226 



NATURE 



[November 6, 19 19 



RADIUM AND THE ELECTRON. 



Bv Sir Ernest Rutherford, F.R.S. 



"l^THEN we view in perspective the extra- 

 "'' ordinarily rapid progress of physics during 

 the last twenty-five years, we cannot 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 

 importance, but also from the marked stimulus 

 it gave to investigations in several directions. In 

 fact, this discoverv marks the beginning of a new 

 and fruitful epoch in physical science, in which 

 discoveries of fundamental importance have fol- 

 lowed one another in almost unbroken sequence. 



It does not fall within my province to discuss 

 the great advances in our knowledge that have 

 followed the close study of this penetrating type 

 of radiation, but to indicate, I am afraid very 

 inadequately, the progress in two other directions 

 of advance which were opened up by the discovery 

 of X-rays, and have revolutionised 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 phos- 

 phorescence set up in the glass of the discharge 

 tube by the impact of cathode rays, and experi- 

 ments were consequently made by several ob- 

 servers to test whether' substances which phos- 

 phoresced under ordinary 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 pene- 

 trating type of radiation, and thus laid the 

 foundation of the new science of radioactivity, the 

 further development of wliich 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 was 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 of ultra-violet 

 light on metals and copiously from glowing 

 bodies, while they were ejected with high speed 

 spontaneously from the radioactive bodies. 



The interpretation by Lorentz of the Zeeman 

 effect in which the spectrum lines were displaced 

 by placing the source of light in a magnetic field 

 showed that electrons of the same small mass 

 were present in all atoms, and that their vibrations 

 constituted visible light. Sir J. J. Thomson early 

 pointed out the significance of the electron as one 

 of the units of atomic structure and its importance 

 in the mechanism of ionisation in gases, and the 

 rapid growth and acceptance of electronic ideas 

 NO. 2610, VOL. 104] 



owes much to his work and teaching. An im- 

 portant stage in advance was the proof by Kauf- 

 mann that the mass of the electron was entirely 

 electrical in origin. Sir J. J. Thomson had shown 

 in 1881 that a charged particle acquired additional 

 or electrical mass in virtue of its motion. The 

 variation of mass with speed has been shown to 

 be in accord with general theory, but is in best 

 agreement with the formula based on the theory of 

 relativity. It would be of great interest I0 com- 

 pare theory with experiment for the highest at- 

 tainable speed of the electron from radium which 

 is so near to the velocity of light that the varia-. 

 tion of mass with velocity is very rapid. 



The proof that the electron was a disembodied 

 atom of negative electricity was a great step in 

 advance in electrical ideas. Information as to th<; 

 nature of positive electricity is far less precise and 

 definite, for no positive electron, the counterpar-; 

 in mass of the negative electron, has ever beer: 

 observed. In all experiments with positive rays 

 and with radioactive transformations where the 

 processes are very fundamental in character, no 

 positive charge has ever been found associated 

 with a mass less than that of the atom of 

 hydrogen. While it is well to keep an open mind 

 on this fundamental question, the evidence as a 

 whole suggests that there is an essential difference 

 in mass between the carriers of positive and nega- 

 tive electricity. In fact, such a difference seems, 

 to be essential to fit in with our knowledge of the 

 structure of atoms. The nucleus of the lightesl: 

 atom hydrogen may prove to be the positive elec- 

 tron and its much greater mass than that of the 

 negative electron would then be ascribed to the 

 greater concentration of the electrical charge in 

 the former. 



From consideration of the passage of electricity 

 through gases, it had long been surmised that 

 electricity, like matter, was atomic in character. 

 The study of the deflection of the cathode rays 

 and a-rays in magnetic and electric fields- 

 showed that the carriers of each type had all the 

 same charge, and the atomic nature of electricity 

 was implicitly assumed by all workers. Townsend 

 showed that the charge carried by the ions in 

 gases was equal to the charge carried by the 

 hydrogen atom in the electrolysis of water and 

 made the first measurements of this fundamental 

 unit. Other methods of attack were developed 

 by Sir J. J. Thomson and H. A. Wilson, and by 

 a skilful adaptation of methods Millikan was able 

 to demonstrate in a very direct way the unitary 

 nature of electricity and tc measure the value of 

 the unit charge, probably the most important and 

 fundamental constant in physics^ with an accu- 

 racy, it is believed, of one in a thousand. By 

 combining the value of this constant with electro- 

 chemical data, the number of molecules in a cubic 

 centimetre of gas and the mass of the atoms can 

 be deduced with equal accuracy. The convincing 



