98 ABSORPTION SPECTRA OF SOLUTIONS. 



It may be said in general that the absorption spectra of the different 

 salts of the same metal resemble each other very closely; and it is only 

 when careful attention is paid to the structure of each individual band, 

 or group of bands, that the differences are brought out clearly. In the 

 process of printing and reproducing the spectrograms illustrating this 

 chapter, a great deal of the finer detail has been lost, and as it is just this 

 detail which shows the differences alluded to above, it is clear that in 

 many cases the reproductions fail entirely to show the important points. 

 In cases of special importance a rather full description of the appearance 

 as seen on the negatives has been given in the text. A full description 

 of this nature covering all the spectrograms of the present chapter would 

 require an amount of time and space that would be quite prohibitive, 

 and unnecessary. 



It is hoped, however, that the plates, together with the description given 

 in the text, will make clear the points which we have tried to emphasize 

 most strongly, viz: 



1. That the absorption spectra of different salts of the same metal in 

 the same solvent are different if the concentration is great, or, more gener- 

 ally, if the dissociation is only slight; and that as the dissociation becomes 

 more and more complete, they become more and more alike. 



2. That the absorption spectra of the same salt in different solvents 

 are in general different. 



3. That with change in dissociation of the salt in any one solvent, the 

 change in the absorption spectrum of salts having anions containing only 

 a few atoms, such as the chloride and bromide, is very slight; but that 

 as the complexity of the anion increases, the change becomes more and 

 more pronounced. 



4. That when a salt is dissolved in mixtures of two solvents, the rela- 

 tive percentages of which are varied, there is not a gradual change of one 

 spectrum into the other; but the spectrum given by the mixture is a super- 

 position of the two spectra, the two sets of bands existing together. If 

 the salt is one whose spectrum changes considerably with its state of 

 dissociation, we have in addition to the above phenomena the changes due 

 to the varying dissociation of the dissolved salt produced by the vaiying 

 composition of the mixture. 



The explanation of these points on the working hypothesis which has 

 guided the present work, has already been given in the discussion of Plates 

 70 and 71. 



In the introduction to the present chapter the work of Helen Schaeffer 

 was referred to. It will be recalled that she studied the spectrum of the 

 nitrate of neodymium in various solvents, and also in mixtures of two 

 solvents; the case to which she calls special attention being mixtures in 

 various proportions of water and acetone. She did not come to the con- 

 clusion which we have reached, that in such mixtures we have two distinct 

 sets of absorption bands, since she considers the bands as shifting gradu- 

 ally, some in the direction demanded by Kundt's law, and some in the 

 opposite direction. There are two reasons why she did not come to the 

 same conclusion that we have reached. In the first place she worked with 



