SALTS OF NEODYMIUM, PRASEODYMIUM, AND ERBIUM. 89 



The corresponding nitrate solution also shows a band at A 5090, but it is 

 much wider and hazier than in the chloride solution, while the A 5125 band 

 is, if anything, narrower. The two bands are not clearly separated in the 

 first strip of B, Plate 70. With dilution, however, the X 5090 band narrows 

 up and becomes a little fainter, while the X 5125 band widens a little towards 

 the red; so that in rather dilute solutions the bands present the same 

 appearance as they do in the corresponding chloride solutions. The region 

 A 5200 to A 5240 shows practically continuous absorption with very hazy 

 edges in the first strip of B, Plate 70 ; with dilution this changes rapidly, 

 indicating bands somewhat similar to those of the chloride solutions belong- 

 ing to A, Plate 60. 



In A, Plate 71, the band has broken up, and instead of showing two 

 narrow intense bands at A 5205 and A 5222 it shows the following : There 

 is a deep, narrow band at A 5205, a wider and very much more intense one 

 at A 5225, and a rather narrow, intense band at A 5235. With increasing 

 dilution the A 5235 band diminishes in intensity, practically disappearing 

 in the most dilute solution used in making B, Plate 71. At the same time 

 A 5225 decreases somewhat in intensity, and rather more on the red than 

 on the violet side; so that when the most dilute solution of B, Plate 71, is 

 reached its intensity is only slightly greater than that of the A 5205 band 

 and its center is at about A 5222. Here, then, we find also the same general 

 tendency for the spectrum of the nitrate solutions to change with dilution so 

 as to become more and more like that of the chloride and bromide solutions. 



We might go on and give in detail the changes taking place in the bands 

 located in the yellow, orange, and red, since the changes here are just as 

 well marked as those we have already described. But they all point to 

 the same thing, namely, the dissimilarity of the spectra of concentrated 

 solutions, and the gradual change of the nitrate spectrum into that of the 

 chloride or bromide with decreasing concentration. That the spectra of 

 dilute solutions should become more and more alike with increasing dilu- 

 tion was, of course, to be expected from the theory of dissociation; but 

 on the simple theory of dissociation no one could have predicted that the 

 chloride and bromide should give spectra which are practically identical, 

 both in concentrated and in dilute solutions, while the nitrate should behave 

 so differently, especially as it is well known that the three dry salts have 

 quite different absorption spectra. 



Our work on the spectrum of neodymium chloride in mixtures of alco- 

 hol and water made it seem very probable that the molecules as well as 

 ions of the salt in solution are solvated, that is, have combined with them 

 a relatively large number of molecules of the solvent. On this view, the 

 results with aqueous solutions of the chloride, bromide, and nitrate are 

 just about what we ought to expect, if we assume that the absorption 

 bands are due to electrons which are located in or closely associated with 

 the neodymium atom. Let us consider this a little more fully, even at the 

 risk of repeating certain things we have said before. 



Let the neodymium atom contain electrons, which if the atom is by 

 itself would respond to light-waves of certain definite frequencies. White 

 light, after having been acted on by a number of such atoms, would, when 



