witness to the stand:

In the great work of Amédée Guillemin, already cited, we read:

"On the other hand, it seems proved that the light of the comets is,
in part, at least, borrowed from the sun. But may they not also
possess a light of their own? And, on this last hypothesis, is this
brightness owing to a kind of phosphorescence, or to the state of
incandescence of the nucleus? Truly, if the nuclei of comets be
incandescent, the smallness of their mass would eliminate from the
danger of their contact with the earth only one element of
destruction: _the temperature of the terrestrial atmosphere would be
raised to an elevation inimical to the existence of organized
beings_; and we should only escape the danger of a mechanical shock,
to run into a not less frightful

[1. Gratacap, "The Ice Age," in "Popular Science Monthly," January,
1818, p. 321.]

{p. 101}

one of being _calcined in a many days passage through an immense
furnace_."[1]

Here we have a good deal more heat than is necessary to account for
that vaporization of the seas of the globe which seems to have taken
place during the Drift Age.

But similar effects might be produced, in another way, even though
the heat of the comet itself was inconsiderable.