INTRODUCTION. 15 



series. A series of lines comprises those whose intensity and Zeeman effect 

 vary in the same way when the conditions outside the atom are changed. 

 The work of Wood is important as showing that spectrum lines are due to 

 different systems of vibrators inside the atom. By using monochromatic 

 light of different wave-lengths he has been able to excite different series of 

 lines which constitute altogether the fluorescent spectrum of the element. 



Present theories of the atom usually regard the electrons and other 

 vibrators that are the sources of arc and spark lines as being well within the 

 atom, and as affected by external physical conditions only under very special 

 circumstances. Stark believes that these electrons may rotate in circular 

 orbits, the locus of the centers of these orbits being a closed curve, say a 

 circle. This system will be equivalent to a positive or negative charge 

 according to the sense of rotation of these electrons. These electrons we 

 will call ring electrons. Supposing these systems to be positive charges, it 

 will require electrons to neutralize these charges. Several of these neutral- 

 izing electrons may be in the outer parts of the atom and under certain 

 conditions might be knocked off from the atom. These easily removable 

 electrons will be called "valency " electrons, and can exist under different 

 conditions of "looseness " of connection with the atom. Most of the neu- 

 tralizing electrons will probably lie far within the atom. For instance, we 

 would expect that in the uranium atom the charged helium atoms are 

 neutralized by negative electrons. 



Our theory is that the finer absorption bands of such salts as neo- 

 dymium, erbium, and uranium are due to vibrations of these neutralizing 

 electrons, and that the forces acting upon these are considerably different 

 from those acting on the ring electrons, which, in many cases, give a normal 

 Zeeman effect. It is probable that these neutralizing electrons play the 

 greatest role in the optical properties of bodies, such as the properties 

 determining the index of refraction, the extinction coefficient, etc. 



Usually the equation of motion of such an electron is given by an equa- 

 tion like the following when a light-wave of an electric field E cos pt is pass- 

 ing by it: 



d 2 x dx 

 m-y-; +k-r-+ n 2 x = E cos pt 

 dt 2 at 



where m is the total mass (electromagnetic and material) of the electron, 

 k dxjdt is the damping or frictional term, and n 2 x is the quasi-elastic force. 

 It is an experimental fact, as shown by the above work and the work of 

 other investigators, that k and n 2 are not only functions of the electron 

 and the atom, but that they are also functions of the physical and chemical 

 conditions existing in the neighborhood of the atom. 



On the other hand, the effect on k and n 2 for a ring electron, when 

 external physical and chemical conditions are changed, is very small. It 

 is for this reason, and the probable fact that there are relatively few neu- 

 tralizing electrons, that we believe that much greater progress can be made 

 in determining some of the properties and constitution of various intera- 

 tomic systems of atoms and molecules by the study of the absorption spectra 

 of uranium and neodymium than by a study of the arc- or spark-spectra of 

 the same. 



