"On the sunlit side, yes," said Kinney. "On the dark side it is as cold
as space itself--four hundred and sixty below, Fahrenheit."

They considered this in silence for some minutes. The builder went to
another window and looked at Venus, at that time about sixty million
miles distant, on the far side of the sun. They were intending to visit
"Earth's twin sister" on their return. After a while he came back to the
group, ready with another question:

"If Mercury ever was inhabited, then his day wasn't as long as it is
now, was it?"

"No," said the doctor. "In all probability he once had a day the same
length as ours. Mercury is a comparatively old planet, you know; being
smaller, he cooled off earlier than the earth, and has been more
affected by the pull of the sun. But it's been a mighty long time since
he had a day like ours; before the earth was cool enough to live on,
probably."

"But since Mercury was made out of the same batch of material--"
prompted the geologist.

"No reason, then, why life shouldn't have existed there in the past!"
exclaimed the architect, his eyes sparkling with the instinct of the
born antiquarian. He glanced up eagerly as the doctor coughed
apologetically and said:

"Don't forget that, even if Mercury is part baked and part frozen, there
must be a region in between which is neither." He picked up a small
globe from the table and ran a finger completely around it from pole to