100 THE ABSORPTION SPECTRA OF SOLUTIONS. 



the neodymium bands are shifted to the red with rise in temperature, whereas 

 aqueous solutions of pure neodymium chloride do not show this effect at all. 

 Exactly what takes place in this case is not evident. 



(6) The effect of rise in temperature on solutions showing the solvate 

 bands of equal intensity has been to cause a change in the intensity of the 

 solvate bands; very little if any change, however, in the wave-lengths of the 

 bands takes place. In the case of water and alcohol, the alcohol bands increase 

 in persistency as the temperature is raised. 



(7) In the work with the closed cell at high temperatures it was found 

 that precipitates were formed in practically every case, probably due to hy- 

 drolysis, alcoholysis, etc., precipitation taking place in dilute as well as in con- 

 centrated solutions. Several examples were tested of concentrated solutions 

 of colored salts mixed with calcium or aluminium chloride, and precipitation 

 was found to take place under these conditions at comparatively low tem- 

 peratures. It would be very interesting to learn whether acid aggregates of 

 uranyl, neodymium, erbium and such salts are less likely to form precipi- 

 tates than the neutral salt solutions. 



In the case of neutral uranous solutions these precipitates form at 70 

 or 80. The presence of acid prevents this precipitation at temperatures 

 below 100. 



Whether the salt precipitation at high temperatures is complete or not, 

 can not be decided in general at present. In the case of several neodymium 

 and erbium solutions this seemed to be the case. In one of the uranyl solu- 

 tions, however, some uranyl salt remained in the solution after the precipitate 

 had settled. 



(8) It may be said, in general, that there is a very great increase in the 

 absorption of all solutions in the short wave-length region of the spectrum 

 as the temperature is raised. How great this increase in absorption would 

 be if pure solutions were used has not yet been determined. The formation 

 of precipitates is usually preceded by a very great increase in the short wave- 

 length absorption. 



(9) The problem as to whether a rise in temperature produces a perma- 

 nent change in the structure of the aggregates in solution has not been studied 

 to any great extent. In the case of the existence of two solvent spectra it is 

 found that the spectra on cooling the solution are exactly the same as before 

 heating. In the case of an acetone solution of uranyl chloride it was found 

 that the bands are apparently single after the precipitate was formed during 

 the heating. Whether these bands would have become double again when the 

 solution was cooled was not tested. It would be interesting to learn whether 

 selective solvate precipitation would take place on heating solutions. 



(10) It seems worth while in conclusion to call attention to the promising 

 and important application of spectrophotography at low temperatures to 

 the study of the nature of chemical reactions. As soon as the changes in 

 breadth, intensity, and frequency of individual absorption bands and the 

 constitution of the groups of bands can be interpreted, our knowledge of the 

 relation between absorption centers or aggregates and the other physical and 

 chemical units of matter will be much more fully understood; and the changes 

 which these particles undergo will be much better comprehended. 



