144 CONCEPTION OF THE CHEMICAL ELEMENT 



atom. I think it important to keep the two matters 

 distinct. Our knowledge of electricity, which in its 

 modern phase may be considered to start from the 

 relatively recent discovery of the electron, is still far 

 too imperfect to enable any complete theory of 

 atomic structure to be formulated. My task would 

 be incomplete, however, if I did not refer briefly to 

 the nuclear atom of Sir Ernest Rutherford, which 

 may be regarded as the logical descendant of the 

 earlier electronic atom of Sir J. J. Thomson. The 

 weakness of the latter was that it took account 

 essentially only of the negative electrons, and its 

 attempt to ascribe the whole mass of the atom to 

 these nearly massless particles involved the supposi- 

 tion that a single atom may contain hundreds of 

 thousands of electrons. The actual number is now 

 known to be rather less, as an average, than half 

 the numerical value of the atomic weight. Although 

 unsatisfactory in accounting for the mass of the 

 atom on an electronic basis, it was much more in 

 line with present views in accounting for chemical 

 character and the arrangement of elements in the 

 periodic table. The root idea that the successive 

 elements in the table are distinguished by the 

 increment of one electron in the outermost electronic 

 ring, followed, as period succeeds period, by the 

 completion of this ring and the formation of a new 

 external one, so that members of the same chemical 

 family have similar external ring systems, is still the 

 most probable view yet advanced. In conjunction 

 with the conception of the nucleus and the gradual 

 unravelling of the various series of characteristic 

 X-radiations, both experimentally and by mathe- 

 matical analysis, it bids fair soon to give a definite 

 concrete picture of the structure of all the different 

 elements (compare L. Vegard, Phil. Mag., 1918, [vi.], 

 35, 293). 



