Air in Closed Vessels and its Causes. 569 



The results for the lead screen may be more briefly 

 tabulated thus : — 



, r l • ] Ionization Ionization Diminution Diminution 



(unscreened), (screened). (absolute), (proportionate). 



Lead '541 '488 '053 10 p. cent. 



Tin -496 "384 -112 22± „ 



Iron -287 -220 '067 23J „ 



Aluminium... -255 '220 -035 13J „ 



Zinc -228 -1(38 -060 26± „ 



From those figures some facts about the cause of the 

 ionization in the vessels may be at once deduced. If part of 

 the ionization were due to the external radiation, and the 

 remainder to an intrinsic radiation from the walls of the vessel, 

 then the first would be cut down by the same amount in each 

 case by the screen, while the second would remain unaltered. 

 Hence the absolute diminution of ionization would be the 

 same for all the materials. We see, however, from the above 

 table that it varies from '035 in the case of aluminium to 

 •112 in the case of tin. Further, as the ionization pro- 

 duced by the external radiation would be the same in all the 

 vessels, the greater the ionization in a vessel the greater 

 would be the proportion of it due to intrinsic radiation, and 

 consequently, the smaller the ionization the greater would be 

 the proportionate diminution due to a screen. We find, 

 however, that in the case of aluminium, a material for which 

 the ionization is small, the proportionate diminution is small 

 also. If, on the other hand, part of the ionization were due 

 to the external radiation and the remainder to a secondary 

 radiation from the walls excited by it, then we should expect 

 both primary and secondary radiation to be diminished in the 

 same ratio by a screen. Therefore, although the amount of 

 secondary radiation excited by the primary might, and 

 probably would, vary with the material, still, whatever its 

 amount, we should expect it to be cut down in the same pro- 

 portion as the primary, and so we should expect the ionization 

 in all the vessels to be diminished in the same ratio by a 

 screen. One glance at our table shows us how far this is 

 from being the case, the proportionate diminution for lead 

 being little more than one third of that for zinc. Our results 

 then point to a threefold origin for the ionization in closed 

 vessels. Part of it is due to the external radiation, part to a 

 secondary radiation from the walls of the vessel excited by 

 it, while the remainder is due to an intrinsic radiation from 

 the walls, altogether independent of the external radiation. 

 Considering only the radiation from the walls, where that 



Pin!. Marj. 8. 6. Vol. 9. No. 52. April 1905. 2 P 



