291 JONES AND ANDERSON— ABSORPTION SPECTRA OF [April 25, 



relative percentages of which are varied, there is not a gradual 

 change of one spectrum into the other, but the spectrum given in 

 the mixture is a superposition 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 varying composition of the 

 mixture. 



A study of all the plates (eighty in number) obtained in this 

 work shows that deviations from Beer's Law is the rule rather 

 than the exception ; only a limited number obey Beer's Law even 

 approximately. Beer's Law could only hold in cases where the 

 relative concentrations of the different kinds of absorbers in solu- 

 tion do not change with the dilution, or where the different kinds 

 of absorbers have the same kind of absorption. The first condition 

 is perhaps never realized, while the second is undoubtedly closely 

 approached with such salts as neodymium chloride and praseody- 

 mium chloride. 



The rule is that the different absorbers have different absorbing 

 powers, and the problem of absorption spectra is to determine which 

 kind of absorbers in solution are responsible for the dift'erent bands. 



The theory of Ostwald, which would refer absorption in solution 

 mainly to the ions present, has been found to be entirely insufficient 

 to account for the facts established in this investigation. 



The other theories which aim to account for the deviations are 

 of two kinds, viz. : 



1. Those that assume that the increased absorption in concen- 

 trated solutions is due to the formation of aggregates of the mole- 

 cules of the dissolved substance, or of the molecules and the ions 

 into which they break down in dissociation. 



2. Those that assume that the deviation is due to the formation 

 of solvates, that is, combinations of the parts of the dissolved sub- 

 stance with the molecules of the solvent. 



Now, it has been shown by Hartley and other workers, who 

 have studied the change in the absorption with change in tempera- 

 ture, that the bands which widen with increase in concentration 

 (conditions for Beer's Law assumed to obtain) also widen with 



