Passage of ol Particles through Hydrogen. 827 



a microscope M. The source consisted o£ radium emanation 

 in a conical tube with a thin mica window. The tube was 

 not sealed off, but its end dipped into a small vessel of 

 mercury which also extended into the tube, so as to confine 



Fiff. 2. 



To Gauge 

 and Hydrogen Bottle 



v—\ 



the emanation in a volume at atmospheric pressure about 

 four times the volume of the conical portion of the tube. In 

 this way, when the pressure was raised to about four atmo- 

 spheres, the emanation just filled the cone and the a particles 

 were fired through the mica window. The disk D had four 

 openings, three of which were covered with foils of the 

 metal whose absorption was under investigation. A piece 

 of soft iron I was soldered to the disk and nearly counter- 

 poised by a piece of lead L. Thus by the attraction of I, 

 any of the four openings could be brought at will over the 

 zinc-sulphide screen by a cycle of operations of short-cir- 

 cuiting in turn the coils A and B of the electromagnet, the 

 successive positions of I being denoted by 1, 2, 3, 4 in the 

 figure. 



In an experiment the source and disk were first placed in 

 position and the end plates screwed on to the lead joints. 

 The vessel was then swept clear of air by hydrogen from the 

 high-pressure bottle. The valve was then closed and the 

 hydrogen let in gradually until the pressure was about 4'5 

 atmospheres, this pressure being nearly 0'75 of an atmosphere 

 greater than was necessary to just prevent the a particles 

 reaching the zinc-sulphide screen. 



After four hours the emanation had attained equilibrium 

 with its products on the walls of the conical tube, and obser- 

 vations of the scintillations were commenced. Several 

 hundred scintillations were counted in each of the four 

 positions of the disk, i. e. with no foil over the screen and 



