barometric light

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...