Fluorescent excitation of mercury by the resonance frequency and by lower frequencies.―IV

Abstract

The present paper deals entirely with “core” excitation of mercury vapour, that is to say, fluorescent excitation by the atomic resonance line λ 2537, the primary action of which is, without doubt, to produce 2 3 P 1 excited atoms in the vapour. 1. Experimental Arrangements. The source was a small commercial mercury lamp in silica, of the standard pattern made by the Thermal Syndicate of Wallsend. It was immersed in a tank of water, kept cool by a constant flow through. A magnetic field from a bar electromagnet kept the discharge pressed forward. The magnet was placed vertically above the lamp, and the iron core alone dipped a short distance into the water. The beam made parallel by a quartz condenser was filtered through 5 cm. of saturated bromine vapour and 15 cm. of chlorine gas at atmospheric pressure. This combination of filters approximately isolated the resonance line. It is true that the group of lines near λ 2652 was largely transmitted, but the other strong mercury lines between this and the green line were completely suppressed. Reproduction No. I is a direct spectrogram of the source thus filtered. The exposure has been pushed as far as possible, short of fogging the plate by the diffuse light of the instrument, and it is seen that even so no trace of the strong mercury lines is obtained. The importance of this will appear later.