within closed vessels due to Rontgen and Gamma Rays. 433 



Ionizations. 



Metal. 



Lead 



Copper .... 



Zinc 



Iron 



Aluminium 

 Cardboard . 



Thickness. 



Density. 



X Rays. 



y Rays. 

 100 



2'1 mm. 



113 



18 



•54 „ 



8-9 



82 



69 



•44 „ 



71 



111 



09 



! 55 „ 



7-8 



100 



68 



•60 „ 



26 



126 



58 







125 



49 



207 

 63 6 

 65 4 

 55-9 

 27-1 



It will at once be seen that with Rontgen rays the heavier 

 metals permit ut' less ionization, within the testing vessel, 

 than do the lighter metals. But with y rays the metals with 

 the higher atomic weight give rise to stronger ionization, 

 within the vessels, than do those with lower atomic weight. 



Experimental Details. 



Three different methods were employed and these gave, 

 broadly speaking, concordant results. 



(1) A very thin-walled electroscope, supported on light 

 pillars, was covered in lurn with cylinders, having the upper 

 end closed, made of the metals named above. After taking 

 the initial reading the electroscope was covered with the 

 inverted vessel and exposed for a known time to the action 

 of X or 7 rays. The vessel was then removed and the fall 

 of potential determined. 



(2) An axial rod was placed in turn within the cylindrical 

 vessels, and this rod was connected to a small electroscope 

 above a lid covering the vessel. 



(3) Inverted cylinders, 30 cm high, 17 cm. in diameter, 

 were placed in turn on the upper platform of the well-known 

 Wul£ electrometer- The long axial rod, about 30 cm., sold 

 with the instrument, was charged initially to 200 volts, and 

 the fall of potential due to the respective rays was measured 

 and the natural leak deducted. 



For the 7 rays 14 mg, of pure radium bromide in a test- 

 tube was placed within a lead cylinder, 6 mm. thick, between 

 the poles of a powerful electromagnet. In this way the 

 /3 rays from the radium were absorbed and the emergent 

 /3 rays due to the 7 rays passing through the lead cylinder 

 were for the most part deflected from the electroscope which 

 was a metre and a half awiiy. 



The results given below to an arbitrary scale, without and 



