2 ABSORPTION SPECTRA OF SOLUTIONS. 



way through the layer of gold laid down on a layer of copper electrolytically, 

 rusted the steel, and caused the separation of the gold from the steel walls. 

 To avoid this, the apparatus, which was designed by Dr. Strong, 1 was made 

 of brass and will be described in some detail in this monograph. It was 

 plated electrolytically with gold and this adhered firmly to the brass, even 

 when the aqueous solution contained in the apparatus was heated to 200 C. 

 We could work as satisfactorily with this apparatus with aqueous solutions 

 as with the former apparatus with nonaqueous solutions. 



The work described in this monograph on absorption spectra of aqueous 

 solutions at high temperatures was all carried out in the gold-plated brass 

 apparatus. The results obtained and the bearing of these results on the 

 nature of solution will be discussed later in this monograph. Suffice it to 

 say here that up to 200 the effect of temperature on the absorption spectra 

 of aqueous and nonaqueous solutions has now been studied pretty exten- 

 sively on a large number of salts and a fairly large number of solvents. 



The effect of dilution on the absorption spectra of solutions was taken 

 up with the following idea in mind: It was long a question as to what is 

 the nature of the absorber of light, say in aqueous solutions. It was at one 

 time supposed that chemical molecules were the absorbers, since these were 

 regarded as the ultimate units in solution. It was supposed that the mole- 

 cules were thrown into resonance by certain wave-lengths of light, and that 

 these were, consequently, stopped ; while the remaining wave-lengths passed 

 through the solution and gave to it its characteristic color. 



When the theory of electrolytic dissociation was proposed in 1886, the 

 view as to the nature of solution of electrolytes underwent a serious change. 

 When electrolytes were dissolved in water, or in any other dissociating 

 solvent, they dissociated into charged parts or ions, and these were the 

 ultimate units in solution. If the solution was fairly concentrated we had 

 both ions and undissociated molecules in the solution, and the question in 

 such cases was, which is the absorber? 



It was further recognized that a dilute solution of salt often has very 

 different color from a concentrated solution; and, moreover, solutions of 

 nonelectrolytes are often colored, i. e., have the power to absorb certain 

 wave-lengths of light and to allow others to pass on through. It was sup- 

 posed, then, that molecules have the power to absorb light, and ions also 

 have absorbing power. When a concentrated and a dilute solution of an 

 electrolyte had the same absorption spectrum the same color it was 

 supposed that the chemical molecule and the ions resulting from it had the 

 same absorption. When the dilute solution had a different color from the 

 concentrated solution, it was thought that the ions were the chief absorbers 

 of light. And since it frequently happens that a dilute solution of an elec- 

 trolyte has a very different color from a more concentrated solution, it 

 was supposed that in dilute solutions of electrolytes the ions are the chief 

 absorbers of light; since in very dilute solutions of electrolytes there are 



1 Cam. Inst. Wash. Pub. 160. Amor. Chem. Journ., 47, 30 (1912). 



