II.
Physical Description of Comets.
21
the tails are gaseous, the gaseous matter in the tails being the result of the volatilisation of the solid matter of the heads or of some of it. The connection between Comets and Meteors (to be unfolded in a later chapter) seems to imply the presence in comets of solid matter; and the spectroscope shows that gases also are a constituent of many, or of all, comets. To say what is the size of the solid particles is impossible: paving stones, brick-bats, and grains of sand have in turn been suggested by people fond of speculation.
