THE SMALLEST PARTICLES OF MATTER 13 



nation of uranium radiations, and found that there are two types: 



(1) alpha rays, now known to be helium nuclei, which produce 

 intense ionization but are absorbed in a few centimeters of air; 



(2) beta rays, now known to be high-speed electrons, which pro- 

 duce less ionization, but are more penetrating. The still more 

 penetrating gamma rays, now known to be "hard" or short-wave 

 length x-rays, were discovered by Villard in 1898. Rutherford, 

 then at McGill University in Montreal, found that thorium gives 

 off a material "emanation," which passes through paper but is held 

 back by a thin sheet of mica. When in contact with substances, 

 this "emanation" made them radioactive. Radon, the gaseous 

 emanation from radium, is nowadays collected in tiny tubes which 

 may be inserted into cancerous growths; it loses half of its radio- 

 activity in 3.823 days and emits alpha particles at about b\ million 

 electron-volts. 



Ramsay and Helium 



A few years later (1903) Sir William Ramsay (Nobel prize, 

 1904), who had discovered the inert rare gases of the atmosphere 

 (argon, krypton, xenon, neon, now used in red electric lamps, and 

 helium, identified in the sun by Sir Joseph N. Lockyer in 1868), 

 showed that helium is being continuously formed in radioactive 

 minerals in amounts that can be recognized in a spectrograph, and 

 that it is another product of radioactive transformations. Alpha 

 particles are helium nuclei carrying two positive charges. 



The rare gases of the atmosphere had actually been isolated by the 

 distinguished chemist and physicist Henry Cavendish (1731-1810), who 

 demonstrated the composition of air in 1793 and of water a year later. 

 But he failed to discover why a tiny residue of "nitrogen" could not be 

 oxidized: it consisted of the rare atmospheric gases. Much later the 

 distinguished American chemist, W. F. Hillebrand, 5 in the course of 

 his analysis of the mineral Cleveite, a variety of uraninite rich in 

 uranium oxide, isolated a small quantity of a gas which, from its inert- 

 ness, he assumed to be nitrogen. Spectrographic examination would 

 have proved it to be helium, as Ramsay later found. Actually, Dr. A. 

 P. Hallock, later Professor of Physics at Columbia University, did 

 make a spectrographic examination of Hillebrand's "nitrogen" and 

 observed in it the bright yellow spectral lines of helium. But he, too, 

 was outfaced from his prize by the skepticism then prevailing against 

 "new elements." Dean George B. Pegram, Professor of Physics at 

 Columbia University, informs me that Professor Hallock often spoke 



