298 Cooksey — Corpuscular Rays produced in 



Rontgen rays were able to get through. In order to compare 

 two metals, say lead and zinc, the lead was covered with about 

 •0O± mm of aluminium and clamped to the chamber, " A ". The 

 Rontgen rays were then turned on and the leak balanced as 

 before ; but this time not only was the ionization due to the prim- 

 ary rays eliminated, but also that due to all the secondary Ront-- 

 gen rays which got through the aluminium. After the system 

 was balanced, the aluminium was removed, and the ionization 

 measured which was due to all the copuscular secondary rays 

 which escaped from the lead. Then the zinc was tested in the 

 same way, care being taken to keep the intensity and hardness 

 of the primary rays constant throughout the comparison. This 

 method was pursued with all the metals ; lead was taken as the 

 standard, and all the other metals compared to it separately 

 in order that the intensity and hardness of the primary rays 

 might be the same throughout the comparison. Hardnesses 

 were used as nearly as possible the same as those used in the 

 first experiments. 



It might be well here, by the way of parenthesis, to -give 

 some idea of the hardness of the tube used. The softest rays 

 obtainable were those equal to a hardness of about 0*4 in the 

 arbitrary scale, the vacuum being down to about the point 

 where Rontgen rays begin, the green fluorescence on the walls 

 of the tube just beginning to show. It was found impossible 

 to keep the tube as constant as necessary with so low r a vacuum, 

 the vacuum changing very rapidly at this point ; therefore a 

 hardness of about 0*6 was the softest used, and this could be 

 kept very constant. A thin iron screen was always used when 

 hard rays were wanted, but this does not really make the rays 

 harder, only cutting out the softer pulses in the beam, and 

 making the proportion of hard pulses greater. As well as using 

 the iron screen, the tube was run for a while with a longer 

 spark gap in the regulator and with higher potential from the 

 coil. This did make the rays harder, as the cathode particles 

 producing them traveled faster in the tube. 



To resume : the above comparison of different metals gave 

 only the relative ionization due to the corpuscles which escaped 

 from the metals, but not the number of corpuscles produced 

 in the metal by the primary Rontgen rays. This latter can 

 be roughly calculated as follows : 



The intensity of the primary rays after passing through a 

 thickness x of the metal, if we assume that for thin layers the 

 rays are absorbed exponentially, is I e-% x , where X is the coef- 

 ficient of absorption of the primary rays in the metal. The 

 number of corpuscles produced per cm. of path of the primary 

 rays is proportional to the intensity of the primary. The num- 

 ber produced in a layer of the metal between x and x + dx is pro- 



