Fluorescein

C2H2O. Resorcin phthalein; substance prepared by heating phthalic phthalic anhydride with resorcin to 200° C.; prepared by red powder, soluble in alkalies and alcohol, imparting a wonderful fluorescence to the solution. By the action of bromine, chlorine, and iodine, it furnishes the magnificent red coloring matters known as the eosins.