Remarks on Radioactivity. 483 



very penetrating rays can be detected in a few minutes, 

 through 1 cm. of lead, by the increased rate at which the 

 leaves of an electroscope fall together. 



The objection raised by M. Becquerel that the effects ob- 

 served may have been due to secondary rays set up by the 

 ^3 or " cathode " rays striking the metal boundaries is in 

 direct opposition to the data given in my paper. I have 

 there shown that 89 per cent, of the discharge in the electro- 

 scope was due to a rays, since the discharge was diminished 

 by that amount by placing a thin layer of mica '01 cm. 

 thick over the active material. It is well known that such a 

 thickness of mica completely cuts off the a. rays, but allows 

 the passage of the j3 rays through it with but little absorption. 

 The effect in the electroscope, due to the j3 rays and the 

 secondary rays set up by them, was thus only slightly altered 

 by the addition of the mica plate, and therefore could not have 

 l>een initially much more than 11 per cent, of the total. As 

 u matter of fact, 1 showed that the 11 per cent, was hardly 

 affected by an intense magnetic field, and was due chiefly to 

 the very penetrating rays from radium. With the uncovered 

 active specimen the rate of discharge in very intense fields 

 was reduced by 89 per cent, of the original, showing that all 

 the a rays were deviated in passing through the narrow slits. 

 It has been shown in previous papers that, with a thin layer 

 of active material, the ionization due to the j3 rays (including 

 that due to the secondary rays produced by them) is very 

 small compared with that due to the a rays under ordinary 

 experimental conditions. On the other hand, the ^3 rays are 

 photographically very active compared with the a. rays ; and 

 M. Becquerel, in several of his papers, has drawn attention 

 to the marked photographic action of the secondary radiation 

 set up by them. This may have led him to believe that their 

 electrical effect would be equally marked, but such is not 

 the case. 



M. P. Curie in his paper, after giving some experimental 

 results showing that the decay of activity of the radium 

 emanation is unaltered by variations in temperature between 

 •150° C. and -180° C., proceeds as follows :— 



" Pour expliquer les phenomenes de la radioactivity 

 induite et la transmission de l'activite par les courants des 

 gaz, M. Rutherford a admis que le theorium et le radium 

 emettent une emanation radioactive qui provoque la radioac- 

 tivite des corps sur lesquels elle vient se fixer. C'est cette 

 emanation qui entretient l'activite induite dans une enceinte 

 fermee activee. M. Rutherford semble croire a la nature 

 materielle de Pemanation et, dans Pun de ses Memoires les 



