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Published online by Cambridge University Press: 14 August 2015
Five years ago, at the previous IAU Symposium (No. 60) on the galaxy, most of us pictured the interstellar medium (ISM) as a mainly quiescent medium which evolved by orderly processes–the “steady state” ISM. Radhakrishnan reviewed the “intercloud medium” which was warm (some few thousand degrees), rarefied (some few tenths cm−3), uniformly - distributed, partly ionized, and responsible for the smoothly-distributed pulsar dispersion measures and Faraday rotations. Heiles reviewed the “clouds,” which had densities of tens cm−3, temperatures of tens K, and nonspherical shapes. These two components were in approximate pressure equilibrium; Grewing, in his review of heating mechanisms, emphasized pervasive, nonvarying processes.