CHAPTER III. 



MAPPING THE ABSORPTION SPECTRA OF VARIOUS 



SALTS IN SOLUTION. 



An accurate knowledge of the absorbing power of solutions is the first 

 requisite in understanding the nature of absorption, and, accordingly, the 

 mapping of the spectrum is the first thing to be done. This has been accom- 

 plished for a large number of solutions, but there are quite a few salts that 

 have thus far been omitted, and it is the purpose of this chapter to give the 

 results with some of these solutions. This has been made possible largely 

 through the kindness of Professor Urbain, who has loaned us the oxides of 

 samarium, dysprosium, and gadolinium. Dr. Guy has converted these oxides 

 into the various salts such as the chloride, nitrate, etc., and has dissolved these 

 in various solvents. The other salts whose absorption spectra have been inves- 

 tigated have been obtained, for the most part, from Kahlbaum. The numbers 

 of plates described in this chapter run from 1 to 34 inclusive. 



Jones and Anderson have shown that the absorption bands of a given 

 salt in any solvent were characteristic of that solvent. For this reason the 

 photographing of the absorption spectra of a given salt in different solvents 

 will be considered as the mapping of characteristic spectra. On the other 

 hand, when a salt is gradually changed to another salt by the addition of an 

 acid to the solution, or by the addition of a foreign salt, the absorption bands 

 show gradual changes. This has been interpreted as being due to changes in 

 the molecular aggregates of the salts, and to the formation of intermediate 

 compounds. These changes are considered as being of a chemical nature, 

 and will, therefore, be taken up in the chapter dealing with the spectrophotog- 

 raphy of chemical reactions; and the facts there described will be regarded as 

 furnishing strong evidence for the existence of molecular clustering in liquids, 

 and also for the theory that the absorption and emission centers of spectrum 

 bands consist of more or less complex atomic or molecular aggregates, probably 

 in a process of ionization. The full development of this view appears in the 

 summary. 



The experimental methods are essentially those described in Publication 

 130 of the Carnegie Institution of Washington, and in the chapter on experi- 

 mental methods in this monograph. Nothing more need be said here, except 

 that it would probably be desirable in some cases to place the negative film 

 of an absorption spectrum just below the photographic film that is being 

 exposed. Then, by lengthening the time of exposure very greatly, it should 

 be possible to get a spectrogram containing a great many bands. 



THE ABSORPTION OF CERTAIN CYANIDES AND CHROMATES. 



The early workers on absorption spectra supposed that if two salts, dis- 

 solved in the same solvent, had absorption bands that were close together, 

 the wave-lengths of the bands would be modified by the salts being 

 together in the solution. Experiments of this kind have been made without 



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