Secondary Radiation. 241 



Curve A' to the primary rays with the foil placed close to 

 the ionization-tube ; Curve B to the secondary rays. Curves 

 A and A' show the importance of the part played by the 

 secondary rays, and how they may influence the value cal- 

 culated for the coefficient of absorption. Comparing Curves 

 A' and B. in both of which the foil is placed close to the 

 ionization-tube, the secondary rays appear to be more easily 

 absorbed than the primary; but the conditions of experiments 

 are not quite the same in the two cases, as the secondary rays 

 are not confined to a narrow pencil as in the case of the 

 primary, and therefore the relative position of the absorbing 

 tinfoil is not so important. The difference in penetrating 

 power is probably, therefore, not so great as a comparison of 

 the Curves A' and B would suggest. 



It will be observed that the primary Curves A and A 1 

 become practically horizontal, which is of course due to the 

 presence in the primary pencil of the very penetrating y rays. 

 The secondary curve B, on the other hand, continues to 

 descend with increasing thicknesses of foil; this is because 

 the secondary pencil consists entirely of ft rays. The mag- 

 netic experiments showed that it was possible to deflect all 

 the secondary rays. The experiments, therefore, did not 

 detect any y rays produced by the stoppage of the primary 

 ft particles, or by the starting of the secondary ft particles ; 

 such rays may have existed in small amount and escaped 

 detection, as we must remember that in every case a cor- 

 rection has to be made for the small normal ionization in the 

 tube T. 



The importance of the secondary rays in the above ex- 

 periments suggest a more detailed consideration of the manner 

 in which matter is penetrated by a stream of charged par- 

 ticles like ft rays ; whether, in fact, there is great penetra- 

 tion by the primary particles, or whether it is not a successive 

 stopping of one set and starting of another set of particles. 

 The secondary rays from the above experiments are not very 

 different in penetrating power from the rays that produce 

 them, and the magnetic experiments described below indicate 

 that the secondary rays contain some even more penetrating 

 than the primary. The point is under further consideration. 



Magnetic 1 h'jiexion of the Secondary Rays. 



The nature of the secondary rays has been examined by 

 passing them through a strong magnetic field. The apparatus 

 ( fig. 1, p. 231) was modified by interposing between the plate P 

 and the ionization-vessel T the poles of an electromagnet so 



Rial. May. S. 6. Vol 9. No. 50. Feb. 1905. K 



