194 REPORTS OF INVESTIGATIONS AND PROJECTS. 



Beer's law says that when the total amount of coloring matter in the path 

 of the light remains constant the absorption should be constant independent 

 of the concentration. The exceptions to this law are very numerous, only a 

 few salts conforming to it. This is exactly what should be expected in terms 

 of Jones's hydrate theory, since solutions almost always contain more than 

 one kind of "absorber," and the relative proportions of these would con- 

 tinually change with change in concentration. Beer's law could hold only 

 when the relative concentrations of the different kinds of absorption did not 

 change. The theory of Ostwald, which refers absorption in solution mainly 

 to the ions, is insufficient to account for the deviations from Beer's law. 



The other two theories of absorption in solution are: (i) That the in- 

 creased absorption in concentrated solution is due to the formation of aggre- 

 gates of the molecules of the dissolved substance, or of the molecules and the 

 resulting ions; (2) that absorption is largely referred to the formation of 

 solvates, i. e., combination of the dissolved substance with the molecules of 

 the solvent. 



It has been shown that those bands that widen with increase in concen- 

 tration also widen with rise in temperature — rise in temperature producing 

 very much the same effect as increase in concentration. Rise in temperature 

 renders aggregates simpler, while increase in concentration makes them more 

 complex, and this argues against the association theory as an explanation 

 of the well-established facts in connection with absorption in solution. 



These facts are, however, in perfect accord with the solvate theory of ab- 

 sorption, since both rise in temperature and increase in concentration produce 

 the same effect on solvates — rendering them simpler. 



Some of the most interesting and important results were obtained with 

 salts of neodymium and praseodymium. These salts have not only a large 

 number of absorption bands, but they are unusually narrow and sharp, espe- 

 cially for the neodymium compounds. 



The absorption spectra of aqueous solutions of the chloride and bromide of 

 neodymium are nearly identical, and change very little with change in con- 

 centration. These facts can not be accounted for on the ionic theory of ab- 

 sorption. The absorption spectrum of the nitrate is also somewhat different 

 from that of the chloride and bromide. It was found that the addition of 

 large amounts of calcium or aluminium chloride to a solution of neodymium 

 chloride does not appreciably affect the spectrum. 



Solutions of the salts in non-aqueous solvents give spectra which are not 

 only different for the different salts, but the spectrum of any one salt is dif- 

 ferent in one solvent from what it is in any other solvent. 



One of the most important facts brought out in this work is the following: 

 When a salt like neodymium chloride is dissolved in mixtures of water and 

 one of the non-aqueous solvents, and the relative amounts of the two solvents 



