﻿434 cox. 



that any emanation would be drawn with it into the apparatus. In all 

 eases excepting one, the air leak was perfectly normal for a period of 

 hovirs, being identical with that produced by the natural activity of the 

 air. In the first experiment with clay number 32 the conductivity was 

 observed to be nearly double for a period of two hours, when the 

 increase fell away. 



The apparatus was sufficiently sensitive as is shown by test experi- 

 ments with a sample of black uranium oxide (Kahlbaum) and one of 

 pitchblende from Joachimthal which contained 26 per cent of uranium. 

 One gram of the oxide was in turn uniformly spread over the bottom 

 of platinum containers of various sizes, from a platinum crucible the 

 bottom of which is 2 centimeters in diameter to a dish 4.5 centimeters 

 in diameter and put immediately in the electroscope jar. The rate of 

 fall in these was from 80 to 107 scale divisions per minute, 4,800 to 

 6,420 per hour, depending on the thinness of the layer of powder. 

 Similarly, the rate of fall for one gram of the pitchblende varied from 

 100 to 203 scale divisions per minute, 8,000 to 12,180 per hour, depend- 

 ing on the spreading. The larger of these rates in each case is about 

 the same as that obtained by Bacon 36 who used a dish of 4 or 5 centi- 

 meters diameter as the container in his experiments. 



These numbers are not necessarily the maximum value because of the 

 unavoidable absorption of the emanation at the surface of the powder, 

 but with reasonably large spreading surface and working under constant 

 conditions, the results are roughly comparable. The greatest activity of 

 pitchblende yet observed is about five or six times that of uranium, and 

 when they are considered with reference to the uranium content, ap- 

 proximately, that is the relation which exists between the above samples. 

 By dissolving a mineral and then setting it aside in a closed vessel, the 

 amount of emanation present reaches a maximum value after a month's 

 interval. This was done with a gram of the pitchblende, the emanation 

 being drawn into the apparatus by sucking air through the solution and 

 a rate of fall of 640 scale divisions or 38,400 per hour was obtained as 

 the average of eight closely agreeing readings under different cbargings. 

 The apparatus therefore was so sensitive that this pitchblende gave a 

 rate of fall of the gold leaf more than 12,000 times as fast as that 

 produced by the leakage through normal air. 



From the work of Elster and Geitel the conclusion may be drawn that 

 no primary radioactivity can be present in the above samples of Philip- 

 pine clays, since they suffered no diminution in eight months. If our 

 clays contained primary radioactive bodies this effect would have been 

 noted since the samples were all investigated in less than eight months 

 from the time they were taken. An extremely small amount of induced 



3,1 Ibid. Gen. Set., Sec. A (1007), 2, 124. 



