is added to the filtrate a little iron solution, 

 and the precipitation repeated, the separation of the radio- 

 thorium from the thorium X is more complete ; but some of 

 the thorium X is precipitated at the same time. Care has to 

 be taken to use pure chemicals and good distilled water, since 

 all solid and non-volatile impurities remain with the thorium 

 X. The presence of such impurities renders it difficult to 

 obtain the very thin layer which is required for the experiment. 



After drivino; off the ammonia salts, the thorium X was 



