THE failure, thus far, to detect gaseous emission lines from circumstellar dust clouds1–3 and from Bok globules2 raises the possibility that such clouds consist solely of dust grains. The possibility has previously been largely discounted on the grounds that relative diffusion of the grains and gas is slow and inefficient requiring time scales which are too long and resulting in only partial separation of the components4,5. This communication points out that a process of forced diffusion will occur in clouds of grains and gas in which stars have formed and will result in a complete separation of the components in a relatively short time.