Concerning the bone, the _os coccygis_, there is an interesting
story in Midrash Kohelet (fol. 114, 3), which may be
appropriately inserted here. Hadrian (whose bones may they be
ground, and his name blotted out) once asked Rabbi Joshua ben
Chanania, "From what shall the human frame be reconstructed when
it rises again?" "From Luz in the backbone," was the answer.
"Prove this to me," said Hadrian. Then the Rabbi took Luz, a
small bone of the spine, and immersed it in water, but it was
not softened; he put it into the fire, but it was not consumed;
he put it into a mill, but it could not be pounded; he placed it
upon an anvil and struck it with a hammer, but the anvil split
and the hammer was broken. (See also Zohar in "Genesis," 206,
"A window shalt thou make to the ark" (Gen. vi. 16). Rabbi Amma says,
"It was a real window." Rabbi Levi, on the other hand, maintained that
it was a precious stone, and that during the twelve months Noah was in
the ark he had no need of the light of the sun by day nor of the moon by
night because of that stone, which he had kept suspended, and he knew
that it was day when it was dim, and night when it sparkled.
THE MIDRASHIM, _Bereshith Rabbah_, chap. 31.