CHAPTER I. 



INTRODUCTORY. 



The work on absorption spectra of solutions, of which this forms a 

 part, was begun in the fall of 1905 and continued during the year 1905-6 

 by Jones and Uhler. The results obtained are given and discussed in 

 "Hydrates in Aqueous Solutions," by H. C. Jones, Publication No. 60 of 

 the Carnegie Institution of Washington. The first part of that work had 

 to do with the effect of adding various dehydrating agents to solutions of 

 colored salts; the second part dealt with the change in the absorption 

 spectra produced by adding water to non-aqueous solutions. In the 

 latter phase of the work, however, the concentration of the colored salt 

 was varied in the solutions used in making any one spectrogram; but as 

 the change in the spectrum observed was always the same qualitatively 

 as would be expected from the change in concentration, it was deemed 

 advisable to carry out again this part of the earlier work, keeping the 

 concentration of the colored salt constant. 



The salts used in the previous investigation were: Cobalt chloride, 

 copper chloride, and copper bromide. In this investigation cobalt bromide 

 was added to the above list of compounds. The resulting spectrograms 

 show the same general change in the absorption as in the earlier work, 

 thus proving that this was not due to change in concentration, but to 

 some action of the water added. 



If we assume that the absorption of light is due to vibrating, charged 

 particles, or electrons, which are associated with ions, molecules, or groups 

 of one or both of these, it is natural to expect that the character of the 

 absorption will, in general, depend upon the nature of the system with 

 which the vibrating, charged particle is associated. In what follows, the 

 system made up of the charged particle and whatever it is associated 

 with will be spoken of simply as the "absorber." 



The simplest case of an absorbing solution would be one containing 

 only one kind of ''absorber," and we shall speak of it as a "simple" ab- 

 sorbing solution. Such a solution does not in all probability exist, but is 

 perhaps closely approached in such cases as the very dilute solutions of 

 the salts of permanganic acid studied by Ostwald and others. In such a 

 solution the absorption of light of a given wave-length would be simply 

 proportional to the number of absorbers in the path of the light (Beer's 

 law); and if the absorbers are not changed by adding more of the sol- 

 vent, it follows at once that if the product of concentration and thickness 

 of layer of the solution is kept constant, the absorption will be unchanged. 

 Also, if we have a solution containing several kinds of absorbers, each 

 acting independently of the others, the same statement would be true; 



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