128 Dr. Hans Schrader on the Existence of Ch 



lemica 



along a constant resistance R (1*48 ohm) connected in series 

 with the wire. The resistance of the wire was determined 

 at 0°, 100°, and 443°. From these data the temperature o£ 

 the wire was calculated according to Calendar's formula. 

 The melting-point of some particles of pure sodium sulphate, 

 which were heated on the wire in vacuo, was observed by 

 means of a microscope, and found to be 895°. This was 

 considered a sufficient test for the accuracy of measurements 

 at high temperatures. 



Before rendering the wire active, it was heated in vacuo 

 for several minutes at 960°. It was then exposed to the 

 emanation from an actinium preparation for 2 minutes. 

 After 20 minutes, which is sufficient for actinium B, C, and 

 D to obtain transient radioactive equilibrium, the activity of 

 the wire was carefully determined in an a-ray electroscope. 

 The amount of the activity was usually about 80 divisions 

 per minute. 



The wire was then inserted in the vessel AB, and heated 

 at a constant temperature for several minutes at a very low 

 pressure. The amount of active material volatilized was 

 given by the difference in activities of the wire before and 

 after heating, a correction being made for the decay of 

 activity during the time which had elapsed between the two 

 measurements. 



in vacuo. — The following are 



Volatilization of Actinium B 

 the results of experiments on the volatility of actinium 

 B at different temperatures, at a pressure of '003 mm. 

 of air. The duration of heating was four minutes in all 

 these experiments. 



Temperature. 



Per cent, of actinium B volatilized. 



600° 

 700° 

 800° 

 900° 



Volatilization commences. 

 10 to 20 

 30 to 50 

 90 to 98 



Complete volatilization can only be effected at con- 

 siderably higher temperatures. While the wire was being 

 heated, the pressure in the vessel increased somewhat owing 

 to the presence of the gas given off by the platinum. 

 Special experiments showed, however, that this increase of 

 pressure caused only a slight diminution in the amount 

 volatilized at any temperature. 



Pressures greater than 0*1 mm. were not used, since then 



