BAROMETRIC LIGHT

luminous glow appearing in the vacuum above the mercury in a barometer tube when the tube is shaken, first noticed in 1675 by a French astronomer, Jean Picard. The electrical discharge takes place with a variety of rarefied gases trapped in the tube (neon glows with its characteristic red light even at atmospheric pressure). The shaking is essential; electrification is probably produced both by the splashing of the mercury and by its movement over the glass surface. Impurities in the mercury considerably influence the phenomenon of barometric light, never exhaustively investigated. Francis Hauksbee in 1709 proved that the effect occurred in other than glass containers, did not require an extremely low pressure, and was similar to discharges produced by his electrical machine.