XX. Note on the spectrum of sodium

On examining the spectra of different metals, there is one point which is striking in the extreme, viz., the absence of any very marked lines in the region between λ 7000 and λ 7600, which latter number we may take as the visible limit of the spectrum. With the exception of the well-known pair of lines of potassium, I am not aware that any lines in metallic spectra, which have been carefully studied, have been found below this limit, though recently, in the spectra of some of the rarer earths, I believe some few lines have been recorded. Having photographed the emission spectra produced in the arc of several metals, it appears, so far as examination has been made, that only those which can be volatilised at a low temperature have any lines in the infra-red region. Sodium is an example of this. It has a pair of lines at wave-length of about 8187 and 8199 of an intensity of about 3, taking the intensity of D lines as 10. It will be noted that the difference in wave-length between this pair is greater than that of the D lines. They do not seem to have any corresponding dark lines in the solar spectrum, though there are three faint lines which lie close to these wave-lengths.