52 THE ABSORPTION SPECTRA OF SOLUTIOX>. 



One of the purposes of the present research is to study one group of 

 bands throughout a long series of reactions. The first group of bands to be 

 studied in this way was the uranyl bands. Although not completely followed, 

 and not identified with certainty in every case, yet for aqueous solutions, at 

 least, it has been possible to designate these bands as a, b, c, d .... and to 

 follow each band as the salt is changed from acetate to nitrate, from nitrate 

 to chloride, from chloride to sulphate, etc. It is found in all these changes 

 that the individual uranyl bands undergo a gradual shifting of their 'position 

 in the spectrum. Various chemical changes have been accompanied by charac- 

 teristic changes in the absorption spectrum; e.g., a narrowing of the bands, a 

 shifting of the bands to the violet, etc. It is natural to try to correlate as 

 many of these characteristics as possible, but as yet not enough work has 

 been done to furnish many generalizations. It may be said, qualitatively, that 

 in many instances a shifting of the bands towards the violet is often accom- 

 panied by a narrowing and sharpening of the edges of the bands. This is a 

 general characteristic also of the temperature effect, since a lowering of the 

 temperature of an absorbing compound is nearly always accompanied by a 

 narrowing of the absorption bands and a slight shifting towards the violet. 



Again, it is natural to inquire whether the same chemical reaction produces 

 the same changes in the absorption spectra when this reaction takes place 

 in different solvents. Unfortunately there are not many examples of this 

 kind that can be tested. In general, the absorption bands of different salts 

 in the same solvent are practically the same, so that a chemical reaction 

 can not be photographed spectroscopically. It was for this reason that a 

 considerable number of absorption spectra have been mapped. A number 

 of solvents showing a difference in the absorption spectra of different salts 

 are not suited very well for this kind of work. Some very good examples, 

 however, have been found, and these will soon be studied. Acetone was 

 found to give a greater difference between the absorption spectra of uranyl 

 nitrate and the other uranyl salts than water; and as the uranyl acetone 

 bands are very sharp and these solutions can be studied at very low temper- 

 atures, this solvent offers exceptional advantages for making a comparison 

 between the same chemical reactions in the two solvents. 



The closely related problem as to whether apparently similar chemical 

 reactions for different salts in the same solvent are really similar has already 

 been answered in some cases in the negative. We have seen that the chem- 

 ical reaction that takes place when uranyl chloride is changed to uranyl 

 nitrate has resulted in a narrowing and sharpening of the uranyl bands, 

 and a gradual shifting of these bands towards the violet. The same reaction 

 of neodymium chloride to neodymium nitrate in water results in the bands 

 being made broader and more diffuse, and in some cases these bands are 

 slightly shifted towards the red. 



It must be stated that the study of the changes that take place in the 

 neodymium bands has been too complicated for the present research. No 

 other element shows such an extremely wide diversity in the structure of its 

 various groups of bands, and at present no examples have been found where 

 any bands can be traced throughout chemical reactions. The changes seem 

 rather to be connected with a change in the relative intensity of bands in the 



