38 JouRXAL OF THE MiTCHELL SOCIETY [September 



The most notable workers in the field of radioactivity have been 

 the Curies and Sir Ernest Rutherford. The Curies examined a 

 great many elements and their compounds to see whether they had 

 the radioactive property of uranium. Thorium was the only ele- 

 ment that was found to show the effect in a degree comparable to 

 that of uranium. They found that all minerals containing uranium 

 or thorium were radioactive but that several specimens of the 

 uranium ore, pitchblende, were several times more active than uranium 

 itself. Hence they concluded that pitchblende contained some un- 

 known substance or substances which were the cause of this activity. 

 Inspired by this idea they sought for them in pitchblende, using 

 chemical methods. In 1898 their efforts were rewarded by the dis- 

 covery of two, which they named polonium and radium, each of 

 which is vastly more radioactive than their common parent uranium. 



The story of the successive steps by which the Curies were finally 

 led to the discovery of these new elements Fleming characterizes "one 

 of the most fascinating chapters in the history of science." 



In 1899 Rutherford showed that the radiation from ui-anium was 

 complex and consisted of rays easily absorbed, which he called alpha 

 (a), and more penetrating rays which he called beta (/?). The 

 beta (/?) rays have been thoroughly studied. While their velocities 

 are different for different sources and in the case of radium, as 

 already noted, the particles have unequal velocities, in general their 

 velocities are greater than the velocities observed for electrons set 

 free in vacuum tubes. The swiftest moving beta (/8) particles have 

 a velocity closely approaching that of light; but that they are elec- 

 trons, similar in all respects to the electrons set free in a vacuum 

 tube save in velocity alone, seems to be indisputable. 



Rutherford made a thorough study of the alpha (a) particles. 

 He showed first that they carried positive charges. Then he deter- 

 mined the — ratio for them and showed that this Avas onlv one-half 

 m 



the — ratio for hydrogen got in electrolysis. Under the assumption 

 of equal charges for the alpha (a) particles and the hydrogen ions, 

 the mass of the alpha (a) particle would be twice that of the 

 hydrogen ion ; but under the assumption of twice the charge, the 

 mass would be four times that of the hydrogen ion. In a classical 

 experiment in wliicli cvciy precaution was taken to eliminate any 



