﻿90 Dr. 0. Halm on tome Properties 



repeated, when the active matter was hermetically covered by 

 a thin sbeet of mica of absorbing power equal to 2*0 centims. 

 of air. Allowing for the absorption of the mica, the maximum 

 range was found to be practically identical with that observed 

 with the unscreened source. 



We may, therefore, conclude that the a rays of thorium X 

 cease to ionize the air at a distance of about 5' 7 centims. 

 Carves b and c, in fig. 3, show the ionization curves 5 and 2$ 

 days later respectively. It is clearly seen that in this case, 

 where the active material was strongly heated, not such a 

 large fraction of the emanation escapes as in the preparation 

 used to determine the curves of fig. 2. The part of the 

 ionization curve due to thorium C is very clearly marked, 

 showing that there is a greater proportion of the active deposit 

 present in the heated thorium X than in the unheated radio- 

 thorium (fig. 2c). As the thorium X decreases in activity, 

 the ionization due to its successive products decreases in like 

 ratio. The very small residual activity, represented in the 

 curve c, fig. 3, shows that the thorium X was nearly free 

 from radiothorium or radium. 



Ionization Range of the a Particles of the Thorium 

 Emanation. 



On account of the strong emanating power of preparations 

 of radiothorium or thorium X, the electrical method cannot 

 be used with advantage to determine accurately the range of 

 the a particles from the emanation. Even when the emanating 

 preparation is completely covered by means of a mica screen, 

 no distinct break in the curve due to the emanation could be 

 observed, indicating that the range of the a particles of the 

 emanation was not very different from those of the other 

 a-ray products with a range below 6 centims. 



The range of the a particles due to the emanation could, 

 however, be measured by observing under special conditions 

 the distance of air through which the a. particles produced 

 scintillations on a sensitive zinc-sulphide screen. 



The general arrangement was the same as that used for the 

 determination by the scintillation method of the maximum 

 range of the a rays of the active deposit of thorium, as 

 described in the first paper on this subject. The active wire 

 was replaced by a shallow metal dish about 2*5 centims. square 

 and a few millims. high. A thin mica screen, which is im- 

 pervious to the emanation, was waxed down on the upper 

 surface of the dish. Two narrow tubes were attached to the 

 metal. A slow current of air, which was passed over a 



