630 BELL SYSTEM TECHNICAL JOURNAL 



exploration which in 1911 Rutherford imposed upon them, and of which 

 they reported to him that in the atom there is a massive particle 

 positively charged, like themselves less than IQ-^^ centimeter in diam- 

 eter. They could not perceive the electrons which surround this 

 "nucleus," bearing charges of which the sum compensates its own; 

 but other evidence makes us secure of the existence of this flock, and 

 of the general theorem that the atom of the Nth element of the periodic 

 table consists of N electrons surrounding a nucleus of the tiny dimensions 

 aforesaid, having a charge + Ne and a mass almost the same as the entire 

 mass of the atom. 



This last, then, is the entity which anyone must attack who wishes 

 to transmute the atom. It takes no part (as I remarked above) in 

 chemical phenomena, in the emission of light or of X-rays, in the 

 electrical effects which atoms can achieve when they lose charge and 

 so become ions. This for the wouldbe transmuter is a fact of serious 

 import; for if the nucleus has no influence on these, no more have they 

 on it. Radioactivity, indeed, is a quality of nuclei — radioactivity is 

 transmutation, natural and spontaneous; and it is not affected by 

 anything chemical or electrical, by any temperature or any illumina- 

 tion which has ever been applied to a self-transmuting substance. 

 These kernels of the atoms are well sheltered and highly resistent; they 

 seem as oblivious of the world around them, as the interior of the earth 

 is unconscious of the life upon its surface. 



But the properties of the alpha-particle which enable it to penetrate 

 to the neighborhood of the nucleus — extreme minuteness, high momen- 

 tum, enormous store of concentrated energy — may they not also qual- 

 ify it to impinge directly on the atom-kernel, to invade the nucleus and 

 disrupt it, to shatter it if it be shatterable at all? We may be sure that 

 the nuclei of atoms, hydrogen perhaps excepted, are complex. They 

 cannot be the ultimate and irreducible particles of matter; for radio- 

 activity proves that some of them disintegrate of themselves into 

 smaller and lighter bits, while as for the rest, the facts that the charges 

 of all are multiples of a common charge and the masses of all are nearly 

 multiples of a common mass must surely be taken as meaning that all 

 of them are structures built of electrons and protons. In principle, 

 therefore, they must be breakable, if only they can be struck with 

 sufficient force by hammers of suitable size. Now of all known vehicles 

 of available force, alpha-particles best combine the qualities of small- 

 ness and great energy, and therefore seem the best adapted to the task. 

 Such must have been the ideas of Rutherford ; it may be presumed 

 that he was meditating them during the war, since in the first year 

 thereafter he put them to the test, and so became the first to achieve 



