Radioactivity of Lead and other Metals. 769 



supposition that no absorption of the rays took place on 

 traversing the walls of the vessel. 



If absorption did occur, and the absorption constant for 

 the cylinder was known, the value 36*3 could be modified 

 accordingly, and the ionization produced by the gamma rays 

 alone within the cylinder be deduced. 



Suppose, for example, that the cylinder was an aluminium 

 one 0*73 mm. in thickness, the absorption from Table III. could 

 be neglected, and 36'3 would represent the ionization pro- 

 duced by the gamma rays in the air which it enclosed. From 

 the results given in Column 5, Table V. the corresponding 

 ionization due to the secondary radiation excited in the 

 aluminium by the gamma rays would amount to 57 per cent, 

 or 36*3 or 20'7, so that the total ionization within the 

 aluminium cylinder due to the gamma rays from the radium 

 and to the secondary rays which they excited, could be 

 represented by (36*3 + 20'7) or 57 would be the estimated 

 reading. 



In an actual experiment with an aluminium cylinder of 

 the dimensions given above, and situated approximately in 

 the position indicated., the reading 62 was obtained as the 

 mean of a number of observations. This difference between 

 the experimental and the calculated values for the ionization 

 is not more than 8 per cent.; and it is not surprising when 

 it is remembered that no special precautions were taken to 

 place the aluminium cylinder exactly in the position occupied 

 by the lead cylinders with which the measurements were 

 made upon which the present calculations are based. It is 

 possible that the aluminium cylinder may have been as much 

 as a centimetre out from the position it was supposed to 

 occupy during the measurements. From the agreement 

 presented by these measurements, it seems warrantable to 

 conclude that the relation which has been established between 

 the relative amounts of ionization produced by primary and 

 secondary radiations within a mass of air confined in lead or 

 • aluminium cylindrical vessels with the dimensions described 

 above, is a reliable one. 



III. On the Character of the Radiation from different Metals. 

 From the foregoing discussion it is evident that with the 

 cylinders examined, a definite proportion existed between 

 the ionization produced by the gamma rays and that pro- 

 duced by the secondary rays which they excited. With lead 

 cylinders the amount contributed by the secondary rays was, 

 as we have seen, twice that arising from the passage of the 

 But with aluminium cvlinders the relation 



