The absorption spectra of vapours of the alkali metals

Abstract

Prof. R. W. Wood, of Johns Hopkins University, has investigated the absorption spectrum of sodium vapour and has shown that its most characteristic feature is the appearance of lines of the principal series of Rydberg and Kayser and Runge. He was able, by employing a spectrograph of considerable dispersion, to observe and measure the wave-lengths of 48 lines of the principal series, thus giving by far the most complete series that has yet been observed. Wood stated, in a paper of his on “The Anomalous Dispersion of Sodium Vapour,” that sodium vapour behaved in a very special manner. It appeared that a cloud of sodium vapour could preserve its boundaries in a closed tube, even when the rest of the tube was exhausted to a very high vacuum, and that in this way sodium was peculiar, other metals showing no corresponding behaviour. This conclusion is, however, not correct. The author of the present communication has shown that the sodium cloud does not exist in a vacuum, but that it behaves like any ordinary vapour, diffusing in a vacuous space instantaneously from a heated to a colder part of the containing vessel. The enormous optical density of the vapour for light in the region of the D lines gives the sodium cloud an appearance of density which is very deceptive. It has also been shown that potassium and lithium behave very much like sodium, so that similar effects of anomalous dispersion have been observed at some of the principal series lines in the case of these two metals. In the investigation of potassium vapour for anomalous dispersion, the author found that the absorption spectrum of the vapour was of great interest in itself, as it showed a number of the lines of the principal series, together with some channelled space spectra, but not lines of the associated series or lines which are not connected with any series.