d, like most of the rooms behind closed doors,
comparatively free from dust. The students, it appeared, had sat with
their backs to the door, facing a low platform, but their seats and the
lecturer's table and equipment had been removed. The two side walls bore
inscriptions: on the right, a pattern of concentric circles which she
recognized as a diagram of atomic structure, and on the left a
complicated table of numbers and words, in two columns. Tranter was
pointing at the diagram on the right.

[Illustration]

"They got as far as the Bohr atom, anyhow," he said. "Well, not quite.
They knew about electron shells, but they have the nucleus pictured as a
solid mass. No indication of proton-and-neutron structure. I'll bet,
when you come to translate their scientific books, you'll find that they
taught that the atom was the ultimate and indivisible particle. That
explains why you people never found any evidence that the Martians used
nuclear energy."

"That's a uranium atom," Captain Miles mentioned.

"It is?" Sid Chamberlain asked, excitedly. "Then they did know about
atomic energy. Just because we haven't found any pictures of A-bomb
mushrooms doesn't mean--"

She turned to look at the other wall. Sid's signal reactions were
setting away from him again; uranium meant nuclear power to him, and the
two words were interchangeable. As she studied the arrangement of the
numbers and words, she could hear Tranter saying:

"Nuts, Sid. We knew about uranium a long time before anybody found out
what could be done with it. Uranium was discovered on Terra in 1789, by
Klaproth."

There was something familiar about the table on the left wall. She tried
to remember what she had been taught in school about physics, and what
she had picked up by accident afterward. The second column was a
continuation of the first: there were forty-six items in each, each item
numbered consecutively--

"Probably used uranium because it's the largest of the natural atoms,"
Penrose was saying. "The fact that there's nothing beyond it there shows
that they hadn't created any of the transuranics. A student could go to
that thing and point out the outer electron of any of the ninety-two
elements."

* * * * *

Ninety-two! That was it; there were ninety-two items in the table on the
left wall! Hydrogen was Number One, she knew; One, _Sarfaldsorn_. Helium
was Two; that was _Tirfaldsorn_. She couldn't remember which ele