The existence of intermetallic compounds in the vapour state. The spectra of the alkali metals, and of their alloys with each other

It has been uncertain until recently, whether any of the many intermetallic compounds which have been established in liquid and solid alloys can persist into the vapour state, or whether they invariably dissociate into mere mixtures of the individual metal vapours. Volatile compounds have occasionally been suspected (e. g ., between magnesium and zinc) on account of the deposition of typical crystals of compounds during the distillation of an alloy of the two metals, but such evidence is so indirect that it carries little weight. The most promising way of approaching this question is by a study of the absorption spectra of mixtures of metallic vapours. The metals that are known to possess a measurable proportion of diatomic or polyatomic molecules in the vapour state invariably indicate this property by the appearance of bands in their absorption spectra. In fact, since it has been accepted that band spectra cannot originate from single atoms, the observation of such bands has become the most delicate test for the existence of association in the vapours. For example, the usual methods of molecular weight determination have led to the belief that the alkali metal vapours are solely monatomic. This cannot, however, be true, as each of these metals possesses an extensive band spectrum, which appears at quite low vapour pressures. There can be no doubt, on the basis of this spectroscopic evidence, that an appreciable fraction of the molecules in the vapour of boiling sodium, etc., are di- (or poly-) atomic. This point will be discussed fully below.