C20H12O5

from The Collaborative International Dictionary of English v.0.48
fluorescein \fluo`res"ce*in\, fluoresceine \fluo`res"ce*ine\,
fluorescein \flu`o*res"ce*["i]n\, n. (Chem.)
   A yellowish red, crystalline substance, {C20H12O5}, produced
   by heating together phthalic anhydride and resorcin; -- so
   called, from the very brilliant yellowish green fluorescence
   of its alkaline solutions. It has acid properties, and its
   salts of the alkalies are known to the trade under the name
   of {uranin}.

   Syn: fluorescein, fluorescent dye, resorcinolphthalein.
        [1913 Webster]
    

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