36 STUDY OF ABSORPTION SPECTRA 



case of the I/* band, table 6; therefore, no curve was plotted for this 

 salt. The results for the 1.25/-1 band, table 7, show differences between 

 the absorption of the solution and of the solvent. The solution has the 

 greater absorption at the center of this band, which is in accord with 

 the results in general for non-hydrated salts. Otherwise the results are 

 of the same general character as those for a very weakly hydrated salt. 



The curves for ammonium bromide and sodium nitrate (figs. 17 and 

 18) present several new features. One of these makes it necessary 

 to modify the statement 1 that solutions of non-hydrated salts have 

 practically the same absorption as that of a layer of water equal in 

 depth to the water in the solution. This is nearly true for potassium 

 chloride, but not so for the two salts ammonium bromide and sodium 

 nitrate. In all four of the curves expressing the relations between the 

 transmissions of the solutions and of the solvent, as we approach and 

 pass through the centers of the bands, the solution is much less trans- 

 parent than the pure solvent. This is exactly opposite to the effect 

 noted in the case of hydrated salts. Regions of the spectrum which, 

 in the case of hydrated salts, showed that the solution was as much as 

 30 to 40 per cent more transparent than the solvent, show in the cases 

 of both ammonium bromide and sodium nitrate, that the solution is 

 30 to 40 per cent less transparent. In both of these cases, however, 

 a slightly deeper layer of solution was used than in the case of strongly 

 hydrated salts. The 1/x band for ammonium bromide (fig. 15) shows 

 that the solution is less transparent than the solvent. It will be 

 recalled that with calcium chloride (figs. 11 and 12) atypical hydrated 

 salt, the solution had the greater transparency for every wave-length 

 of light investigated. 



There seems to be fairly good evidence that there is a shift in some 

 of the absorption bands. If there is any shift of the solution band for 

 a hydrated salt, it is towards the red. The 1/z bands for magnesium 

 chloride (fig. 1), magnesium nitrate (fig. 13), and zinc nitrate (fig. 9) 

 are displaced slightly towards the red, and the centers of both bands 

 for sodium nitrate (figs. 17 and 18) seem to be shoved slightly towards 

 the violet end of the spectrum. There does not seem to be any justi- 

 fication for stating that there is a shift in either direction for any 

 absorption band of any of the other solutions. 



The question arises, why are concentrated solutions of hydrated 

 salts more transparent than a layer of water equal in depth to the 

 water in the solution? The answer involves, in our opinion, the solvate 

 theory of solution. From earlier work 2 done in this laboratory, it was 

 calculated that in a solution of magnesium chloride of the concentra- 

 tion 2.3 normal, about 65 per cent of the water present is at ordinary 



'Phys. Zeit., 15, 447 (1914). 

 "Carnegie Inst. Wash. Pub. No. 60. 



