The 20th century saw continued progress in cometary science. Spectroscopy revealed many of the molecules, radicals, and ions in the comae and tails of comets. An understanding began to develop about the nature of cometary tails, with the ion (Type I) tails resulting from the interaction of ionized molecules with some form of “corpuscular radiation,” ppossibly electrons and protons, from the Sun, and the dust (Type II) tails coming from solar radiation pressure on the fine dust particles emitted from the comet.