384 Mr. P. W. Burbidge on the Absorption of the 



Thus arranged the vessel withstood the strains due either to 

 evacuation or to an internal pressure of 2 atmospheres, with- 

 out developing leaks. 



The main ionization vessel was a short brass cylinder 

 placed so that the rays entered through the aluminium 

 potential plate and fell on the brass electrode connected to 

 the measuring electroscope. The front aluminium plate was 

 *4 mm. thick, to absorb any soft L radiation from the lead 

 used in screening. 



The standard ionization vessel was a long brass cylinder 

 with parallel aluminium plates (14x4 cm.) between which 

 the beam of rays passed. Both ionization vessels were 

 sealed with air inside. The difference in structure between 

 the two vessels resulted in surprisingly good detection of 

 changes in the homogeneity of the X-rays, this being shown 

 by the variation in the ratio of the ionizations in the two 

 vessels. 



The electroscopes used were of the Wilson tilted type, and 

 for greater constancy of readings were enclosed in wooden 

 boxes to minimize convection currents in the leaf-chamber, 

 and had earthing keys consisting of weighted sharp knitting- 

 needles pressing into brass or copper. These keys were both 

 connected to a potentiometer, the middle of which was 

 earthed, and were arranged to lift simultaneously, so as to 

 isolate both measuring systems at once. Corrections were 

 made for natural leak and for insulation leaks. 



(3) Gases. 



The gases used were air, sulphur dioxide, hydrogen, carbon 

 dioxide, sulphuretted hydrogen, oxygen, and methyl iodide. 

 In all cases but the last they were passed slowly through 

 two calcium-chloride tubes (15 x 1*5 cm.). The absorption 

 chamber was evacuated with a water-pump, filled with dried 

 gas, and the operation repeated once at least, generally twice. 

 The pressure of gas in a U-tube mercury manometer gave 

 the proportion present. No rubber connexions were used 

 in standing contact with the gases. 



The sulphur dioxide was obtained from the commercial 

 liquid ; the carbon dioxide from marble and pure hydro- 

 chloric acid solution, acid spray being absorbed in a tube of 

 glass wool moistened with sodium-carbonate solution ; the 

 methyl iodide was evaporated into the evacuated chamber 

 from liquid in an attached air-free bulb, the liquid boiled 

 at 41°*9 and the pressure of vapour in the chamber kept 

 constant after the tube connecting with the liquid was closed. 



