22 ABSORPTION SPECTRA OF SOLUTIONS. 



results show that difference in exposure can not account for the changes in 

 the widths of the bands in question. 



Spectrograms B and C of this plate give the absorption of solutions of 

 neodymium sulphate. On account of the slight solubility of this salt, we 

 observe only slight changes in any of its absorption bands. 



The concentrations in B were 0.1, 0.004, and 0.0008 normal; the corre- 

 sponding depths of cell being 2, 50, and 250 cm. The concentrations in C 

 were 0.1, 0.001, and 0.0002 normal, and the corresponding depths of cell 

 were 0.5, 50, and 250 cm. 



In both B and C, those bands having their center near X3500 appear well 

 defined and remain unchanged both in position and intensity as the dilution is 

 changed. The band A5750 widens with concentration in A as much as 25 a.u. ; 

 it remains practically unchanged in B, where the solutions are more dilute. 



The plate brings out the fact already mentioned, that only the more con- 

 centrated solutions show marked change, either with change in temperature 

 or with change in dilution. 



NEODYMIUM ACETATE IN WATER. (See Plate 17.) 



The concentrations of solutions used in making A of this plate were sat- 

 urated, one-hundredth saturated, and five-hundredth saturated, the corre- 

 sponding depths of cell being 0.5 cm., 50 cm., and 250 cm., respectively. 

 The most dilute solution is nearest the numbered scale. 



This plate was made with very long exposures, to see if the apparent 

 widening of the bands could be due to the difference in the amounts of light 

 falling upon the photographic plate. In such a procedure the most concen- 

 trated solution was given the longest exposure and yet had the broader 

 bands. It is possible to narrow any given absorption band by lengthening 

 the time of exposure, but this can not account for so large a difference as is 

 shown by strip 3 of section A. Even in this section it is seen that the third 

 strip has wider bands than either of the other two strips of this section, not- 

 withstanding the fact that the actual exposure of the strip is greater. Thus 

 we see that the difference of exposure can not account for the changes in 

 the width of bands such as we have noted. 



In section A the violet group of bands in the region X3500 came out beau- 

 tifully. Such is only the case when quite a long exposure is made. Indeed, 

 in order to show these lines clearly, the exposure must be long enough to 

 destroy those fine, sharp lines in the region of X3800 to X4600. Hence, in 

 this plate the latter group of lines do not appear distinctly, though traces of 

 them can be seen on the original film. 



The hazy bands X3300 and X3400 appear on this plate and remain un- 

 changed by dilution. 



The three bands, X3460, X3500 and X3540, remain perfectly constant 

 throughout the section. Band X5120, which appears broad and diffuse, 

 shows no change. Band X5210 narrows about 10 a.u. from the third to the 

 second strip, and remains unchanged with the next dilution. The broad 



