"Sir," replied Ardan's antagonist, "there are many and
incontrovertible reasons which prove the absence of an
atmosphere in the moon. I might say that, _a priori_, if one
ever did exist, it must have been absorbed by the earth; but I
prefer to bring forward indisputable facts."

"Bring them forward then, sir, as many as you please."

"You know," said the stranger, "that when any luminous rays
cross a medium such as the air, they are deflected out of the
straight line; in other words, they undergo refraction. Well!
When stars are occulted by the moon, their rays, on grazing the
edge of her disc, exhibit not the least deviation, nor offer the
slightest indication of refraction. It follows, therefore, that
the moon cannot be surrounded by an atmosphere.

"In point of fact," replied Ardan, "this is your chief, if not
your _only_ argument; and a really scientific man might be
puzzled to answer it. For myself, I will simply say that it is
defective, because it assumes that the angular diameter of the
moon has been completely determined, which is not the case.
But let us proceed. Tell me, my dear sir, do you admit the
existence of volcanoes on the moon's surface?"

"Extinct, yes! In activity, no!"

"These volcanoes, however, were at one time in a state of activity?"

"True, but, as they furnish themselves the oxygen necessary for