ELECTRICAL STRUCTURE OF MATTER RUTHERFORD 165 



the common unit in the structure of atoms which the periodic va- 

 riation of the chemical pioperties had indicated. It gave for the 

 first time some hope of the success of an attack on that most funda- 

 mental of all problems — the detailed structure of the atom. In 

 the early development of this subject science owes much to the work 

 of Sir J. J. Thomson, both for the boldness of his ideas and for 

 his ingenuity in developing methods for estimating the number of 

 electrons in the atom, and of probing its structure. He early took 

 the view that the atom must be an electrical structure, held together 

 by electrical forces, and showed in a general way lines of possible 

 explanation of the variation of physical and chemical properties 

 of the elements, exemplified in the periodic law. 



In the meantime our whole conception of the atom and of the 

 magnitude of the forces which held it together was revolutionized 

 by the study of radioactivity. The discovery of radium was a great 

 step in advance, for it provided the experimenter with powerful 

 sources of radiation specially suitable for examining the nature of 

 the characteristic radiations which are emitted by the radioactive 

 bodies in general. It was soon shown that the atoms of radioactive 

 matter were undergoing spontaneous transformation, and that the 

 characteristic radiations emitted, viz, the a, % and y rays, were an 

 accompaniment and consequence of these atomic explosions. The 

 wonderful succession of changes that occur in uranium, more than 

 30 in number, was soon disclosed and simply interpreted on the 

 transformation theory. The radioactive elements provide us for 

 the first time with a glimpse into Nature's laboratory and allow us 

 to watch and study but not control the changes that have their ori- 

 gin in the heart of the radioactive atoms. These atomic explo- 

 sions involve energies which are gigantic compared with those in- 

 volved in any ordinary physical or chemical process. In the ma- 

 jority of cases an a particle is expelled at high speed, but in others 

 a swift electron is ejected often accompanied by a y ray, which is 

 a very penetrating X ray of high frequency. The proof that the a 

 particle is a charged helium atom for the first time disclosed the im- 

 portance of helium as one of the units in the structure of the radio- 

 active atoms, and probably also in that of the atoms of most of the 

 ordinary elements. Not only then have the radioactive elements had 

 the greatest direct influence on natural philosophy, but in subsidiary 

 ways they have provided us with experimental methods of almost 

 equal importance. The use of a particles as projectiles with which 

 to explore the interior of the atom has definitely exhibited its nu- 

 clear structure, has led to artificial disintegration of certain light 

 atoms, and promises to yield more information yet as to the actual 

 structure of the nucleus itself. 



