564 Mr. A. Wood on Spontaneous Ionization of 



the material or a secondary radiation excited by the more pene- 

 trating kind. This latter point was attacked by determining 

 the action o£ a lead screen on the ionization in vessels of 

 different materials. If the radiation from the walls be primary 

 and an intrinsic property of the material, it will be unaffected 

 by an external screen, and so, in the case of a metal possessing 

 this property to any extent, we should expect a lead screen 

 to produce comparatively little effect on the ionization, while 

 in the case of a metal possessing the property to a less 

 extent, or even lacking it altogether, we should expect the 

 same screen to produce a greater effect. Thus the pro- 

 portionate reduction of the ionization due to a screen would, 

 on this view, vary considerably with the material of which the 

 vessel was constructed. On the other hand, if the radiation 

 from the walls be secondary in character, excited by the 

 more penetrating kind, then we should expect both to be cut 

 down in the same proportion, and hence, even if the absolute 

 amount of secondary radiation excited by the penetrating 

 radiation varied with the material, the proportionate reduction 

 due to a given screen would still be the same for all. Thus, 

 if we measure the proportionate reduction of ionization due 

 to any particular screen in vessels of the same dimensions but 

 of different materials, our (results ought to enable us to say 

 whether the radiation coming from the walls is primary or 

 secondary in character. 



The apparatus used was similar to that employed in the 

 first experiments. The vessels were all of the same dimen- 

 sions — 6*25 cms. in diameter and 13 cms. high. The electrode 

 was of the form previously described, and each vessel was 

 fitted to receive it, so that the same electrode and insulation 

 was used throughout the whole series of experiments. As 

 the electrostatic screens for the electroscope and electrode 

 wire were kept in the same positions, the capacity remained 

 the same all through, and the ionization in any one vessel, as 

 measured by the Wilson electroscope, could be directly com- 

 pared with a similar measurement for any other. Instead 

 of determining the sensitiveness of the electroscope at 

 frequent intervals, and finding the potential to which the 

 electrode had been raised, from the deflexion of the gold- 

 leaf, on the assumption that the sensitiveness was uniform 

 over a range of one volt, a modification of the method described 

 on page 561 was used. The potentiometer-wire was inserted 

 in the mercury-cup and the electrode system brought to zero. 

 The wire was then withdrawn and, after twenty minutes had 

 elapsed, the position of the gold-leaf was carefully read. The 

 potentiometer-wire was then once more inserted, and the 

 resistance adjusted until the gold-leaf was brought back to 



