John Henry Pepper born 1821: science educator, popularizer, and showman; inventor of "Pepper's ghost," a stage special effect. [Thanks to Bill Palmer at Charles Darwin University for drawing my attention to Pepper.]
L. B. Magnusson and T. J. LaChappelle isolated first microscopic amount of a neptunium (Np, element 93) compound at wartime Metallurgical Laboratory, University of Chicago, 1944.
William Higginsdied 1825 (uncertain; birth date unknown in 1762 or 1763): atomic notions of chemical combination; advocate of antiphlogistic theory of combustion.