SUMMARY AND CONCLUSIONS. 103 



to the solvated molecules, the absorbing power of which increases rap- 

 idly with decrease in the complexity of the solvate. 



The band in the red belongs in the same class with the green cobalt 

 band. But, as mentioned above, Miiller came to the conclusion that dis- 

 sociation is unable to account for its deviation from Beer's law, which 

 also agrees with what we found in studying its behavior in mixtures of 

 alcohol and water for the case of the chloride. Hence we assume that 

 solvates here also play a r61e, which, however, is not quite so apparent, 

 owing to the fact that both the solvated ions and the molecules absorb 

 light in this region. 



Another fact which supports our view is that the absorption in the red is 

 not widely different in different solvents, provided the concentrations are 

 about the same; while in the ultra-violet the absorption is many times 

 greater in the non-aqueous than in the aqueous solvents; the reason for the 

 latter being, first, that the dissociation in aqueous solutions is much greater 

 than in non-aqueous, and hence, for equal concentrations, the number of 

 molecules in the latter is much greater than in the former. Secondly, the 

 solvating power of water is much greater than that of the non-aqueous solv- 

 ents used, and hence the comparatively few molecules present, by forming rel- 

 atively complex hydrates, have their absorbing power still further reduced. 



The only salt of iron studied was ferric chloride. It shows only one 

 region of absorption, namely, the one which cuts off the entire ultra-violet, 

 and usually also the violet and blue portion of the spectrum. In aqueous 

 solutions this absorption band narrows very rapidly with dilution, even 

 when molecules are kept constant, indicating a marked effect of hydration. 

 In alcoholic solutions the band remains of sensibly constant width, indicat- 

 ing that in this case the solvation is probably very slight. The difficulty 

 in drawing any definite conclusions from solutions of this salt is that the 

 solutions are not very stable, and hence the effects may very often be 

 marked by chemical changes of unknown amount. 



Chromium salts behave very much like those of nickel. Only two of 

 them were studied in this work and these only in aqueous solution. The 

 behavior of their bands is quite analogous to that of the green cobalt band, 

 and hence calls for spectrophotometric study. Their diffuse character also 

 makes them rather unfit for spectrographic investigations. 



The most interesting and important results were obtained from the 

 study of the salts of neodymium and praseodymium, especially those of 

 the former. These substances have not only very many absorption bands, 

 but they are remarkably narrow and sharp, and hence peculiarly suitable 

 for spectrographic study. 



The chief experimental results were the following: 



1. The absorption spectrum of aqueous solutions of the chloride and 

 bromide of neodymium changes very little with change in concentration, 

 and the two are nearly identical throughout, excepting for the fact that 

 the absorbing power of the bromide appears to be somewhat greater than 

 that of the chloride. 



2. The absorption spectrum of aqueous solutions of neodymium nitrate 

 is somewhat different from that of the chloride or bromide, especially if 



