Section III deals with a phenomenon typical of much of interstellar space. When a gas is hot, velocity differences of the order of the sound speed are established. If the ionization later disappears and the gas cools, the same velocity gradients lead to strong gas compression. Upon reionization, the compressed gas may become visible as a filamentary H ii region. In particular, some very thin shells of ionized gas observed around planetaries might be the consequence of double ejections of planetary nebulae. A shell is the ''signature'' of the expansion of an older nebula into the ambient gas, with its gas distribution altered by the ejection and expansion of a second nebula. (AIP)