absorption spectra of higldy concentrated solutions, etc. 211 



absorption of the more refrangible rays by the concentrated 

 solution. Again, just as in the case of the salts of Didymium, 

 the concentrated solution of Erbium chloride, the dilute solution 

 of Erbium chloride, and the dilute solution of Erbium nitrate 

 are alike in the characters of their absorption bands, and the only 

 apparent difference is that mentioned above. 



With regard to the differences between the concentrated and 

 diluted solutions of the nitrates of Didymium and Erbium it may 

 be noticed that the photographs do not seem to show that each 

 band is equally affected in diffuseness and intensity. Perhaps 

 this is the most noticeable in the concentrated solutions of 

 Didymium nitrate, where the band in the blue at A, 476 is so very 

 diffuse that it is only just visible, and appears very well marked 

 and strong in the diluted solutions, whilst the adjoining bands 

 A 483 and A. 469 do not seem to be affected to the same extent. 

 Also the intensity of the band at about A 531 is not so highly 

 changed on dilution ; the most apparent change being a shift 

 towards the red end of the spectrum. The same remark applies 

 to the faint band at about A 595. 



Also in the solutions of Erbium nitrate the two bands at about 

 A536 and A541 do not seem to be affected on dilution to any great 

 extent, and this applies to the faint band at about A 441. 



These experiments, therefore, seem to prove 



(1) that the absorption bands of highly concentrated solutions 

 of the chlorides of Didymium and Erbium are not altered when 

 these solutions are highly diluted. 



(2) that the absorption bands of highly concentrated solutions 

 of the nitrates of Didymium and Erbium are considerably less 

 diffuse when these solutions are highly diluted. It is noteworthy 

 that this is an effect analogous to that produced in the spectra of 

 some gases and vapours by diminishing the density of the gas or 

 vapour. 



(3) that the absorption bands of concentrated and diluted 

 solutions of the chlorides of Didymium and Erbium are precisely 

 similar to those observed in diluted solutions of the nitrates 

 of these earths. 



It is an obvious speculation to suggest that the various 

 elements which compose the molecules of Didymium and Erbium 

 form different complex groupings with nitric acid than with 

 hydrochloric acid, and that this complexity is broken down when 

 the concentrated solutions are diluted. The fact that these 

 changes are gradual on gradual dilution may lend support to this 

 view. It may also be mentioned that Praseodym and Neodym, 

 two of the constituents of Didymium, are separated by repeated 

 fusion and rapid cooling of the nitrates ; whilst there is no such 

 separation when the chlorides are similarly experimented with. 



