54 



The recent determination by Cooksey of the want of 

 symmetry between the emergence and incidence radiations 

 due to X-rays is the last experiment required to show that 

 all the properties in the summary of § II. are true for X-rays 

 as well as for y rays, mutatis mutandis. All the properties 

 except the first have been already shown to be true (see our 

 first paper on this subject). The complete parallelism be- 

 tween X- and y rays stands out more strongly than ever. 



In conclusion, there is one aspect of the problem which 

 seems to invite a little further consideration. The charac- 

 teristics of the secondary )8 ray are independent, as we have 

 seen, of the nature of the atom in which it arises, and de- 

 pend only on the nature of the y ray to which it is due. 

 This is all the more remarkable when we consider that the 

 characteristics of primary ^ rays are peculiarly dependent 

 on the nature of the atoms whence they emerge, and are ab- 

 solutely independent of physical agencies acting from with- 

 out. In the one case, that of secondary ^ radiation, we can 

 determine that a given material shall emit jS rays of definite 

 speed and direction, and can carry out our determination by 

 the use of suitable agencies and dispositions. In the other 

 case, that of primary $ radiation, the whole process is com- 

 pletely beyond our control. It is one example of this con- 

 trast that the radio-active substances do not emit secondary 

 radiations to an abnormal extent. It is clear that there is 

 a sharp line of distinction between the emission of an elec- 

 tron from an atom as a primary )8 ray, and the emergence 

 of an electron from an atom as a secondary ^ ray. On our 

 hypothesis the origin of the distinction is simply that in the 

 former case the electron was part of the atom which ejected 

 it ; in the latter case, it was no part of the atom : it came 

 in with the exciting ray. All the experimental evidence ac- 

 cords with this view. We come very close to the complete 

 realization of an anticipation made twelve months ago (Trans. 

 Roy. Soc. of S.A., May 7, 1907, pp. 84, 85), "All secondary 

 radiation, other than the 5 rays, seems to be in general a 

 rough reflection or scattering of the primary . . . The 

 only cases in which a secondary radiation appears, that is 

 neither S radiation nor reflected primary rays, are those in 

 which 13 rays are produced at the impact of X- or y rays, and 

 in which X-rays are produced by cathode rays. ... It 

 may well be that further research will bring these cases into 

 better agreement with the rest." On the neutral-pair hypo- 

 thesis the exceptions mentioned here practically disappear. 

 There remains a broad generalization, which, with all the 

 faults natural to its kind, seems to us to be applicable to 

 every case of which ^e have knowledge, and to be an import- 

 ant principle of the tiieory of secondary radiation. 



