the Corpuscular Hypothesis of the y and X Rays. 391 



called a new ray. We may call it a secondary ray if we 

 please, but we may just as well say that every molecule of a 

 gas is a primary molecule before a collision, a secondary 

 afterwards, a tertiary after two collisions, and so on ; and it 

 would be worse than useless to do so. Again, when Geiger 

 shows that an a particle maybe deflected or scattered he does 

 not speak of a secondary a. ray. When an X ray entity is 

 transformed by an atom's action into a cathode ray, or a 

 7 ray into a /3 ray, we may speak of the new rays as secondary 

 rays, and now the term is really convenient ; but it must not 

 be taken to mean too much. There is a change of form of 

 the entity, and that is all. When an X ray entity is deflected 

 in passing through an atom, or is " scattered " in the usual 

 phrase, the term secondary radiation is really inappropriate, 

 because it is but the X ray entity swinging off in a new 

 direction. Barkla has shown that when primary X radiation 

 falls upon any metal (from Or to Ag at least), so long as the 

 penetrating powers of the primary exceed a certain limit 

 peculiar to that metal, a homogeneous X radiation is emitted 

 which is characteristic of the metal, and is less penetrating 

 than the primary. Here the term secondary would seem to 

 have a real meaning, for we wish to describe the fact that 

 a primary X ray entity possessing energy of any amount 

 above a certain minimum is replaced by a secondary X ray 

 entity possessing an energy characteristic of the particular 

 metal, and always less than that of the primary. The effect 

 is simple enough to be described in this way, for energy con- 

 siderations show that it can only be a case of one entity 

 replacing another, not of two or three replacing one, nor of 

 one being added to the original. It is not clear, however, 

 that a transformation of this kind actually occurs, a trans- 

 formation, that is to say, which makes the primary differ so 

 much from the secondary that a real difference is to be re- 

 cognized by the use of different terms. I hope to be able to 

 show later that there are good grounds for presuming a 

 double transformation, the first stage being a conversion of 

 the primary X ray into a cathode ray stage, during which a 

 loss of energy occurs, and the second a reconversion into the 

 X-ray form. In any case it is enough for the present that 

 the secondary must draw its energy from the primary, and 

 the appearance of the former implies the disappearance 

 of the latter. 



There is another case which must be considered specially. 

 McClelland * has explained certain of his experiments on the 



* Troc. Roy. Soc. lxxx. p. .501 (1908). 



