88 THE ABSORPTION SPECTRA OF SOLUTIONS. 



scope, it is found that the finer absorption bands are different for the same 

 salt in different solvents, and for different salts in the same solvent. This 

 difference may be a difference in the number of the bands present, a difference 

 in the intensity and width of the bands, or a difference in wave-length. 



It can be said, in general, that the absolute differences of intensity, 

 diffuseness, and wave-length, under these conditions, is greater for the larger 

 and the more intense bands. Unfortunately the very large bands are usually 

 so situated in the spectrum that only small changes in the amount of salt 

 can be made before the band becomes one-sided on account of the compara- 

 tively small region of the spectrum that can be investigated by photographic 

 methods. But it can be said that the uranous bands show greater changes 

 with change of salt or solvent than the uranyl bands; that the uranyl bands 

 show greater changes than the neodymium bands, and these in turn seem to 

 show much greater changes than the dysprosium and samarium bands; and 

 these, in turn, probably greater changes than the erbium bands. 



When we consider the minute structure of the bands and groups of bands 

 under high dispersion we find very great differences, especially in the case of 

 neodymium. The different salts of neodymium in the same solvent, especially 

 in some of the organic solvents, give entirely different absorption spectra. 

 The same is true of the same salt in different solvents. Isomeric solvents very 

 often show characteristic spectra. The absorption bands of neodymium have 

 been divided into groups a, (3, y, 8, e, etc. When there is a large amount of 

 salt in the path of the beam of light, each group usually forms a single broad 

 band. It is found that the relative intensities and characteristics of these 

 groups is very much the same for different salts in different solvents. The 

 minute structure of the bands in the same group is usually more widely 

 different than the absorption spectra of dysprosium and samarium; so that 

 it is quite probable that if the region of spectrum that we could study were wide 

 enough to include one of these groups and sufficient dispersion was at hand, 

 we would consider that we were dealing with different elements whenever the 

 salt and the solvent were changed. 



The spectrograms and the descriptions in detail give a large number of 

 examples illustrating the above. Very much more work of this kind remains 

 to be done with solutions like those of neodymium at low temperatures, using 

 very high dispersion. 



From the above description of absorption spectra it follows at once that 

 any law such as the supposed law of Kundt, connecting the wave-length of 

 the absorption band with the value of the dielectric constant of the solvent, 

 is impossible. It is probable that the so-called Melde effect is equally chi- 

 merical. Looked at from the point of view of the aggregate theory the Melde 

 effect has very little if any meaning. 



Beer's law is found to hold approximately for nearly all solutions of a 

 single neutral salt in a single solvent. Exceptions are found when very con- 

 centrated solutions are used, and with only one known exception (solutions 

 of uranyl acetate give a Aery large negative deviation) the deviation is such 

 that the absorption is greater than would be given by Beer's law when the 

 concentration is increased. The fact that Beer's law holds indicates that, as 

 far as our knowledge of absorption spectra is concerned, there is no difference 



