170 DISCUSSION OF EVIDENCE. 



rubidium should lower the viscosity of glycerol is in keeping with 

 what was found in aqueous solutions. Salts of rubidium and csesium 

 and some salts of potassium lowered the viscosity of water. This has 

 already been explained as due to the large atomic volumes of these 

 elements. The same explanation holds for solutions in glycerol. 



Davis 1 continued the work of Guy, studying especially the effect 

 of salts on the viscosity of glycerol. He repeated the work with 

 ammonium iodide and obtained the same result that had been earlier 

 found by Guy. He studied rubidium chloride, bromide, iodide, and 

 nitrate, and showed that these lowered the viscosity of glycerol. The 

 rubidium salts lower the viscosity of glycerol to such an extent that 

 they appreciably increase their own conductivity in this solvent. 



Comparing the effects of the chloride, bromide, and iodide of rubid- 

 ium on the viscosity of glycerol, Davis found that the chloride has the 

 least effect, the bromide next, the iodide the greatest. He showed that 

 this was in the same order as the molecular volumes of the salts in 

 question. The results obtained with glycerol were, then, analogous to 

 those obtained with water, both with respect to viscosity and solution. 



SPECTROSCOPIC EVIDENCE BEARING ON THE SOLVATE THEORY OF SOLUTION. 



WORK OF JONES AND UHLER. 



Work on the absorption spectra of solutions has now been in progress 

 in my laboratory continuously for eight years. This work was under- 

 taken in connection with its bearing on the solvate theory of solution. 

 What connection is there between solvation and the power of solu- 

 tions to absorb light? 



It is well known that absorption of light means that the wave- 

 lengths of light set something vibrating with periods the same as their 

 own. Selective absorption of light or the absorption of certain wave- 

 lengths of light means that the wave-lengths absorbed set something 

 vibrating with their own periods. Absorption of light is, then, a 

 resonance phenomenon. Absorption of light by a dissolved substance 

 means that something in the solution must be thrown into resonance 

 with the light must be set vibrating with the same periods as the 

 light-waves. Many dissolved substances absorb only certain wave- 

 lengths. This means that those particular wave-lengths of light find 

 something in the solution which they can set vibrating with their own 

 periods. Transparency means lack of resonance, opacity means reso- 

 nance. The color of any given solution is determined by the wave- 

 lengths of light which are not absorbed. A red solution is one which 

 allows the long wave-lengths to pass through. A blue solution is one 

 which allows the short wave-lengths to pass through. That particle 

 in solution which is thrown into resonance by the light is called the 



'Zeit. phys. Chem., 81, 68 (1912). 



