184 Britain's Heritage of Science 



at a perfectly definite rate, and the life of any radio-active 

 substance can, therefore, be foretold. The ejected particles 

 consist either of one or more negative electrons (/3 particles), 

 or positively charged corpuscles (a particles); frequently 

 both are emitted. The a particle carries twice the charge 

 of an electron, and weighs about twice as much as an 

 atom of hydrogen : that is to say, as much as a helium atom. 

 Rutherford formed the idea that the two might be identical 

 and this was experimentally confirmed by Sir William Ramsay. 

 The emanation of radium which emits an a particle in its 

 decay was introduced into, and kept in an exhausted tube for 

 several days, when it was found that the spectrum line of 

 helium could be clearly seen, though no helium had originally 

 been present. This experiment, which gave the proof of 

 Rutherford's surmise, was an historical event, as it supplied 

 the first definite example of the decomposition of a so-called 

 chemical element. For the emanation possesses all the 

 characteristics of such an element and was shown to decom- 

 pose spontaneously, helium being one of the products. 

 The subsequent development of radio-active experiments 

 and theories confirmed the original ideas, and many new 

 and interesting facts were brought to light. 1 These must be 

 passed over, and we might here close our account, were it 

 not for the brilliant researches of a young man, who promised 

 to become one of the great investigators of his time. 



Henry Moseley (1887-1915) was the grandson of Canon 

 Moseley, a distinguished mathematical physicist, and the 

 son of Professor H. N. Moseley, at one time Linacre Pro- 

 fessor of Zoology at Oxford. He took his degree at Oxford, 

 but received his scientific training mainly from Rutherford 

 at Manchester. After Laue, at Munich, had proved the 

 existence of a diffraction effect of crystals on X-rays, and 

 Professor William Henry Bragg had developed and improved 

 the methods of observation, Moseley set himself the task of 

 determining the fundamental vibrations of the atoms which 

 give rise to the X-rays. The research required exceptional 

 experimental skill, and great powers of devising new methods 



1 For a detailed account of these investigations see Rutherford, 

 " Radio-activity." 



