che;mistry — JONES. 195 



in the mixture are varied, no marked change in the spectrum is observed 

 when the amount of water is varied from 100 per cent to about 15 or 20 

 per cent. As the amount of water is still further reduced we find that the 

 solution gives a spectrum which consists of a superposition of the spectra be- 

 longing to the aqueous and the non-aqueous solutions, the former decreasing 

 in intensity, while the latter increases, as the amount of water is decreased. 

 The composition of the mixed solvent which will show the two spectra with 

 about one-half their normal intensity depends upon the concentration of the 

 salt in solution; and a constant ratio between the number of molecules of 

 water and those of the dissolved salt was indicated by the experiments, this 

 ratio having the value ten. 



Neodymium nitrate dissolved in mixtures of water and one of the non- 

 aqueous solvents shows exactly the same change as the chloride, but there are 

 also shown the changes in the spectrum produced by the great change in the 

 state of dissociation of the salt. The result is that the whole change is a 

 much more gradual one, and hence is not nearly so striking as it is in the 

 chloride or bromide solutions. 



Praseodymium chloride dissolved in mixtures of water and methyl or ethyl 

 alcohol shows in general the same kind of change in the spectrum as neo- 

 dymium chloride ; but in addition there appears in the alcoholic solutions an 

 entirely new band having no analogue in the aqueous solution. In the former 

 this new band in the ultra-violet is by far the most intense in the entire spec- 

 trum. It disappears entirely on addition of water, having about half its nor- 

 mal intensity for a half-normal solution when the water content of the solvent 

 is about 8 per cent. 



The points emphasized most strongly are, then : 



( 1 ) That the absorption spectra of different salts of the same metal in the 

 same solvent are different if the concentration is great, or, more generally, if 

 the dissociation is only slight ; and that as the dissociation becomes more and 

 more complete, they become more and more alike. 



(2) That the absorption spectra of the same salt in different solvents are 

 in general different. 



(3) That with change in dissociation of the salt in any one solvent, the 

 change in the absorption spectrum of salts having anions containing only a 

 few atoms, such as the chloride and bromide, is very slight, but that as the 

 complexity of the anion increases the change becomes more and more pro- 

 nounced. 



(4) That when a salt is dissolved in mixtures of two solvents, the relative 

 percentages of which are varied, there is not a gradual change of one spec- 

 trum into the other ; but the spectrum given by 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 



