INTRODUCTION. 9 



ATOMIC STRUCTURE AND SPECTRA. 



The greatest interest, however, lies in the relation between chemical 

 constitution and absorption or emission spectra. The relation between 

 the flame, spark, or arc spectra and the chemistry of the emitters is not 

 known. The source of spectra, like that from the blue cone of a Bunsen 

 burner, the Swan spectra, is at present unknown. It is very probable, 

 however, that chemical reactions play an important role in the emission 

 or absorption of light and especially of band-spectra. We usually think of 

 most spectra like the sodium D lines as coming from the metallic atoms 

 of sodium. Fredenhagen 1 points out that under most conditions oxygen 

 is present. In chlorine, hydrogen, or fluorine flames, calcium, strontium, 

 thallium, sodium, barium, and copper give spectra that are very different 

 from those obtained when oxygen is present. Thallium under these condi- 

 tions does not show the characteristic green line, and the D sodium lines 

 are completely absent. Work upon the absorption of sodium, mercury, 

 potassium, and various other vapors shows that the presence of foreign gases 

 modifies the character of the absorption very much. Many believe that 

 certain series or groups of lines are due to chemical reactions of various 

 kinds. 



Chemical reactions and processes of ionization and recombination are 

 believed to place the atom or molecule in a peculiar condition by means of 

 which it can emit energy to the ether or absorb energy from it. Under 

 ordinary conditions the atom does not seem capable of doing this. In 

 sodium-vapor, for instance, theory indicates that only one in several thou- 

 sand of the sodium atoms are taking part in the absorption of the D lines 

 at any particular instant. The problem as to how energy is transferred 

 to and from matter is then one of the fundamental problems of science. 



Our present theory of the mechanism of absorption and emission of 

 radiations is very simple. Light and heat are electromagnetic radiations, 

 and hence the emitter or absorber must be either an electric charge or a 

 magnetic pole. As free magnetic poles are not known to us and free electric 

 charges are, theory makes the electric charge the origin of all electro- 

 magnetic phenomena. At present no positive electrical charge is known 

 to be associated with portions of matter smaller than the hydrogen atom, 

 whereas negative charges or electrons are known to be associated with 

 charges of about one seventeen-hundredth that of the hydrogen atom. As 

 far as experiment shows, these electrons always have the same properties 

 and the same charge, no matter from what element they may come. It is 

 for these reasons that the electron is made the basis of all electromagnetic 

 theory, and at present there are but few phenomena that can not be ex- 

 plained, if explained at all, by this theory. 



Radiations, and especially light-radiations, have, then, their origin in 

 electric charges. Continuous spectra like those of the metals are due to 

 free electrons in the metals and have little connection with chemical con- 

 stitution. Fine line- and band-spectra apparently are due to different 

 systems of electrons within the atom, and are greatly affected in intensity 



1 Phys. Zeit,, 8, 404, 729 (1907). 



