English organic chemist whose research in substitution reactions of naphthalene was a major service to the synthetic-dye industry.

Armstrong studied at the Royal College of Chemistry, where he developed a method of determining organic impurities (sewage) in drinking water, which was used in the sanitary surveys of water supplies and helped to control typhoid fever. In 1867, he went to study under Hermann Kolbe at the University of Leipzig in Germany and secured his Ph.D. there in 1870. In 187l he became professor of chemistry at the London institution Finsbury Circus (later Finsbury Technical College) and in 1879 was appointed to help organize the teaching of chemistry and physics at the City and Guilds of London institute. From 1884 he taught at the Central Technical College, retiring in 1911. He had been elected a fellow of the Royal Society in 1876.

Armstrong's classic researches in chemistry dealt in part with subsitution in the hydrocarbon naphthalene, a problem on which he published some 60 treatises, which, with the work of his collaborators, securely established the chemistry of a substance and its derivatives that were of the greatest technical importance in the dyestuffs industry. He also pioneered in organic crystallography, contributed much to the understanding of the chemical composition of camphor and related terpene compounds, and devised a centric formula for benzene.