RADIUM AND THE ELECTRON. 
By Sir Eknest Rtjthebfokd, F.R.S. 
When we view in perspective the extraordinarily rapid progress of 
physics during the last 25 years, we can not fail to be impressed 
with the great significance to be attached to the discovery of X-rays 
by Rontgen in 1895, not only from its intrinsic interest and im- 
portance, but also from the marked stimulus it gave to investigations 
in several directions. In fact, this discovery marks the beginning 
of a new and fruitful epoch in physical science, in which discoveries 
of fundamental importance have followed one another in almost un- 
broken sequence. 
It does not fall within my province to discuss the great advances 
in our knowledge that have followed the close study of this pene- 
trating type of radiation, but to indicate, I am afraid very in- 
adequately, the progress in two other directions of advance which 
were opened up by the discovery of X-rays, and have revolutionized 
our ideas of the nature of electricity and the constitution of matter. 
Following Rontgen's discovery, attention was concentrated on two 
aspects of the problem. On the one side it was thought that the 
excitation of the X-rays might be connected with the phosphor- 
escence set up in the glass of the discharge tube by the impact of 
cathode rays, and experiments were consequently made by several 
observers to test whether substances which phosphoresced under ordi- 
nary light emitted a type of penetrating X-rays. By a fortunate 
combination of circumstances, H. Becquerel in 1896 tried the effect 
of a phosphorescent uranium salt, and this led to the discovery of 
the emission of a penetrating type of radiation, and thus laid the 
foundation of the new science of radioactivity, the further develop- 
ment of which has been attended by such momentous consequences. 
On the other side, the problem of the nature and origin of the 
X-rays led to a much closer study of the cathode rays and to the 
definite proof, as Sir William Crookes had long before surmised, 
that the cathode rays consisted of swift charged particles of mass 
small compared with that of the hydrogen atom. It Avas soon shown 
that these corpuscles of small mass or negative electrons, as they are 
now termed, could be set free by a variety of agencies, by the action 
* Eeprinted by permission from Nature, Nov. 6, 1919. 
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