DISCUSSION OF EVIDENCE. 171 



resonator. This was formerly supposed to be the molecule or the ion, 

 but is now thought to be the electron. Whatever the nature of the 

 resonator, the absorption of light by dissolved substance is due to it. 



The line of thought which led us to take up the study of the absorp- 

 tion spectra of solutions in connection with the solvate theory of 

 solution is the following: The absorption of light being due to a 

 resonator, this would have different resonance when anhydrous than 

 when combined with molecules of the solvent. In general, the reso- 

 nance would be different when the resonator was unsolvated than 

 when it was sol vat ed. The color of the solution being due to the 

 resonator, the solution could reasonably be expected to have different 

 color when the resonator was solvated than when it was unsolvated. 

 The study of the color of solutions, and the changes in the color when 

 the resonator underwent changes in solvation, might give some clue 

 to the changes in solvation. 



It is a comparatively simple matter to change solvation in solution; 

 it is only necessary to change the concentration of the solution. The 

 more dilute the solution the more complex the solvates formed. We shall 

 see that this often produces a marked change in the absorption spectra. 

 We can diminish the complexity of the solvates by raising the temper- 

 ature. This also frequently produces marked changes in the absorption. 

 Addition of a dehydrating agent will change the hydration of any given 

 salt. This frequently changes the absorption spectra and the color of a 

 solution; and there are many other ways of changing solvation. These 

 frequently produce concomitant changes in the absorption spectra. 



A salt dissolved in water may form hydrates, in alcohol alcoholates, 

 in acetone acetonates, in glycerol glycerolates, etc. We should expect 

 these different solvates to affect the resonator or resonators differently. 

 We shall see that this is true. 



With this idea in mind, work was begun in my laboratory on the 

 study of the absorption spectra of solutions. The first investigation 

 was carried out by Dr. Uhler and myself. Our work consisted largely 

 in devising a method and apparatus for studying the property of 

 solutions to absorb light. The key to the method consisted in using 

 a grating instead of a prism spectroscope. This gave much greater 

 dispersion, and brought out many new lines and bands. A form of 

 cell was devised for holding solutions in non-aqueous solvents which 

 avoided the use of all cement. The details of this phase of the work 

 are all given in Publication No. 60 of the Carnegie Institution of 

 Washington. We studied the effect on the absorption spectra of 

 increasing the concentration of the solution, and found that, in general, 

 the effect of increasing the concentration of the solution was to widen 

 the absorption bands. As the solvates became simpler the absorption 

 bands became broader. 



Another method of simplifying the hydrates existing in an aqueous 

 solution was to add a dehydrating agent in the form of a second salt. 



