SUMMARY AND GENERAL DISCUSSION OF RESULTS. 89 



between ions and molecules. It therefore can not be said, in general, that for 

 solutions showing characteristic bands there is any part of the spectrum due 

 to ions or any part due to molecules. Yet the work of Jones and Anderson 1 

 showed that certain bands are probabfy due to certain constituents of the 

 molecules, such as the atom, the hydrate, etc. 



A THEORY OF ABSORPTION SPECTRA. 



It is natural to try to correlate as many spectra as possible, and this has 

 been done in many cases. For organic compounds, benzene for example, it is 

 found that the ultra-violet absorption spectra in the gaseous, in the liquid, 

 and in the dissolved states are very similar. It is therefore natural to endeavor 

 to test colored salts in the same way. Unfortunately no metallic salts having 

 a characteristic absorption spectrum can be obtained in the gaseous state. 

 As pure salts, in solutions, and in transparent solids, however, the absorption 

 spectra of salts of the same element are very much alike as far as the grosser 

 structure is concerned. None of the vapors of the metals of these salts can 

 be obtained unaccompanied by the effects of high temperature and intense 

 ionization. It is for this reason that the branch of spectroscopy dealing with 

 the absorption spectra of inorganic solutions is quite completely separated 

 from the other branches of spectroscopy. 



However, a study of other branches of spectroscopy will be of considerable 

 importance in helping to give us a working hypothesis concerning the mate- 

 rial centers that are absorbing or emitting light. 



An emission or absorption center of light and heat will be denned as the 

 smallest particle existing by itself that is capable of emitting or absorbing a 

 given characteristic spectrum. Until proved to be an ion, an atom, a molecule 

 or an aggregate of these we can not assume that the division of matter into 

 emission and absorption centers is at all identical with the division of matter 

 into ions, atoms, and molecules. In a study of these light centers one of the 

 most difficult problems that confronts us is the complete separation of indi- 

 vidual light centers, and it is owing to this fact that our knowledge of these 

 light centers is so meager. An example of this kind might be taken in the 

 more or less independent series of lines as classified by Kayser and Runge, 

 Ritz, and others. It has never been proved that there are separate emitters 

 and absorbers for each one of these series of lines, neither can it be said that 

 these centers correspond to atomic, ionic, or molecular units. It may be that 

 each series of lines is due to a system, and that several systems are joined 

 together to form an absorption or emission center, but that these systems 

 can not exist as separate units and still act as light centers. The same con- 

 dition probably applies to the systems that give rise to the simple series of 

 fluorescent bands discovered by Wood, so that there would not be a different 

 absorption center for each one of these series of bands, but that there is one 

 type of absorbing centers that contains several absorbing systems that can be 

 separately excited, and that as soon as the absorbing center is broken into 

 parts it loses its characteristic absorption spectrum. 



It seems quite probable that many of the systems in the light centers 

 contain electrons, and it is probable that it is these electrons that give the 



1 Publication No. 110, Carnegie Institution of Washington. 



