EFFECT OF TEMPERATURE ON ABSORPTION SPECTRA. 83 



THE EXISTENCE OF AGGREGATES AND THEIR PROPERTIES, AND 

 THE EFFECT OF RISE IN TEMPERATURE ON THE AGGREGATES. 



The presence of free acid 1 or of other salts has been found, in many cases, 

 to modify very considerably the uranyl, uranous, and neodymium bands of 

 a given salt in solution. Up to the present, work of this kind has been practi- 

 cally restricted to aqueous solutions, although similar changes take place in 

 other solvents. One of the most pronounced cases is that of the uranyl nitrate 

 and the uranyl chloride bands. Most of the nitrate bands have shorter wave- 

 lengths than the chloride bands. As increasing amounts of hydrochloric acid 

 are added to an aqueous solution of uranyl nitrate, the nitrate bands are found 

 to shift gradually into the position of the chloride bands. Furthermore, the 

 addition of nitric acid to an aqueous solution of uranyl nitrate causes most 

 of the uranyl bands to shift towards the violet, whereas the addition of hydro- 

 chloric acid to an aqueous solution of uranyl chloride causes most of the bands 

 to shift towards the red. These changes are quite different from those that 

 take place when the solvent is changed; and if it is supposed that a character- 

 istic absorption spectrum corresponds to a more or less stable compound, 

 then these changes indicate a series of compounds which will be referred to 

 as aggregates. We would have, then, nitric acid aggregates of uranyl nitrate, 

 or hydrochloric acid aggregates of neodymium chloride. 



Whether there is an actual change in the frequency of vibration for a 

 series of uranyl or uranous aggregates, or whether there is simply a relative 

 change in the intensity of a number of finer bands which blend into the 

 rather broad and diffuse bands that are photographed, can not be decided at 

 present. In the case of neodymium salts 2 the various spectrophotographs 

 indicate the latter effect. It is unlikely that the case can be settled very easily 

 for uranyl and uranous salts, since the greatest shifts take place in the absorp- 

 tion of aqueous "solutions, and these can not be studied at very low tempera- 

 tures. Subsequently, it will be assumed that a spectrogram of a chemical 

 reaction showing a gradual shifting of bands indicates the presence of a series 

 of closely related aggregates existing in the solution. 



The mixture of varying proportions of two neutral salts in a solution may 

 result in a gradual change from the bands of one salt into the bands of the 

 other salt (mixtures of uranyl nitrate and uranyl chloride in water). On the 

 other hand, there are cases in which each salt seems to have its own definite 

 spectrum. In this case there will be no shifting of the bands but only a change 

 in intensity, and we may assume that, in this case, there are no double salts 

 formed. In the former case, however, it seems probable that there are aggre- 

 gates formed which contain one or more molecules of each salt. 



The addition of salts 3 containing the same cation as the corresponding 

 uranyl or uranous salt, has an effect similar to that of the addition of free 

 acid, and may be considered as indicating the presence of aggregates. In the 

 case of uranyl chloride the effect seems to be due largely to the chlorine present. 



Acid aggregates of uranous salts are found to be much more stable than 

 the neutral aggregates. Uranous nitrate, for example, is very unstable, but 



1 Phvs. Zeit., 11, 66S (1910), 12, 269 (1911). 



2 Ibid., 11, 671 (1910), 12,269 (1911). 



:i Publication No. 130, Carnegie Institution of Washington, p. 91. 



