Basic laboratory studies of grains.

Laboratory investigations of grain mantles are shown to have a direct application to observations of interstellar grains. The experiments have made it possible to show how infrared spectra may be used as a probe not only of the grains as they are but also as they have evolved both chemically and physically. Growth by accretion, ultraviolet photoprocessing, gradual heating, explosions, and the general effects of grains on interstellar chemistry provide a consistent basis for understanding the evolution of grains in terms of interactions with the gas, and ultraviolet radiation in both diffuse and dense regions of space.