CH. xxvi.] "SHIFTS." 369 



stances there is evidence to suggest that the refrangibility of 

 lines is slightly changed by change of chemical composition or 

 physical condition. My attention was first called to this by 

 Professor Stokes, who had observed that one of the bands in 

 chlorophyll changes its refrangibility when different solvents of 

 the chlorophyll are employed. 



Much more evidence of the same kind has in fact necessi- 

 tated the introduction of the word " shifts " to define these slight 

 changes of refrangibility, or want of coincidence, of apparently 

 the same line under different chemical and physical conditions. 



The following quotation from a paper by Herr W. Vogel 

 supplies a reference to some of the work done on this 

 subject : 



"It is known that the position of the absorption-band of a sub- 

 stance depends very essentially on the dispersion of the medium in 

 which it is dissolved or incorporated. One often observes that in 

 strongly dispersive media the absorption-bands of a substance are 

 displaced towards the red. 1 Now, the remarkable case often here 

 occurs that certain absorption-bands are displaced with the increase 

 of dispersion of the solvent, while others are not. Thus Hagenbach 

 observed that, e.y., the chlorophyll bands I. III. and IY. lie more 

 towards red in alcoholic than in etheric solution, while the band II. 

 in both solutions shows exactly the same position. I observed 

 similar cases with uranium protoxide salts 2 and with cobalt com- 

 pounds. 3 



" Now Kundt has already called attention to the fact, that for 

 absorption-spectra of gases the same rule holds good as for the 

 absorption-spectra of liquid substances. He adds, indeed : ' It is 

 only questionable whether, if, e.g. hyponitrate gas be mixed with 

 various other transparent gases, the displacements of the absorption- 

 bands are so considerable, that they can be perceived.' This doubt, 

 however, does not affect the rule supposed, but merely its experi- 

 mental verification. The supposition then is permissible that, in 



1 Kundt, Jubelband Pogg. Ann. p. 620. 



2 Vogel, "Pract. Spect. analyse," p. 248. 



3 Monatbr. der Akad. der Wiss. of May 20, 1878. 



B B 



