The interaction in a galaxy between the stars and the interstellar medium (ISM) through mass loss, accretion, and the formation of new objects is exceedingly important for its evolution. The composition of its ISM tells us about its past history and, correctly understood, also about its future. The ISM represents the youngest generation and has therefore, and as a consequence of experience from our Galaxy, been considered to be basic in studies of the kinematics of external galaxies. It has played, and plays, a fundamental role in all discussions of the Magellanic Clouds. Historically, the Lick Expedition radial velocities of a few emission nebulae in the LMC and one in the SMC (see Chap. 1) served chiefly to prove that the Clouds were outside the main body of the Galaxy. An indication of a gradient of velocities in the LMC pointed towards rotation or translation. The existence of this gradient was put beyond doubt in the 1950s, when 21 cm HI line observations began. Since then, other components of the ISM have been observed, making important contributions regarding the composition and kinematics of the Clouds.

The neutral hydrogen

The continuous radio emission from the Magellanic Clouds was first detected by Mills in 1953. Radio isophotes at 3.5 m were obtained in 1954 (Mills 1954). The line radiation from neutral hydrogen in the Clouds was detected in 1953 and a preliminary survey of its distribution was presented (Kerr et al. 1954).