DURING the last few years it has been established, on the basis of spectroscopic and other evidence, that certain metals, for example, the alkali metals and mercury, are capable of forming di-or poly-atomic molecules in the vapour state. It is also known that the vapours of certain pairs of metals (for example, sodium-potassium, rubidium-caesium, and their analogues, also magnesium + alkali, and mercury + alkali) contain molecules of volatile intermetallic compounds. Both types of molecule are most conveniently studied by their absorption spectra, and most of them have been discovered in this way.