90 ABSORPTION SPECTRA OF SOLUTIONS. 



analyzed by a prism or grating, show a certain number of absorption bands 

 whose wave-lengths could be determined. If, now, the atoms, instead of 

 being free, are each united to 3 chlorine atoms, since these foreign atoms 

 would affect the periods of the neodyrnium electrons, we should expect to 

 find the absorption spectrum modified. If instead of 3 chlorine atoms we 

 had united the neodymium atom with 3 bromine atoms, we should expect a 

 somewhat different spectrum again, and so on for the various salts ; each 

 one would be characterized by its own absorption spectrum. If these salts 

 could be dissolved in some medium which had no action on it except to 

 allow its molecules to move about freely, we should not expect any mate- 

 rial change in the spectrum; while if the solvent united with it, forming 

 solvates, we should expect the spectrum to be modified. 



In a solvent like water, where it is probable that rather complex hy- 

 drates are formed, the effect of the solvent might even become the most 

 important factor in determining the character of the absorption. To take 

 a concrete case, suppose each molecule of a salt of neodymium in aqueous 

 solution is united with 10 molecules of water. If the salt is the chloride 

 or bromide, each neodymium atom has only 3 foreign atoms to disturb 

 the periods of its electrons besides the 30 atoms in the combined water; 

 while if the salt is the nitrate, it would have 12 foreign atoms besides those 

 of the water. Evidently these 12 atoms would have a very much greater 

 effect than the 3 in the case of the chloride or bromide, if we assume that 

 the general arrangement in space is not very different in the two cases. 

 We see, then, that the fact that the spectrum of the nitrate in aqueous 

 solutions of considerable concentration is different from that of the chloride 

 or bromide is what we should expect, and we also see that the very slight 

 change in the spectrum of the bromide and chloride on dilution, as com- 

 pared with the great change in case of the nitrate, might almost have 

 been predicted. 



The change taking place with dilution is, of course, due to dissociation, 

 each neodymium atom after dissociation being simply united with, say, 10 

 molecules of water, the anion of the molecule having left it. The neodym- 

 ium ions in dilute solutions are, therefore, the same, no matter what salt 

 is in solution, if we assume that the presence of the anions in the solution 

 does not influence the hydrating power of the metallic ion. Other things 

 being equal, therefore, we should expect that salts whose molecules are 

 made up of only a very few atoms united with a neodymium atom, in 

 aqueous solution, should show the least change in the spectrum when the 

 concentration is varied; since the removal of the few atoms making up the 

 acid radical from the hydrated molecule would in general have but a slight 

 effect on the periods of the absorbing electrons in the metallic atom. Salts 

 whose molecules consist of a great many atoms united with a neodymium 

 atom, like the nitrate, acetate, or sulphate, when dissolved in water, ought 

 to show considerable change in their spectra as a result of dissociation, since 

 the removal of the great number of atoms forming the acid radical would un- 

 doubtedly have a marked influence on the periods of the absorbing electrons. 



It is plain, therefore, that the theory outlined above furnishes a per- 

 fectly simple and rational explanation of all the phenomena that have 



