BAROMETRIC LIGHT, the luminous glow which appears in the vacuous space above the mercury in a barometer tube when the tube is shaken. It is a special case of the general effect that, if a sealed tube containing mercury and a rarified gas be shaken, flashes of light are produced. With the gas neon bright flashes of red light can be excited by shaking it with mercury, even when the pressure of the gas is atmospheric.
The barometric light was first observed in 1675 by Jean Picard, a scholar of Gassendi's, who noticed flashes of light in the Torri cellian vacuum when his barometer was moved about. It was made the subject of many experiments by Francis Hauksbee, who pub lished an account of his work in 1709. He proved that the phe nomenon was produced by the motion of mercury in either a glass or a varnished vessel, so that the glass was not a necessary factor, and that a very low pressure of the air was not necessary, as the light was strong when the air in contact with the mercury was at a pressure of an inch or two of mercury. He recognized that the rarefaction of the air was one of the conditions of the phenom enon, and he further stated that the light was electrical in origin, a conclusion suggested by the similarity in appearance of the light to that which he produced in exhausted vessels by the help of his electrical machine. The phenomenon roused much interest among his contemporaries. In 1745 Ludolf the younger clearly demon strated that the agitation of the mercury produced an electrifica tion of the barometer tube, by showing that the tube attracted, and then repelled, bits of paper in the familiar way. A little later Aepinus and Deluc brought the phenomenon again into notice, and further emphasized its electrical nature.
The barometric light is of interest as the first case of electrical discharge in gases at low observed in the laboratory. The electrification is probably produced partly by the movement of the mercury over the glass surface, partly by the splashing of the mercury. The fact that a newly prepared surface of pure mercury acquires a positive charge in contact with glass has been recently (1920) demonstrated by Perucca, while the electrical effects ac companying splashing of mercury, or the bubbling of gas through mercury, have been investigated by Duhme, following the pioneer work of Lenard. Slight impurities in the mercury have a consider able influence on these effects, and the barometric light, as ordina rily produced, is a complicated phenomenon which has not been analyzed in detail.
For the first detailed experiments, see F. Hauksbee, Physico Mechanical Experiments (1709). For the early investigations see J. Priestley, History of Electricity (5th ed., 1794) ; E. Gerland, Geschichte der Physik + '913). For the modern work on the potential differences at mercury surfaces, see A. Coehn, "Kontaktpotential" in Ergebnisse der exacten Naturwissenschaften (1922) .