358 A CENTURY OF SCIENCE 



cubic crystal can be computed at once from its density 

 and molecular weight, and then the wave length of the 

 radiation calculated from the deviation suffered. In this 

 way X-rays are found to have a length less than one 

 thousandth as great as visible light. Further study of 

 this phenomenon, particularly by the two Braggs, father 

 and son, has revealed many of the structural details of 

 more complicated crystals. 



The most significant investigation in the field opened 

 up by Laue's discovery is that undertaken by Moseley 12 

 only a couple of years before he lost his life in the 

 trenches at Gallipoli. Using many different metals as 

 anticathodes in a vacuum tube, he measured the fre- 

 quencies of the characteristic rays emitted. He found 

 that if the elements are arranged in order of increasing 

 atomic weight, the square roots of the characteristic fre- 

 quencies form an arithmetical progression. If to each 

 element is assigned an integer, beginning with one for 

 hydrogen, two for helium, and so on, the square root of 

 the frequency of the characteristic radiation is found to 

 be proportional to this atomic number. Even though 

 Uhler has shown recently that over wide ranges Mose- 

 ley 's law does not hold within the limits of experimental 

 error, there is undoubtedly much significance to be 

 attached to this simple relation. 



Radioactivity. The year following the discovery of 

 X-rays, Becquerel found that a photographic plate 

 is similarly affected by radiations from uranium 

 salts. Two years later the Curies separated from 

 pitchblende the very active elements polonium and 

 radium. Passage of the rays from these substances 

 through electric and magnetic fields revealed the 

 existence of three types. The alpha rays have 

 been shown by Rutherford and his co-workers to be 

 positively charged helium atoms ; the beta rays are very 

 rapidly moving electrons ; and the gamma rays are elec- 

 tromagnetic pulses of the same nature as X-rays but 

 somewhat shorter. In 1902 Rutherford and Soddy 

 advanced the theory of atomic disintegration, according 

 to which the emission of a ray is an indication of the 

 breaking down of the atom to a simpler form. Thus in 

 the radioactive substances there is going on before our 



