"Professor," asked May, "are there more worlds with people on them like this one of ours?"
"That is a hard question," said he. "For many ages it was believed that there could be only one. More recently, when astronomers learned by the aid of their telescopes the countless number of the heavenly bodies, it began to be doubted whether such an immense creation could be destitute of intelligent creatures like man; and it was argued that most likely the Almighty had supplied the heavenly bodies with inhabitants, but had for some good reason thought best not to reveal the fact to us, perhaps because our attention might be too much drawn away from the truths that He wished us particularly to remember. At last, however, men of science, continuing their researches, seem to be settling back in the first opinion."
"Why is that?" asked Joe.
"Because they find reasons for thinking that our earth has had human beings on it only a very little while in comparison with its own existence. And if this world was millions of years without man, then, of course, any or all the heavenly bodies may still be without any such creature on them."
"Is there no better reason than that?" asked Joe.
"Yes, there is considerable evidence that the bodies nearest to us can not be inhabited by any creatures at all like man. On the moon, for instance, there is no air to breathe and no water to drink. And without air and water there can be no grass, trees, or plants of any kind, and no food for any animal. And besides starving, all creatures that we know of would immediately freeze to death; for the moon is excessively cold. The nights are about thirty times as long as ours, and allow each portion of its surface to get so cold that nothing could live."
"How did the moon get so cold?" asked Joe. "What became of the heat?"
"It went off into the surrounding space, which is all very cold. Empty space does not get warmed by the sun, whose heat seems chiefly to lodge in solid bodies and dense fluids."
"But some of the planets are larger than the moon, are they not?" asked Joe.
