Pressure for Hontgen Rays in different Gases. 37 



absorption undergone by them could be measured. This was 

 necessary in order to correct the results obtained, for the absorp- 

 tion of the primary rays before reaching the electrodes, which in 

 order to avoid secondary rays from the aluminium window were 

 placed some considerable distance within the box. This correction 

 (which varies with the pressure) has been applied to all the results 

 given in this paper, A second similar chamber was filled with air, 

 and used as a standard. 



3. The first experiments were made with the plates 6 cms. 

 apart. These failed to give any indication of any departure from 

 the simple pressure law (except, as stated above, in the case of ethyl 

 bromide at the higher pressures). It was thought that this might 

 be due to the distance between the plates being sufiicient to 

 absorb completely any soft secondary rays emitted by the gas, 

 even at the lowest pressures employed. The apparatus was then 

 altered so that the distance between the plates was only 5 mm. ; 

 the aperture of the primary beam being suitably reduced by 

 means of lead slits to a width of about 2 mm., and the experiment 

 repeated. 



The results obtained connecting pressure and ionization are 

 given in Figures 1 and 2, Fig. 1 giving the results for com- 

 paratively high pressures (up to 200 mm.) and Fig. 2 the results 

 for ethyl bromide and methyl iodide at pressures below 20 mm. of 

 mercury. The upward trend of the ethyl bromide curve is evident 

 from Fig. 1. Apart from this all the curves are sensibly straight 

 lines passing through the origin. There is thus no evidence of 

 any appreciable amount of soft secondary radiation from the gas 

 itself. If gases do emit these soft secondary rays they are either 

 too absorbable to pass through 2^ mm. of gas (the half distance 

 between the electrodes) even at the lowest pressures employed, or 

 else they are too small in amount, compared with the ionization 

 produced by the direct action of the primary rays, to be appreciable. 



The former hypothesis is not probable. By allowing the 

 primary rays to graze one of the aluminium electrodes it was 

 shewn that the soft secondary rays emitted by the aluminium 

 under the action of the Rontgen rays passed through the whole 

 distance between the plates without being completely absorbed 

 by ethyl bromide at a pressure of 20 mm. of mercury. It seems 

 probable therefore that practically the whole of the ionization is 

 produced by the direct action of the primary rays. 



