VI. On the absorption-spectra of bromine and of iodine monochloride

Abstract

The element bromine and the compound iodine monochloride possess nearly the same molecular weight. The colours of their vapours appear almost identical, and a rapid glance at the complicated absorption-spectra afforded by the two gases fails to detect any difference between them. It becomes, therefore, a matter of importance to ascertain whether the molecules of the two bodies, when gaseous, vibrate identically or similarly. We have accordingly undertaken a series of exact measurements of the absorption-spectra of these two gases, the results of which we have now the honour to communicate to the Royal Society. The two spectra were compared simultaneously by means of one of Kirchhoff’s model spectroscopes, 4 flint-glass prisms of 60° and 45° and a magnifying-power of 40 being employed. The position of the lines and of the well-defined edges of the bands in both spectra were read off by reflexion upon a fixed arbitrary scale; the positions of 27 air-lines lying between the extremes of the absorption-spectra were then determined upon the same scale, both the scale and prisms remaining untouched during the different series of observations. In order to determine the wave-lengths of each of the absorption-bands in the spectra of bromine and iodine monochloride, the wave-length of each of the 27 air-lines was ascertained by reference to the measurements of Thalén; in three cases, marked H, in the accompanying Table the numbers given by Huggins have been used.