Priestley's oxygen apparatus, 1774.
© Science Museum / Science & Society Picture Library
Reproduction. Joseph Priestley (1733-1804) used this apparatus to prepare oxygen by heating mercuric oxide floating on the surface of some mercury in an inverted phial in 1774. He established its properties in a long series of experiments with mice and plants. The single lens in the picture was Priestley's own. The bowl and the phial are modern reconstructions.