RADIOACTIVITY 45 



that of an electron which as a cathode ray is without its 

 positive, which picks it up at the anticathode and becomes 

 an X ray and which may drop it again in passing through 

 some atom. I venture to think that from this point of view 

 the phenomena of the X and 7 rays are very simple and 

 easily grasped ; and the hypothesis has been a good guide to 

 experiment. Whatever be the outcome, this hypothesis will 

 be found, I think, to contain truth. 



These paragraphs are in a sense a digression from my main 

 procedure, which was to show that 7 and X rays illustrated 

 the penetrability of matter. But it was not possible to show 

 the force of this until something had been said of the properties 

 of these rays. When we realise that so many things point to 

 a corpuscular form, whether the corpuscle be "material" or 

 not, it becomes singularly interesting that they can proceed 

 in straight lines through such great quantities of matter. Let 

 us suppose, if we like, that atoms contain electrons only, each 

 having a radius of io~ 13 cm., the value usually assigned; we 

 shall find if we make the calculation that a 7 ray will go directly 

 through many electrons in crossing an inch of steel. Then 

 what becomes of the idea of the impenetrability of matter? 

 It has indeed been slowly vanishing away. At one time atoms 

 were impenetrable; we put into our visions of them ideas of 

 contact and surface blows borrowed from our observations 

 of colliding balls. Then radioactivity came and the very first 

 experiments were enough to break down the crude idea. As 

 we go further we see no reason for a halt at any particular 

 point and we incline to the belief that there is nothing in the 

 universe with which we are acquainted which possesses the 

 sole right or power to occupy any particular portion of space 

 to the exclusion of everything else. Relative speed and enough 

 of it is all that is wanted to make penetration possible to the 

 last degree. 



