SUMMARY AND GENERAL DISCUSSION OF RESULTS. 91 



of different bands after the excitation has ceased is very different. No relations 

 have, however, been found between fluorescence and phosphorescence and the 

 conductivity, so that the current theories are based on the assumption that 

 the light centers are very complex molecular aggregates (Lenard and Klatt) 

 and that the dissociation or recombination effects take place within these 

 aggregates. 



The absorption spectra of organic compounds supports the view that the 

 absorbing centers in these cases are generally complex. These centers are 

 called chromophores, and as a rule they form only a part of the molecular 

 structure. Unfortunately the absorption spectra of chromophores are not 

 characteristic, so that the whole study is more or less a colorimetric one. 

 The peculiar conditions under which the absorption centers are active has 

 been assumed to be the same as those accompanying phenomena that are 

 explained by the theory of dynamic isomerism, and consist in a supposed 

 change of the interlinking of radicles in the organic compounds. If valency 

 is of an electromagnetic nature, a condition of dynamic isomerism would also 

 be a condition of intramolecular ionization. 



The nature of the absorbing centers of inorganic salt solutions is probably 

 somewhat similar to the absorption centers found in the case of solutions of 

 organic compounds, the phosphorescent compounds of the rare earths studied 

 by Goldstein, Kowalski and others. Every characteristic absorption spectrum 

 will be considered as evidence for the existence of a compound, and these 

 compounds will be called aggregates and may be assumed to consist of one 

 or more molecules or ions of the dissolved salt, and one or more molecules of 

 the solvent. That the number of these aggregates seems to be quite large is 

 no evidence against the theory of aggregates, since there is no reason why 

 an element like uranium should not form a very large number of compounds. 



As a typioal example of an aggregate, we will take a mixture of two kinds 

 of dissolved compounds in a mixture of two solvents. Assuming that the 

 UO2 group can act as an ion and that it carries a double charge, a general 

 formula for aggregates would be the following: 



+ 



a;{U0 2 S0 4 }^{H 2 S0 4 }w{U0 2 }y{H}w;{S0 4 }a{H 2 0} 



4- 



x' { U0 2 S0 4 [ y' \ H 2 S0 4 [ u' { UO, } v' { H } w' { S0 4 } 6 { CH,OH } 



We may assume that at ordinary concentrations u, v, w, u' , v', and w' 

 have small values. Whenever a definite and characteristic absorption spectrum 

 is obtained it will be assumed that the absorption centers are aggregates of 

 a definite composition, and that all the coefficients in the above equation have 

 a definite value. 



From the fact that absorption centers are found in solids such as the 

 various glasses and crystals, it must be assumed that the existence of active 

 absorption centers need not be connected with any conducting particles. On 

 the other hand, the work of Becquerel and others seems to indicate that 

 the number of absorbing centers is much smaller than the number of atoms 

 present. If the aggregates are very complex then the number of absorbing 

 centers would be much less than the number of molecules of the colored salt. 



