CHAPTER I. 



THE ABSORPTION AND EMISSION CENTERS OF LIGHT 



AND HEAT. 



During the last decade a considerable number of physicists have directed 

 their efforts towards solving some of the many problems of spectroscopy. The 

 following chapter will contain a discussion of some of this work, with the view 

 of recording part of our experimental knowledge concerning the nature of the 

 emission and absorption centers of light; the connection between these centers 

 and molecular and atomic structures; the effect of ionization and recombina- 

 tion on these centers, and the effects and changes that can be produced by 

 physical and chemical agents such as temperature, the presence of a mag- 

 netic field, etc., upon the constitution of the emission and absorption centers. 



Emission and absorption centers will be denned as the smallest particles 

 from which we can obtain characteristic absorption or emission spectra. A 

 further division of the centers of any characteristic spectrum would make it 

 impossible to obtain that spectrum, though the resultant particles may possess 

 characteristic absorption or emission spectra of their own. When emission 

 or absorption centers move with reference to an observer, the frequencies of 

 the spectral lines and bands will show the Doppler effect. 



ATOMIC STRUCTURE AND SPECTRA. 



A very important problem is that of the relation between chemical con- 

 stitution and absorption or emission spectra, although the relation between 

 flame, spark, and arc spectra, and the chemistry of the absorption and emission 

 centers may not be known. Even the source of spectra like that from the blue 

 cone of a bunsen burner, the Swan spectra, is at present a much mooted ques- 

 tion. It is probable that chemical reactions play an important role in emis- 

 sion and absorption spectra, and especially in band spectra. We usually think 

 of most spectrum lines, like Di and D 2 of sodium, as coming from the metallic 

 atoms. Fredenhagen points out that under most conditions oxygen is present. 

 In chlorine, hydrogen, or fluorine flames, calcium, strontium, thallium, sodium, 

 barium, and copper emit spectra that are very different from those obtained 

 when oxygen is present. Under these conditions thallium does not emit the 

 characteristic green line, and the lines Di and D2 are completely absent. Work 

 on the absorption of sodium, mercury, potassium, and various other vapors 

 shows that the presence of foreign gases modifies the character of the absorp- 

 tion very much. 



Chemical reactions and processes of ionization and recombination are 

 believed to place the atom or molecule in a peculiar condition, in which it 

 can emit energy to the ether or absorb energy from it. Under ordinary con- 

 ditions the atom does not seem capable of doing this. In sodium vapor, for 

 instance, according to present theories only one atom in thousands is taking 

 part in absorption at any one time. The problem as to how energy is trans- 

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