they found a way to escape to some other stellar system.
They could not travel as fast as light, and they could escape only if
they found some near-by solar system. Their star was dead--black. Let's
call it Nigra--the Black One--since like every other star it should have
a name. Any objection?"

There was none, so Arcot continued:

"Now we come to an impossibly rare coincidence. That two suns in their
motion should approach each other is beyond the point of logic. That
both suns have a retinue of planets approaches the height of the
ridiculous. Yet that is what is happening right now. And the Nigrans--if
that's the correct term--have every intention of taking advantage of
the coincidence. Since our sun has been visible to them for a long, long
time, and the approaching proximity of the suns evident, they had lots
of time to prepare.

"I believe this expedition was just an exploratory one; and if they can
send such huge machines and so many of them, for mere exploration, I'm
sure they must have quite a fleet to fight with.

"We know little about their weapons. They have that death ray, but it's
not quite as deadly as we might have feared, solely because our ships
could outmaneuver them. Next time, logically, they'll bring with them a
fleet of little ships, carried in the bellies of those giants, and
they'll be a real enemy. We'll have to anticipate their moves and build
to circumvent them.

"As for their ray, I believe I have an idea how it works. You're all
familiar with the catalytic effects of light. Hydrogen and chlorine will
stand very peacefully in the same jar for a long time, but let a strong
light fall on them, and they combine with terrific violence. This is the
catalytic effect of a vibration, a wave motion. Then there is such a
thing as negative catalysis. In a certain reaction, if a third element
or compound is introduced, all reaction is stopped. I believe that's the
principle of the Nigran death ray; it's a catalyst that simply stops the
chemical reactions of a living body, and these are so delicately
balanced that the least resistance will upset them."

Arcot halted, and sat puffing furiously for a moment. During his
discourse the pipe had died to an ember; with vigorous puffing he tried
to restore it. At last he had it going and continued.

"What other weapons they have we cannot say. The secret of invisibility
must be very old to them. But we'll guard against the possibility by
equipping our shi