CHAPTER XII. 



GENERAL DISCUSSION OF RESULTS. 



Previous investigators have in many instances spoken of the absorp- 

 tion of light by molecules, ions, or aggregates of these. The present theory 

 of spectroscopy is, however, more and more inclined to consider the nega- 

 tive electron as the chief absorber of light in the visible and ultra-violet 

 portions of the spectrum. Whether the absorbing electron is the same as 

 the electron found in vacuum discharge tubes, or emitted by radioactive 

 elements, is at present a very much discussed subject. It seems to be 

 quite certain that the masses of many of the absorbers are not of molecular 

 magnitude. It is equally certain, as experiments show and as theory indi- 

 cates, that the various coefficients which define the equations of motion 

 of the absorber are functions of the conditions under which the absorbers exist. 

 If the absorbers are electrons we may think of them as being within or in 

 close proximity to the atom. Such absorbers would be expected to have 

 their period, coefficient of damping, and other coefficients greatly modified 

 by the formation of aggregates, solvates, etc., and such seems to be the 

 case. According to the present theory of the conductivity of solutions 

 it would be expected that if the absorbers existed in or about the atom, 

 their properties would be very greatly affected by the given atom existing 

 in an ionic or in a condition as part of a molecule. Consequently, at great 

 dilutions, it would be expected that the absorption of a colored solution of 

 a salt would be entirely independent of the salt when the anion was the 

 carrier of the absorbers. In the absorption spectra of solutions there are 

 but few examples where the absorption spectra of different salts with the 

 same cation are very different from each other. There is one important 

 exception in the case of aqueous solutions of the uranyl salts. The uranyl 

 nitrate bands are all of shorter wave-lengths than the bands of the other 

 uranyl salts. The absorption spectra of these salts have been photographed 

 by us over quite wide ranges in concentration, and no evidence has been 

 obtained that indicates any dependency of the wave-lengths of the uranyl 

 bands on the concentration. 



Although more and more spectroscopic phenomena are being explained 

 by means of the electron theory, yet there is a general tendency to consider 

 that only the electrons in a few atoms are in a condition to absorb or emit 

 light at any moment. What the nature of these conditions is, is at present 

 not well known, but it seems probable that they are exceptional states in 

 some cases at least. For instance, only a few of the sodium atoms take 

 part in the absorption of the D lines at any particular time, and the same 

 is probably true of neodymium and erbium salts in solution or in the solid 

 state. It is supposed by some physicists that absorption or emission may 

 take place during ionization, as, for example, when an electron leaves or 

 returns to an ion. Upon a basis such as this is laid the theories of dynamic 

 isomerism, isorepesis, and Stark's theory of the spectrum of canal rays 



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