Thin Films of Elements exposed to Rontgen Rays. 127 



The ionization chamber is shown in fig. 2. To the bottom 

 o£ the brass cylindrical case a sheet o£ aluminium (a) *75 mm. 

 thick was sealed on with Chatterton compound and fixed 

 with a screwed-on brass ring. The heavy top was ground 



Fig. 2. 



°£fecfras> -ope 



flat for easy removal, the sealing material in this case being 

 a soft mixture of bees'-wax and vaseline. A pure carbon 

 plate (c), about 11 cm. in diameter and 1*5 mm. thick, was 

 mounted flush within a flat brass ring : this ring with its 

 plate rested on three small chairs round the bottom of the 

 case, and when a screen was put in, its legs rested on the 

 brass ring. On the ring there were three small sulphur 

 beads (i), which supported an electrode connected to a 

 Wilson electroscope, the electrode being made of radial 

 carbon filaments. Finally, all the brass parts within the 

 case were painted with a mixture of sugar-water and 

 artificial graphite. Lead diaphragms were arranged so as 

 to keep the X-radiation from falling anywhere but within 

 the prepared surface of the screen. 



The only gas used throughout the experiments was air, 

 and the pressure within the chamber could be varied from a 

 few millimetres up to atmospheric pressure. Before passing 

 into the chamber the air was roughly dried bv bubbling it 



