4 o SCIENCE PROGRESS 



is direct or indirect; may it not be due to the fi rays which 

 they produce ? For we know that the /3 rays ionise and we 

 ought not to provide two causes for one result until we are sure 

 that the one already demonstrated is insufficient. We can only 

 answer this question directly by quantitative experiment. The 

 description and discussion of such work would take too long; I 

 can but state my own opinion of the results obtained. So far as 

 it has gone, no reason is shown why we should suppose the 

 7 rays to be able to ionise as well as the (3 rays which they 

 produce : the whole effect is explained without hypothecating a 

 new property. The same is to be said of the X rays ; these 

 produce cathode rays of various speeds in matter on which they 

 fall. It is perhaps well to mention again that /3 rays and cathode 

 rays are both electrons in motion, the only difference being 

 that the former are going (at first) about three times as fast as 

 the latter. Most of the measurements on this point have indeed 

 been made with X rays, which are easier to handle : and the 

 argument is carried over to the 7 rays in accordance with the 

 reliance we have learnt to place on their being exactly analogous. 



Let us consider the way in which 7 rays produce /3 rays or 

 X rays produce cathode rays. In the first place we have the 

 remarkable fact that the speed of the /3 rays depends on the 

 quality of the 7 rays and not at all on the nature of the material 

 in which they arise. It is exactly the same sort of thing as we 

 saw took place when /3 radiation was scattered by material on 

 which it fell ; only we cannot now say that the scattered 

 radiation is merely the incident radiation scattered without 

 change. Very penetrating 7 rays cause fast /3 rays : as we take 

 less penetrating rays of the one kind, we find the speed and 

 penetration of the other diminish also; until we come to the 

 "softest" X rays, when the cathode rays which they produce 

 can only cross one or two tenths of a millimetre of air. But 

 it does not matter what material we employ to bring about 

 the change from one form of ray to the other ; our choice does 

 not effect the speed of the electron, which is the direct result 

 of the change. 



Now for a second point. When the 7 rays pass perpen- 

 dicularly through a thin plate it is found that the /3 rays having 

 their origin in the plate are far more numerous on the side of 

 the plate from which the 7 rays emerge than on the side upon 

 which they fall. It looks as if the 7 rays drove the /3 rays 



