30 STUDY OF ABSORPTION SPECTRA 



in the other. The solution curve for the 1.25^1 band (fig. 14) was 

 plotted from the data given in the second column of table 13. 



Magnesium nitrate (like zinc nitrate, the other hydrated nitrate 

 which we studied) shows the peculiarity that at the center of the 1/j. 

 band (fig. 13) the solution is less transparent than the solvent. The 

 difference in this case is quite pronounced. At the center of the 1.25^ 

 band (fig. 14) the solution and solvent have about the same trans- 

 mission, whereas the absorption of zinc nitrate (fig. 10) at this point 

 conforms to the general characteristics of the bands for the other 

 hydrated salts. Magnesium nitrate presents another curious feature, 

 which is not present in the results for any of the other hydrated salts. 

 The difference between the percentage transmissions for the solution 

 and for the solvent is at no point very great. The data given in table 

 12 for the lju band show that for wave-lengths of light near 1.2/j. the 

 solution is actually several per cent less transparent than the solvent. 

 This is the region where solutions of hydrated salts in general are much 

 more transparent than the pure solvent. This is clearly shown in both 

 series of measurements given in table 12. Here we have employed 

 a deeper layer of solution than in the series from which the curve for 

 the IM band was plotted. Consequently, the intensity of absorption 

 is greater, and this would have the effect of making more pronounced 

 any peculiarity shown by a more shallow layer. The curves for the 

 solution and solvent cross at a point closer to the violet end of the 

 spectrum when deeper layers are used. This is evident from data 

 given in table 12, and the results clearly show that the solution would 

 not have the greater transparency at the center of the 1.25ju band. 



The 1.25/z band (fig. 14) does not show any special peculiarities, 

 there being several other bands of very similar character in the cases 

 of the other hydrated salts. As might be expected from the nature of 

 the results for the lju band, the solution is less transparent just before 

 the center of the band is reached. 



The transmission curves for ammonium bromide (figs. 15 and 16), 

 being so widely different from those of the solvent, have been plotted 

 to bring out the differences in general between hydrated and non- 

 hydrated salts. Ammonium bromide is a typical non-hydrated salt, 

 and the results for this salt can be compared with those for magnesium 

 bromide, a typical hydrated salt. 



In every case thus far studied, the solution of a non-hydrated salt 

 is less transparent at the center of the band than the solvent. The 

 differences in the case of ammonium bromide are quite marked. It is 

 only through a narrow range of wave-lengths (that is, from 1.1 Q/JL to 

 1.1 8/x, fig. 13) that the solution is the more transparent in the neighbor- 

 hood of the lju band, and even here the difference is very slight. 



Near 1.2/z, where the hydrated salts are much more transparent than 

 the solvent, ammonium bromide is about 30 per cent less transparent. 



