The molecules CS and NH3 are expected on theoretical grounds to trace high density interstellar gas in a similar fashion. However, observations show that CS is often widespread while NH3 appears concentrated in dense cores. This result is interpreted here in terms of a clumpy model of molecular clouds, and a differential rate for chemistry of the two species. Crucial to this model is the accreting role of the dust particles. These CS cores are unresolved and numerous; most of them probably disperse rather than collapse to form a star.
