102 Plant Pigments [March, 



and chemical Constitution began to attract attention. Witt's chro- 

 mogen-chromophore theory, as well as the quinone theory of Arm- 

 strong, held absolute sway f or many years ; and though their use- 

 fulness is far from exhausted, the tendency at the present time 

 seems to be to rely less on the influence of the radical in the altera- 

 tion of color, and more on the relationship of color to the absorp- 

 tion spectra produced. 



As is well known, colored substances exhibit the phenomenon 

 of selective absorption; that is, whenever a body vibrates so as to 

 emit waves of certain definite periods, any waves of these periods 

 falling upon the body will be absorbed. This gives rise to the ab- 

 sorption spectra that have so often been of use in the Identification 

 of complex colored Compounds. Introduction of radicals into a 

 Compound, transforming it from a colorless to a colored substance, 

 with consequent exhibition of absorption in the visible part of the 

 Spectrum, may be explained by assuming that the oscillation-fre- 

 quency has been altered; for example, benzene, though colorless 

 shows absorption bands in the ultra-violet portion. Introduction 

 of the azo group, — N = N — , gives red azo-benzene, with absorp- 

 tion bands in the visible part of the spectrum. What apparently 

 occurs in this case is a change of the short wave-length with its high 

 oscillation-frequency (as found in the ultra-violet region) into a 

 longer wave-length and a consequent slower oscillation-frequency. 



Application of the inductive method in attempts to draw general 

 conclusions has been but partially successful. Even early in the 

 course of these studies it was recognized that unsaturation in a Com- 

 pound is essential to the development of color. Attempts were also 

 made to trace relationships between the molecular weights of Com- 

 pounds and the probable colors produced. This culminated in 

 Nietzski's rule : " The simplest colored substances are in the green- 

 ish yellow and yellow, and with increasing molecular weight the 

 color passes to orange, red, violet, blue and green." Like most of 

 the theories in this field, this is at best highly imperfect. 



Undoubtedly the most fruit ful theory which has so far been 

 advanced connecting color with chemical Constitution is that due to 

 Witt.^ He considered color to be due to the presence of a " chro- 

 mophore" group in the molecule. The resulting "chromogen" 



5 Witt : Ber. d. d. ehem. Gesell., 1876, ix, p. 522 ; 1888, xxi, p. 325. 



