Although Comets have a smaller mass than any other cosmical
bodies--being, according to our present knowledge, probably not equal
to 1/5000th part of the earth’s mass--yet they occupy the largest
space, as their tails in several instances extend over many millions of
miles. The cone of luminous vapour which radiates from them has been
found in some cases (as in 1680 and 1811) equal to the length of the
earth’s distance from the sun, forming a line that intersects both the
orbits of Venus and Mercury. It is even probable that the vapour of
the tails of comets mingled with our atmosphere in the years 1819 and
1823.--_Humboldt’s Cosmos_, vol. i.