"Besides, if I'm not very much mistaken, Pouillet--another countryman of
yours, Ardan, and an Academician as well as Fourrier--esteems the
temperature of interplanetary spaces to be at least 256° Fahr. below
zero. This we can easily verify for ourselves this moment by actual
experiment."

"Not just now exactly," observed Barbican, "for the solar rays,
striking our Projectile directly, would give us a very elevated instead
of a very low temperature. But once arrived at the Moon, during those
nights fifteen days long, which each of her faces experiences
alternately, we shall have plenty of time to make an experiment with
every condition in our favor. To be sure, our Satellite is at present
moving in a vacuum."

"A vacuum?" asked Ardan; "a perfect vacuum?"

"Well, a perfect vacuum as far as air is concerned."

"But is the air replaced by nothing?"

"Oh yes," replied Barbican. "By ether."

"Ah, ether! and what, pray, is ether?"

"Ether, friend Michael, is an elastic gas consisting of imponderable
atoms, which, as we are told by works on molecular physics, are, in
proportion to their size, as far apart as the celestial bodies are from
each other in space. This distance is less than the 1/3000000 x 1/1000',
or the one trillionth of a foot. The vibrations of the molecules of this
ether produce the sensations of light and heat, by making 430 trillions