88 M. E. Wiedemann's Investigations on 



cules almost never continue undisturbed ; in the absorption- 

 spectra (to the examination of which we are here almost ex- 

 clusively directed), sharp absorption-streaks appear only in 

 isolated cases— with the salts of uranium, didymium, and sub- 

 stances of analogous composition to potassio-chromic oxa- 

 late. The last-mentioned shows in its absorption-spectrum, 

 beside a broad band in the orange and yellow, a sharp black 

 line in the red. The absorption of single rays or groups of 

 rays corresponds to the vibrations of the atoms composing the 

 molecules or their proximate constituents. Hence, if a group of 

 atoms occurs in different combinations, it calls forth in general 

 the absorption-spectrum corresponding to it (nitrocompounds, 

 chromates), which, however, may be more or less modified by 

 the presence of other atoms with those occurring in the mole- 

 cules. Thus all the salts of didymium show on the whole the 

 same absorption-spectra; but the individual lines are some- 

 what displaced, sometimes towards the violet, sometimes 

 towards the red, according to the nature of the acid. Upon 

 the atoms of the absorbent part of the didymium group the 

 acid evidently exerts an attraction which does not extend 

 equally to all of them, and therefore does not produce a mere 

 shifting of the centre of gravity of the system. Then, accord- 

 ing to the position of the acid and the strength of its attrac- 

 tion, the vibration-period may be increased or lessened. 



Nevertheless this is not always the case. Thus experiments 

 made at my suggestion by M. Pierce, with the chromic oxa- 

 lates of potassium, sodium, lithium, and silver, showed that 

 the sharp streak in the red, quite independently of the nature 

 of the shifting constituent, the metal, always retains the same 

 position, even when the solution is heated. We must there- 

 fore admit that the action of these metals upon the absorbing- 

 group of atoms is unperceivable. 



Pleochroism. — In a similar way may pleochroism be ex- 

 plained. The vibrations which take place in the molecules 

 that build themselves together in a definite manner to form 

 crystals are affected very differently by the neighbouring mo- 

 lecules, according as they take place in one or the other direc- 

 tion; or else, in molecules thus arranged, the motions in dif- 

 ferent directions have from the beginning different fundamental 

 vibrations. To derive pleochroism at once from differences in 

 the dispersion of the different rays is surely not admissible, 

 since the different dispersion is, according to the newer theo- 

 ries, just a consequence of difference in the strength of the 

 absorption ; on the contrary, a connexion between the two 

 quantities is certainly to be expected. 



