CHAPTER VIII. 



SUMMARY AND CONCLUSIONS. 



It is evident from the spectra of the solutions studied in the present 

 investigation that deviation from Beer's law is the rule rather than the 

 exception. Of the great number of sets of solutions studied, only a very 

 limited number appear to confirm Beer's law, and it is possible that with 

 the more exact spectrophotometric measurements this number would be 

 reduced still further. This is exactly what we should expect, since actual 

 solutions always contain more than one kind of "absorber," and the rela- 

 tive concentrations of these "absorbers" are continually changing with 

 change in concentration of the solution. Beer's law could only hold, as 

 explained in the introductory chapter, in cases where the relative con- 

 centrations of the different kinds of absorbers do not change with dilution, 

 or in the event that the absorption of all the different lands of absorbers 

 is identical. The first one of these conditions is perhaps never fulfilled, 

 while the second one is undoubtedly approached more or less closely in 

 certain cases, such as in aqueous solutions of neodymium chloride or bro- 

 mide or of praseodymium chloride. The rule is, however, that the different 

 absorbers have different absorbing powers, and the problem is, therefore, 

 to decide which absorbers are responsible for the bands observed in the 

 various spectra. 



According to the theory of Ostwald, which is simply Arrhenius's dis- 

 sociation theory applied to the absorption spectra of solutions, we have 

 but two or three kinds of absorbers, namely, the molecules of the dissolved 

 salt and one or both the ions formed from it. In the case of all the salts 

 studied in the present work, excepting the nitrates, the anion has been 

 colorless; so all the absorption, according to Ostwald's theory, should be 

 due to two kinds of absorbers, the molecule and the cation. That this 

 theory fails entirely to account for the deviation from Beer's law observed 

 in the ultra-violet absorption of copper salts, the red bands of cobalt salts, 

 the ultra-violet band of cobalt chloride, and the absorption of iron chloride, 

 has already been pointed out; since all of these bands narrow with dilution, 

 even when the number of molecules in the path of the beam of light is kept 

 constant. Whether this theory is able to account for the behavior of those 

 bands which narrow with dilution when the conditions for Beer's law 

 obtain, but which widen when molecules are kept constant, can only be 

 decided by spectrophotometric measurements. 



The work of Miiller on salts of nickel and copper shows that the behavior 

 of the red absorption band of these substances can not be accounted for 

 on Ostwald's theory, and this makes it at least very probable that the same 

 will be found for salts of other metals. Ostwald's theory may, therefore, 

 be dismissed, not because it is erroneous, but because it is incomplete. 

 It leaves out of account certain changes taking place in solutions, which 

 produce other "absorbers" than those which it considers. 

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