[f. CHROMO- 2 + -GEN.] (See quots.)
1858. Thudichum, The Urine, 328. The chromogen of indigoferous plants is a peculiar colourless substance.
1879. Watts, Dict. Chem., VIII. I. 696. The compound which requires only the presence of a salt-forming group to convert it into a dye-stuff may be called a chromogen (thus NO2 is the chromophore of nitraniline and nitrophenol, and nitrobenzene is their chromogen).
1882. Syd. Soc. Lex., Chromogen, a former term for a supposed vegetable colouring matter which is acted upon by acids and alkalis in producing red, yellow, or green tints.
Hence Chromogenic a.
