SOifE COXTEMPlW.INy .IDf.lXCI.S l\ I'lnSICS ir 487 



thf profiuiiul plusii-al si^jiiiruMiict.' il oiu-t- st-ciiU'd to posst-ss. Hiil this 

 is not all; wt- must further coiu'ede that even thi- mass of thi- atom - 

 or the onseniMe of masses of the atoms -of an element is not !)>■ any 

 means so distinctive and important a ciiialily of the element as one 

 would expect. Not only may one element have atoms of several 

 different masses, but two distinct elements may have atoms of, so 

 far as we can distinguish, the same mass; arjjon and calcium, selenium 

 and krypton, tin and xenon. Now if an atom of the jjaseous and inert 

 .irjjon may have the same mass as an atom of the metallic and active 

 calciimi, we cannot c\'ade the conclusion that the mass of an atom is, 

 in the terms of logic, an accidental property of the atom rather than 

 .in essential one. There must be some fundamental and essential 

 feature or quality of the atom, which determines its ensemble of 

 physical and chemical prof)erties, and which is not the atomic mass; 

 perhaps this quality determines the atomic mass as well, but cer- 

 tainK- not in so rigorous a manner that one value of atomic mass cor- 

 responds invariably to one set of chemical and physical properties, 

 and vice versa. This fundamental feature of the atom we recognize 

 as the charge upon its nucleus, which, expressed as a multiple of the 

 electron-charge e (of which it must be an integer multiple '•'') is also 

 the number of electrons accompanying the nucleus, and the atomic 

 number of the element. 



This nuclear charge, or (cardinal) electron-number, or (ordinal) 

 atomic number, is the same for all the atoms of a single element, and 

 never the same for two atoms of different elements. It is 50 for all 

 of the eight kinds of atoms of tin, and .54 for all of the nine kinds 

 of atoms of xenon. It is 18 for all atoms of argon and 20 for all 

 atoms of calcium, though some atoms of the one have the same weight 

 (within one part in a thousand, Aston says) as some atoms of the 

 other. It is 26 for all atoms of cobalt and 27 for all atoms of nickel, 

 though most of the atoms of nickel are lighter and a few heavier 

 than the atoms of cobalt. It is the true basis for the ordering of the 

 elements, of which the ordering of the atomic masses is but an im- 

 perfect and distorted (though not a badly distorted) imitation. 



Five observations or assemblages of observations, made in fields of 

 physics separated almost as widely as any five fields could be, sustain 

 this principle; and, combined with its philosophical attractiveness 

 for the idea of arranging the elements in a single procession and 

 attaching consecutive integer numbers to their fundamental qualities 

 is as irresistibly attractive as a scientific idea can be — make it about 



"Otherwise the nuclear charge could not be exactly balanced by the charges of 

 the environing electrons. 



