30 FORESHADOWING OF THE ELECTRON [CH. iv. 



an opaque solid such as a sheet of metal, provided 

 it is thin enough and not too massive, it will be 

 penetrated by the rays; and phosphorescent effects 

 will be produced on the other side of it. The rays 

 can also affect photographic plates, and indeed do 

 nearly all the things, though on a smaller scale and 

 with much less penetrating power, that the later 

 discovered Rontgen rays can do. 



The Lenard rays are clearly cathode rays emerged 

 from the tube ; and it was the custom, at the date of 

 their discovery, to think of them as flying charged 

 particles of matter ; though the extraordinary dis- 

 tance they could travel through common air, a 

 distance comparable to an inch, was a manifest 

 objection to such a hypothesis, seeing that things as 

 big as atoms of matter cannot travel so much as xeVfy 

 of an inch in ordinary air without many collisions. 



Lenard accordingly adhered to the view, advanced 

 first by Goldstein, that they were not material but 

 ethereal ; and although, in the sense he probably 

 intended, this is not a tenable view for they are 

 not Qthereal waves or anything of the nature of 

 radiation yet, as we shall see, neither are they 

 ordinary material particles, any more than the 

 cathode rays are. But that is just the point which 

 we are now considering, and we will return to them 

 as observed by Crookes in 1879.* 



Nature of the Cathode Rays. 



We have seen that the impact of the cathode rays, 

 speaking in language appropriate to the assumption 



* The biographical history of this subject is set out largely in the 

 contemporary letters that passed between Crookes and Stokes : these 

 have been supplied by Sir William Crookes, and will shortly be 

 published in the Scientific Correspondence of Sir George Stokes. 



