The after-luminosity of electric discharge in hydrogen, observed by Hertz

In previous papers I have examined certain striking cases of after-luminosity in gases through which the electric discharge has been passed. The cases dealt with fall under two heads, those due to ozone, and those due to active nitrogen. I wish now to pass to case in which neither of these substances is concerned. Hertz described a phenomenon of after-luminosity which he had observed in hydrogen. The method of investigation was somewhat special. A series of jar discharges was passed through a small discharge tube, with an open end, arranged inside a bell jar. It was then observed that at each discharge a stream of luminous gas was squirted from the end of the small discharge tube into the bell jar. This is apparently due to kind of explosive action of the spark—the same, probably, as that described by De La Rue and Muller. The method is well adapted to show the afterglow in other gases, nitrogen or air, for instance, but the immediate concern is with hydrogen. With this gas, Hertz sometimes observed a jet of blue luminosity, which was best developed at a pressure of 100 mm. He considered that he had good evidence that this luminosity showed the hydrogen spectrum, but be found an unaccountable capriciousness of the effect, which sometimes refused to appear at all. He did not succeed in tracing the cause of this uncertainty. Goldstein made similar experiments; he states that the spectrum consists of at least 10 bands, from the green to the ultra-violet, totally unrelated to the recognised hydrogen spectrum. But he believed that the glow was due to pure hydrogen.