a phenomenon of after-luminosity which he observed in hydrogen on passing a series of jar discharges through a small discharge tube with an open end, arranged inside 3, bell-jar. It was observed that at each discharge a steam of luminous gas was ejected from the end of the small discharge tube into the bell-jar. He sometimes observed a jet of blue luminosity, which was best developed at a pressure of IOO ram., and considered he had good evidence that this luminosity showed the hydrogen spectrum, but he found an unaccountable capriciousness of the effect, which sometimes did not appear at all. He failed to trace the cause of this uncertainty. Goldstein made similar experiments; he says that the spectrum consists of at least IO bands, from the green to the ultra-violet, totally unrelated to the hydrogen spectrum. But he believed the glow was due to pure hydrogen. With the same apparatus the spectrum of the glow is now obtained, and is found identical with that of active nitrogen passed over heated sulphur. The blue glow in hydrogen, then, is connected with the presence of sulphur as an impurity in the gas. On cooling in liquid air an annex of the vessel in which the glow is observed, so as to condense hydrogen sulphide (or any sulphur compound), the glow disappears in one