214 PHYSICAL SCIENCE 



matter and may identify them with Thomson's 

 corpuscles. 



Then came the discovery of radio-activity, 

 throwing a new Hght on the problem of atomic con- 

 stitution. Atoms appeared as complex structures, 

 some of the heaviest of which break down spon- 

 taneously, leaving a new and somewhat lighter 

 element as residue, and ejecting charged helium 

 atoms in the form of a particles and electrons as 

 /5 particles. 



The first detailed picture of a modern atom 

 was drawn by J. J. Thomson, who published in 

 March 1904 a mathematical investigation of the 

 conditions of stability of systems of revolving 

 corpuscles, and thereby deduced in a most re- 

 markable manner many of the properties of the 

 different chemical atoms. He supposed any one 

 atom to consist of a uniform sphere of positive elec- 

 trification, the structure of which is not specified, 

 and of a number of negatively charged corpuscles 

 revolving in orbits within that positive sphere, 

 under the influence of the attraction of the positive 

 electricity and of their own mutual repulsions. 



A similar problem was long ago attacked by 

 Mayer by means of experiment. A number of 

 little magnetised needles were thrust through 

 corks, and were allowed to float on the surface of 

 water with their axes vertical. The similar poles of 

 all the magnets were directed upwards, and thus the 

 resultant force between the magnets was a repul- 

 sion. High above the water wa^ placed a powerful 

 bar magnet, with that pole downwards of which 

 the magnetisation was opposite in kind to that of 

 the upward poles of the little floating magnets. 

 This large pole attracted inwards all the little 



