62 ROYAL SOCIETY OF CANADA 



8.14 inms., in thickness respectively are given in columns II to VII of 

 Tables XI and XII. 



The curves A, B, C, D, and E, shewn in Figs. 12 and 13 are plotted 

 from the results given in columns I, II, IV, VI and VII of Table XI, 

 and curves D' and E', Fig. 13, from the results given in columns 

 VI and VII of Table XII. 



On comparing the results obtained when there was no metallic 

 covering over the opening at the top of the ionisation chamber with 

 the results when a covering of .0065 mms. of aluminium was used, it is 

 readily seen that the addition of the covering considerably increased 

 the maximum ionisation in the chamber as the /? rays were deflected 

 into it. This effect is also brought out very clearly by the curves 

 A and B in Fig. 12. This increase in ionisation in the chamber due 

 to the thin covering of aluminium was interpreted as being due to the 

 action of secondary radiation. The small thickness of aluminium foil 

 used would only absorb a very small proportion of the primary /? 

 rays, and, consequently, it would be possible for the excited secondary 

 rays to make a contribution to the ionisation 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 ionisation 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 connection with this explanation, that since the values of the 

 ionisation shewn by curve A were obtained with the opening at the top 

 of the chamber uncovered, these undoubtedly represented the ionisa- 

 tion of a somewhat larger body of air than was used in the experi- 

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

 real increase in ionisation produced by the passage of the /? 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 ionisations 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 examina- 

 tion was deferred. This rise in conductivity residting from the pas- 

 sage of ft 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 ft 

 rays than could be compensated for by the excited secondary radia- 

 tions. This result, it will be remembered, was referred to in Section 



