﻿84 Dr. 0. Halm on some Properties 



this purpose, a wire coated with the active deposit o£ thorium 

 was used. The ionization-distance curve was determined 

 with and without the mica screen, in the manner explained in 

 the previous paper. From these curves the decrease of 

 range of the a rays in passing through the mica screen could 

 be expressed in terms of the corresponding distance of air. 



The ionization-chamber was the same as used in the case of 

 the active deposit, the two plates being - 5 centim. apart 

 from each other. In order to obtain a fairly narrow cone of 

 rays a great number of brass tubes, each 1*1 centim. high and 

 0'4 centim. in diameter, were placed side by side, connected 

 together, and then placed over the active material. The 

 cones of rays were narrow enough to allow the whole cross- 

 section of the cone of rays for the greatest distance employed 

 to enter the ionization-chamber. 



For the reasons described above, it is not easy to determine 

 with accuracy the range in air of the a particles emitted from 

 the different thorium products. Since, however, the experi- 

 ments in every case w r ere repeated several times with freshly- 

 prepared active material and under different conditions,, the 

 final results obtained are probably close to the truth. 



Ionization Curve of a Radiothorium Preparation in 

 Radioactive Equilibrium. 



The ionization-distance curve was first obtained for a strong 

 radiothorium preparation in radioactive equilibrium. A few 

 drops of a radiothorium solution, activity over 100,000, were 

 placed on a little flat porcelain dish and evaporated to 

 dryness. The layer obtained was fairly thin. 



If the a particles, expelled from each of the five a-ray 

 products present in the radiothorium, had different ranges in 

 air, theoretically one should expect to observe five distinct 

 breaks in the ionization curve. The result obtained is shown 

 in fig. 1 a. 



Only two well-marked changes in the curve are to be 

 observed. The upper part, beginning at about S'6 centims. 

 indicates the presence of thorium C, the range of which had 

 been shown previously to be 8' 6 centims. A second set of 

 a. particles appears to enter the ionization-chamber at a 

 distance slightly less than 6 centims., but the point of change 

 of the curve is not very clearly defined. The distance PQ 

 represents the ionization due to thorium C alone (range 

 S'6 centims.). The distance PR represents the ionization 

 due to the five a-ray products of thorium together, and is 

 seen to be more than ten times as great as PQ. Theoretically 

 PR should be equal to 5PQ, since five successive products 



