SOME CONTEMPORARY ADVANCES IN PHYSICS— X 117 



spark and above all Millikan's "Vacuum Spark." To take instances 

 from the work of Millikan and Bowen, Paschen, and F"owler: in the 

 spectra of such discharges lines have been identified which belong to 

 atoms for which Z = 14 and h has the several values 1,2,3,4 (four 

 stages in the building of a silicon atom) ; and to atoms for which 

 Z = 10+& and h has the several values 1,2,3,4,5,6. Many of these 

 spectra of multiply-ionized atoms have not yet been analyzed, but the 

 work is proceeding rapidly. There is reason to hope that within a 

 few years we shall be in possession of interpreted spectra not only of 

 many systems of Z electrons about a nucleus of charge Ze, but also of 

 many systems of fewer than Z electrons about nuclei of charge -\-Ze. 

 This may be highly important, as I will try to show by an illustra- 

 tion. We will consider two consecutive elements of the periodic table; 

 sodium (Z = ll) and magnesium (Z = 12). 



A Mg atom is imagined as 12 electrons around a nucleus of charge 

 + 12 e. It is formed when one electron joins itself to a Mg+ion, 

 which is composed of 11 electrons about a nucleus of charge -^12 e. 

 For this process a spectrum is emitted, the so-called arc spectrum of 

 Mg or "Mgl" spectrum, which is known and analyzed. It shows that 

 the normal state of the Mg atom is an 5-state (probably of total 

 quantum-number 3). It is likewise a singlet-and-triplet-spectrum. 

 The first of these facts is taken to mean that the valence-electron, or 

 twelfth electron (the reader will see the reason for this usage, the electron 

 in question being the last annexed out of the twelve) of the Mg atom 

 moves in a 3i orbit. The second is taken to mean something or other 

 about the residue of the atom, as will be shown in section S. 



This residue of the atom is itself formed when one electron joins 

 itself to a Mg++ion, which is a group of 10 electrons about a nucleus 

 of charge +12 e. In this process the so-called spark-spectrum of Mg, 

 or "Mgll" spectrum, is emitted. It is known and analyzed. It shows 

 that the normal state of the Mg + ion is an 5-state (probably of total 

 quantum-number 3). It is a doublet spectrum. The first of these 

 facts is taken to mean that the valence-electron or eleventh electron of 

 the Mg+ion, moves in a 3i orbit. The second is taken to mean some- 

 thing or other about the residue of the Mg++ion. 



A very interesting question now arises: is the Mg + ion actually the 

 same as the residue of the Mg atom? In other words: when a 12th 

 electron is added to the group of 11 electrons about a nucleus of 

 charge + 12 e, is the group of eleven left unchanged? If so, we have 

 knowledge about this group from two sources. The character ot the 

 Mgl spectrum (the fact that it is a singlet-and-triplet spectrum) 

 teaches something about the group, though what it is is far from 



