208 ANNUAL REPORT SMITPISONIAN INSTITUTION, 1937 



DISCOVERY OF THE ELECTRON 



All through the 1880's and early 1890's a series of most striking and 

 unexpected discoveries followed from investigations of electric arcs, 

 sparivs, and especially the glowing discharges of electricity at high 

 voltages through glass tubes containing various gases at pressures far 

 below atmospheric pressure. The striking color effects, mysterious 

 luminous streamers and entirely bizarre behavior of these discharges 

 made them the most popular, yet most elusive, subject of laboratory 

 research of those days. 



It was these phenomena which led Crookes to postulate the exist- 

 ence of a mysterious "fourth state of matter," different from the soHd, 

 liquid, or gaseous states. (Of course, we now know that Crookes's 

 fourth state is simply the ionized state of matter.) Once, while 

 attempting to photograph the appearance of a discharge at very low 

 gas pressure, Crookes was bothered by the fact that all the photo- 

 graphic plates in the room with his apparatus became fogged, as if 

 light-struck in spite of their opaque wrapping. He avoided the trouble 

 afterwards, however, by keeping his new supply of plates in another 

 room until, one at a tune, they were wanted for use. Thus he solved 

 an experimental difTiculty, and missed making a great discovery. 



At about the same time Rontgen, in Germany, was trying the same 

 experiment, and he too was troubled by the fogging of his photographic 

 plates. But, as the stor}' goes, his laboratory assistant directed his 

 attention to the pecuUar fact that these fogged plates, when developed, 

 showed the image of a bunch of keys which had accidentally been 

 lying on top of the box of plates while the electric discharge experi- 

 ments were in operation. Rontgen immediately looked into this and 

 discovered that the fogging was duo to penetrating radiations pro- 

 duced in the discharge tube where the cathode rays struck the target 

 or anode. Thus by accident were X-ra3'S discovered — that type of 

 accident not uncommon in science when an observant experimenter 

 is at work. 



Wliile on the subject of accidents, I might digress to tell of another 

 accident which did not happen, also in connection with X-rays. 

 For more than 15 years after their discovery, disputes raged as to 

 whether X-rays were radiations, like light but of very short wave 

 length, or electrically neutral particles of small mass and high speed. 

 It was evident that they were not electrically charged, since their 

 paths were unaffected by electric or magnetic fields. The leading 

 advocate of the neutral particle theory was W. H. Bragg. In 1912, at 

 Princeton, O. W. Richardson tried an experiment to see if X-rays 

 could be refracted by a prism. A positive result would support the 

 wave theory of X-rays. People had tried this with X-rays through 

 glass prisms without success, but Richardson had an idea that an 

 iron prism might be more effective. So he passed X-rays for hours 



