142 Mr. V. E. Pound on the Absorption of the 



to the ionization in the chamber greater than the loss incurred 

 by the absorption of the primary rays. Of course it is also 

 possible that the increase in ionization observed could be 

 interpreted as being due to a decrease in velocity impressed 

 upon the primary rays by their passage through the foil. It 

 is to be noted, too, in connexion with this explanation that, 

 since the values of the ionization shown by curve A were 

 obtained with the opening at the top of the chamber un- 

 covered, these undoubtedly represented the ionization of a 

 somewhat larger body of air than was used in the experiments 

 when the opening was covered. It follows, therefore, that 

 the real increase in ionization produced by the passage of the 

 yS rays through the single sheet of aluminium should have 

 been greater than that indicated by the curves A and B 

 of fig. 12. Some measurements were made with screens of 

 two and of three sheets of aluminium ; and as these were 

 found to give maximum ionizations approximately the same 

 as that obtained with a single sheet, it was seen that, in 

 order to investigate more fully this rise in conductivity, 

 it would be necessary to use still thinner sheets of aluminium 

 than the one with which the opening was first closed. As 

 this point was not specially pertinent to the subject under 

 investigation by the writer, its examination was deferred. 

 This rise in conductivity resulting from the passage of /3 rays 

 through a thin layer of aluminium was not observed in the 

 experiments with lead and tin screens, doubtless because the 

 least thicknesses of these metals absorbed more of the primary 

 /3 rays than could be compensated for by the excited secondary 

 radiations. This result, it will be remembered, was referred 

 to in Section III. A, and was given as a reason for drawing 

 the curve shown in fig. 6 with an additional rise, although no 

 determinations were made with which it could be confirmed. 



Curve C shows that while the more deflectable of the 

 /3 radiations were absorbed by 1*184 mm. of aluminium-foil, 

 the more penetrating still passed through it. The slight rise 

 in curve D also indicates that some of the ft radiation was 

 still able to penetrate 4*73 mm. of aluminium. With a 

 thickness of 8*14 mm. of aluminium, however, no rise in the 

 conductivity occurred, and, as curve E (fig. 13) shows, this 

 thickness was sufficient to cut off all the /3-ray effect. 



It will be seen that the curves which are drawn on a large 

 scale for deflexions of /3 rays downwards, and for deflexions of 

 these rays upwards, corresponding to a thickness of 8' 14 mms. 

 of aluminium over the opening at the top of the chamber and 

 denoted by E and E', do not coincide. It will be recalled, 

 further, that the curves drawn for the limiting thicknesses 



