NOVEMBEE 10, 1916] 



SCIENCE 



657 



rays are neither atoms nor molecules, but 

 are negatively charged particles, or cor- 

 puscles as he called them, much smaller 

 than the atom. These experiments showed 

 conclusively that the atom has a complex 

 structure. Thomson measured the amount 

 the particles, or electrons as they are now 

 commonly called, are deflected by magnetic 

 and electric fields of known intensities, and 

 in this way determined the velocity of the 

 particles and the ratio e/m, where e repre- 

 sents the charge and m the mass of a par- 

 ticle. This ratio has been determined by 

 different experimenters, and has been found 

 to be about 1.77 X 10 7 . 



In 1897 Zeeman observed that the lines 

 in the spectrum are separated into two or 

 more lines when the source of the light is 

 subjected to the influence of a strong mag- 

 netic field. Zeeman and later Lorentz 

 showed that such an effect would be pro- 

 duced if the lines in the spectrum are due 

 to the vibration of electrons. They showed 

 that the displacement would be propor- 

 tional to the ratio e/m. Then from the 

 amount of displacement the value of e/m 

 was calculated and found to be approxi- 

 mately equal to the value determined by 

 Thomson and others. The Zeeman effect 

 has confirmed in a remarkable manner the 

 existence of electrons, and proved them to 

 be common constituents of all atoms. 

 Various other phenomena show the exist- 

 ence of electrons, but it is unnecessary to 

 consider all of these evidences. 



Eegardless of the soiirce of the electron, 

 the ratio e/m is constant, and is much 

 larger than the corresponding ratio for the 

 hydrogen ion. The latter ratio is 9,649, or 

 almost 10 4 . From the values of these two 

 ratios, it is evident that the charge of the 

 electron is much larger, or its mass is much 

 smaller, than that of the hydrogen ion. 

 Thomson and others have determined the 

 magnitude of the charge of the electron 

 and found it to be of the same order of 



magnitude as that of the hydrogen ion. 

 The value generally accepted for this 

 charge is 4.7 X 10" 10 electrostatic units, or 

 1.59 X 10" 20 electromagnetic units. If the 

 charge of the electron is equal to that of 

 the hydrogen ion, it is evident from the 

 ratios given that the mass of the electron 

 is only about 1/1800 that of the hydrogen 

 ion or atom. 



It occurred to Thomson that the exceed- 

 ingly small mass of the electron might be 

 entirely electrical in its nature. He had 

 previously shown that a moving electric 

 charge has a certain amount of mass which 

 is independent of any matter with which 

 the charge may be associated. He calcu- 

 lated that the mass due to a moving charge 

 is equal to % e 2 /a, where e represents the 

 charge and a the radius of the sphere of 

 action of the electron. The work of Thom- 

 son indicated clearly that the mass of the 

 electron is purely electromagnetic in its 

 nature, but it was difficult to show this con- 

 clusively with the particles of the cathode 

 rays. Thomson calculated the relation of 

 the mass to the velocity, and showed that 

 the mass would increase rapidly as the ve- 

 locity approached the velocity of light. 

 The phenomena of radioactivity soon made 

 it possible to test this theory. 



Since the discovery of radioactivity by 

 Becquerel in 1896, and the actual separa- 

 tion of radium from its ores by Mme. Curie, 

 many eminent chemists and physicists have 

 contributed to the investigations on radio- 

 activity, and various radioactive substances 

 have been discovered and isolated. In 

 every case it has been found that radioac- 

 tivity is caused by a spontaneous disinte- 

 gration of the atom, a sort of an atomic ex- 

 plosion accompanied by an unparalleled 

 evolution of energy. 



Three types of radiations are given off 

 by radioactive substances, the alpha rays, 

 beta rays and gamma rays. The gamma 



