128 THE ROYAL SOCIETY OF CANADA 



we take this to be the case the average energy in a secondary j8-ray 

 is 30 per cent, of that of a primary /S-ray. The reason that the 

 average energy in a secondary /3-ray is less than that of a primary ray 

 is because a /S-ray loses energy before it has a chance of exciting 

 X-rays and further, when this takes place, the whole of the energy of 

 the /3-ray may not always be given up. 



These results alone prove that secondary /3-rays, at least after 

 their ejection from the parent atoms, can play very little part in the 

 production of the secondary X-rays (see the next paper in these 

 Transactions) which are always formed in any substance struck by 

 any beam of X-rays (the primary X-rays). We know that X-rays of 

 frequency n eject secondary /3-rays of energy hn where h is Planck's 

 constant, and that /S-rays of energy E may excite X-rays of frequency 

 E/h but not of higher frequency. Suppose we now have a beam of 

 primary X-rays of frequency n. This beam will eject ;8-rays of 

 energy hn. These /3-rays will excite secondary X-rays of average 

 frequency much less than n, because, according to the result obtained 

 above, the /3-rays, excited in turn by such secondary X-rays will 

 have an energy much less than hn. When we come to examine the 

 secondary X-rays we find that their frequency, although lower than 

 that of the primary, is of the same order. Consequently the per- 

 centage of them produced by secondary /3-rays must be negligible. 

 Ultimately any beam of X-rays will be transformed into ,S-rays. 

 The /3-rays, as we have seen, give rise to X-rays of smaller frequency. 

 These X-rays in their turn eject /3-rays, which will excite X-rays of 

 still smaller frequency, and so on. The reasons for this have been 

 referred to above. 



It should be explained that most of the results in this paper were 

 obtained in the last week that the writer spent in 1912 in the Physical 

 Laboratories of the University of Manchester, and his thanks are due 

 to Sir Ernest Rutherford for the use of the active material employed. 



