92 THE ABSORPTION SPECTRA OF SOLUTIONS. 



This would probably require very large values for x and x' , and, therefore, 

 makes this view improbable. On the other hand, it is not necessary to assume 

 that the aggregate is an active light center all the time, but that the aggregate 

 is only active while it is in a peculiar condition. This peculiar condition may 

 be assumed to be a kind of internal ionization, somewhat similar to that which 

 is assumed in the theory of dynamic isomerism, or by the theory of Lenard 

 and Klatt. For instance, electrons may pass back and forth within the UOo 

 group, or between the UO2 and surrounding groups. The system whose vibra- 

 tions give rise to the uranyl group is probably located in the uranyl radicle, 

 and possibly in the uranium atom itself. When an electron is ejected, or 

 when it recombines with the U0 2 group, it may be considered that the system 

 absorbs light and is then an active center. When this absorption takes place, 

 the frequency of the absorption bands and their width and intensity will 

 depend on the structure of the aggregate i.e., on the values of x, y, u, v, w, 

 and a. 



In a spectroscopic study of the aggregates found in salt solutions, it may 

 be said that the absorption spectrum indicates only a condition of a small 

 part of the salt. In other words, the values of .r, y, u, v, w, a, etc., may be 

 functions of the time. In order to solve this problem it will be necessary to 

 study the nature of the aggregates by other methods; and if widely different 

 methods give the same values for x, y, u . . ., etc., it may then be assumed 

 that the value of these variables is not a function of the time. 



In the present treatment it will be assumed that these spectroscopic 

 aggregates represent the statistical average condition of the dissolved salt 

 molecules, and although these aggregates may be active absorbing centers 

 for a small part of the time it will be assumed that the difference between an 

 absorbing and a non-absorbing aggregate, if such there be, is not due to any 

 change in the values of x, y, u, v, w, a, etc. The intensity of a given absorption 

 spectrum will be assumed to be a measure of the amount of the aggregate 

 present in the solution that corresponds to the given absorption spectrum. 

 In every known example it has been found that the changes in the values 

 of x, y, z, a, etc., produce changes that indicate that each characteristic spec- 

 trum is due to a system whose parts form one organic whole. 



SOLVATION. 



In the general formula for the structure of the absorbing centers, the 

 coefficient of the solvent part of the compound is represented as being con- 

 stant. The fact that in the examples studied each solvent is characterized 

 by a definite absorption spectrum, and that a salt dissolved in mixtures of 

 varying proportions of two solvents shows only two definite absorption spectra, 

 indicates that definite compounds of salt and solvent are formed. It is a very 

 remarkable fact that one solvent spectrum does not gradually change into 

 the other solvent spectrum, but that only the relative intensities of the two 

 spectra vary as the percentage of each solvent present is changed, and that 

 for a certain percentage of the two solvents the two sets of solvent spectra are 

 of approximately the same intensity. 



Taking as an example uranous bromide in water and methyl alcohol, we 

 have an equilibrium of probably the following type: 



