98 THE ABSORPTION SPECTRA OF SOLUTIONS. 



absorption spectra of uranyl nitrate and uranyl sulphate were photographed 

 at very great dilution. If dissociation is sufficiently complete it would be 

 expected that the uranyl nitrate bands would shift towards the red with 

 decrease in concentration, and the uranyl sulphate bands would shift towards 

 the violet, so that for very great dilution the uranyl bands of both salts would 

 be identical. This was not found to be the case. There appeared a very 

 slight shift of the uranyl nitrate bands to the red, and possibly a slight shift 

 of the uranyl sulphate bands to the violet; but these shifts were extremely 

 small; so small, indeed, that some observers who looked at the spectrograms 

 did not detect it at all. If the shifts occur, it is exactly what would be expected 

 if the uranyl nitrate and uranyl sulphate aggregates break up into complex 

 ions. The loss of an N0 3 group would cause the uranyl nitrate bands to shift 

 only slightly to the red if the aggregate was of some size, and the same would 

 be true of the uranyl sulphate aggregate. If this theory is true it indicates 

 that the aggregates possess quite a high degree of complexity. 



(10) The question has been asked as to whether the aggregates are definite 

 chemical compounds, whether they have a definite composition, and how 

 stable they are. Unfortunately the spectroscopic method itself can not solve 

 these problems, but by studying the composition of the acid aggregate precipi- 

 tates that are often formed, and the chemical composition and absorption 

 spectra of these, much light may be thrown on the subject. The other physical 

 and chemical properties of solutions containing aggregates should also be 

 studied. 



THE EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 



(1) The general effect of rise in temperature is to give a solution of an 

 inorganic salt a deeper color. This deepening of the color signifies that the 

 absorption of light has become more selective, and spectroscopic work indi- 

 cates that this selective absorption is usually due to a widening of the absorp- 

 tion bands. As these bands are never so distributed over the spectrum as to 

 give a colorless solution, it follows that a widening of the absorption bands will 

 intensify the color of the solution. In many cases this widening appears to 

 be quite unsymmetrical, but this need not necessarily mean that the center of 

 gravity of the individual absorption band is shifted. Many examples of the 

 neodj'mium absorption bands show this phenomenon very clearly. For 

 instance, the absorption due to the y and 8 groups of bands may be sufficiently 

 intense to make these groups appear as single bands. If the absorption is 

 not so intense as this, in some solvents it is found that with rise in temperature 

 the shortest wave-length bands may decrease in intensity, and may even dis- 

 appear. The long wave-length bands increase in intensity, and in some cases 

 new bands appear. Knowing this, it is easy to understand that if the absorp- 

 tion is so strong that each of these groups of bands appears as a single band, 

 these broad bands will widen very unsymmetrically towards the red with rise 

 in temperature. It may be that some change like this takes place in the case 

 of the uranyl bands. It is for this reason that a formula calculated for the widen- 

 ing of a band with rise in temperature would not apply to many wide bands. 



(2) In the case of all pure salts dissolved in a single solvent, the bands 

 have been found to widen with rise in temperature, and at the same time the 



