186 DISCUSSION OF EVIDENCE. 



widening with increase in concentration. Solutions of praseodymium 

 salts also show a widening of the absorption bands as the concentra- 

 tions are increased, but these changes are less pronounced than with 

 salts of neodymium. The absorption spectra of uranyl salts change 

 more with change in concentration than the spectra even of salts of 

 neodymium. The changes are in the same direction, the bands increas- 

 ing in breadth with increase in concentration. 



These results are what would be expected from the solvate theory of 

 solution. As the concentration of the solution is changed, the com- 

 plexity of the solvate about the molecules or ions is changed. It would 

 seem that this ought to affect the resonance of the solvated resonator. 

 As the concentration of the solution is increased the solvate becomes 

 simpler and simpler. The vibrating particle surrounded by a simple 

 solvate should show different absorption than when surrounded by a 

 complex solvate. The above results show that such is the case, the more 

 concentrated the solution the wider in general the absorption bands. 



The radiomicrometer not only provides us with a method of study- 

 ing absorption spectra quantitatively, but greatly extends the range 

 of wave-lengths that can be studied. The earlier work with the very 

 sensitive radiomicrometer had to do with the study of solutions of 

 neodymium salts. The effect of dilution on absorption spectra was 

 also investigated quantitatively by means of the radiomicrometer. 

 It was found by this method, as with the grating and photographic 

 plate, that the more concentrated the solution the broader the absorp- 

 tion bands. It was also found that in the more dilute solution, while 

 the absorption bands were narrower, they were more intense. Further, 

 in the more dilute solutions the centers of the bands were displaced 

 towards the longer wave-lengths. 



The most interesting and important result brought out by the work 

 of Jones and Guy was the effect of the dissolved substance on the ab- 

 sorption of light by water. We noted that aqueous solutions of certain 

 hydrated salts are more transparent than pure water. This is obvi- 

 ously a fact which called for careful study. We compared the absorp- 

 tion of aqueous solutions of strongly hydrated salts,with the absorption 

 of a layer of water equal in depth to the water in the solution." Similar 

 experiments were carried out with salts which are only slightly 

 hydrated. The slightly hydrated salts with which we worked were 

 potassium chloride and ammonium chloride and nitrate. It was 

 necessary to select colorless salts which themselves had little or no 

 absorption in the infra-red where water absorbs. It was found, in the 

 earlier work, that the above-named compounds had nearly the same 

 absorption as water having the same depth as the water in the solution; 

 but in subsequent work this conclusion must be modified for certain 

 substances near the bottoms of the absorption bands. 



In terms of the solvate theory of solution, we should expect the 

 absorption of the solution of a slightly hydrated salt in general not to 



