ARTICLES 603 



a number of elements of high atomic weight which disintegrate 

 or break apart, and thus led to the conclusion that, under present 

 terrestrial conditions, elements of higher atomic weight cannot 

 exist. Therefore the radio-active elements indicate the upper 

 limit of the periodic system. 



The continuity and upper limit of the system of matter thus 

 revealed, it remained, before the system became complete, to 

 determine how many individual members there are, and how 

 many of the gaps may be due to undiscovered elements. This 

 problem was solved by the discovery of the high-frequency or 

 X-ray spectra, which permitted the assignment of atomic 

 numbers to the chemical elements, ' and thus indicated that 

 between hydrogen (atomic number 1) and uranium (atomic 

 number 92) only five elements — namely, those of atomic num- 

 bers 43, 61, 75, 85, and 87 — have not yet been found. 2 



With this gradual completion of the periodic system there 

 came the realisation, based upon a study of the properties of 

 the elements, that the periods are not of equal length, but 

 become longer? Thisf fact furnished further evidence for the 

 completeness of the system, for it has been shown that no more 

 than six noble gases, from helium to niton inclusive, are possible, 

 and that therefore all the elements of electro potential ± cc are 

 known, and can serve as the terminals of the periods. 4 



The scientific epoch which comes to a close by the recognition 

 of the system of matter, showing that there are ninety-two 

 different types of atoms, interlaps the new epoch which searches 

 for means of determining the constitution and the structure of 

 these ninety-two types of atoms. No matter what the results 

 of this inquiry, the term " element " for the building-stones of 

 the universe will remain, modified, however, by the conception 

 of subatomic structure, and perhaps the assumption of a single 

 " protyle " or " urstoff." The writer may be pardoned if he 

 attempts at this time to take an inventory of the known dis- 

 tribution of these " bricks " — the chemical elements — followed by 

 some thoughts concerning the practical use of such knowledge. 



The painstaking and excellent work of F. W. Clarke 5 has 



1 Moseley, Phil. Mag., 1913, vol. xxvi, pp. 1924 ff. ; de Broglie, Compt. 

 Rend., 1914, vol. clix, pp. 304 ff . ; Higgs, Phil. Mag., 1914, vol. xxviii, p. 139 ; 

 Rydberg, Phil. Mag., 1914, vol. xxviii, p. 144 ; Siegbahn and Friemann, Phil. 

 Mag., 1916, vol. xxxi, pp. 403 ff. 



2 Astrophys. Journ., 1918, vol. xlviii, p. 241 ; Amer. Journ. Pharm., 1918, 

 vol. xc, p. 478. 



3 E.g. Batschinsky, Zeit. Phys. Chem., 1893, vol. xliii, p. 372 ; Werner, 

 Ber., 1905, vol. xxxviii, p. 914 ; Adams, Journ. Amer. Chem. Soc, 1911, vol. 

 xxxiii, p. 648 ; Hackh, Weltwissen, 1915, vol. iii, p. 63 ; Harkins, Journ. 

 Amer. Chem. Soc, 1916, vol. xxxviii, p. 169. 



4 Amer. Journ. Science, 1918, vol. xlvi, p. 481. 



5 Clarke, " Data of Geochemistry," U.S. Geolog. Survey, Bull. 616. 



