86 ABSORPTION SPECTRA OF SOLUTIONS. 



molecules of the salt in the non-aqueous solutions exist in the free state, 

 that is, not combined with the solvent in any way, they should give about 

 the same spectrum as they do when in the state of the dry powder. Since 

 they do not do this, we must suppose that the solvent plays an important role 

 in determining the character of the absorption, and how it can do this with- 

 out being combined with the salt in some way is not easy to understand. 



NEODYMIUM BROMIDE IN WATER BEER'S LAW. (See Plate 69.) 



The concentrations of the solutions used in making the negative for 

 A, beginning with the one whose spectrum is adjacent to the numbered 

 scale, were 2.3, 1.7, 1.15, 0.77, 0.54, 0.38, and 0.29; the corresponding 

 depths of absorbing layer were 3, 4, 6, 9, 13, 18, and 24 mm. For B the 

 concentrations were 0.57, 0.42, 0.29, 0.19, 0.13, 0.09, and 0.07; the depths 

 of the absorbing layer were the same as in A. 



The bromide solutions are very much redder in color than those of the 

 chloride or nitrate. Judging from the color alone, one would say that the 

 nitrate solutions are much more transparent in the blue and violet than the 

 chloride, and the chloride solutions much more so than those of the bromide. 

 The spectrograms do not show this, at least not very clearly; which merely 

 indicates that where full exposures are given, slight general absorption is 

 not recorded by the photographic plate. A spectrophotometric compari- 

 son of the light transmitted through these solutions, such as is now in 

 progress in the present work, will undoubtedly show this general absorption 

 of the bromide solutions in the more refrangible portion of the spectrum. 



In studying the spectrograms of this plate, A was compared with Plate 

 59 B, and B with Plate 60 B, that is, the spectrum of the bromide solu- 

 tions was compared with that of a chloride solution whose concentration, 

 in each case, was almost exactly 1.5 times that of the bromide solution, 

 the depth of the absorbing layer being the same in both cases. The two 

 spectra were found to be almost identical, except in the extreme ultra- 

 violet, where the bromide solutions absorb much more strongly. The 

 limits of transmission for the most concentrated and most dilute solu- 

 tions of A are, respectively, A 3270 and A 3050; whereas the correspond- 

 ing chloride solutions transmitted to beyond ^ 2500. The ultra-violet 

 absorption shown by B is about the same as that of the chloride solutions 

 used in making Plate 59 B. 



The absorption bands have in general about the same intensity and 

 character in the bromide solutions as they have in the corresponding 

 solutions of the chloride, indicating a considerably greater absorbing power 

 of the bromide, since the concentrations of its solutions were only 0.66 

 of that of the chloride. A small part of this is due to the fact that the 

 negatives for Plate 69 were not as fully developed as those made with the 

 chloride solutions, but even if the development had been exactly the same, 

 the bands of the bromide solutions would only have been very slightly 

 less intense than those of the chloride solutions. We must, therefore, con- 

 clude that in solutions of the same concentration the bands of the chloride 

 solution would have only about 75 per cent of the intensity of the same 

 bands in the spectrum of the solution of the bromide. 



