crystal buildings. They
were in the suburbs of Great New York. Within ten minutes the conveyor
terminal would be reached.

Hilary's eyes flicked speculatively to the tiny cigar-shaped boat in
which the dead guard had flown down to them. Its smooth gray-gleaming
surface was devoid of wings or other lifting devices. Only a
fan-shaped fin projected from the stern like the tail of a fish. The
cockpit, if such it could be called, was tiny, just ample enough to
accommodate the Mercutian's girth. The sunlight dazzled back from a
bewildering jumble of tiny lenses inset in the instrument board.
Arranged along the hull, on either side, were larger disks of the same
quartz-like material.

"Let's get away in the flier," he said.

"Can't," Grim said. "Those lenses you see on the instrument board are
the controls. No one knows how to operate them except the Mercutians.
Our people managed to capture a few, but couldn't do a thing with
them."

Hilary stared at the motionless flier with interest. "What are those
round glass disks stretched along the hull in a double row?" he asked.
"They look like burning glasses."

"That's just what they are," said Grim sadly. "The top row are
sun-lenses, that throw a terrible ray for a distance of two to three
hundred feet. Melts everything in its path--men trees, rocks even. You
saw one in action in the sun-tube with which poor old Peabody was cut
in half. The lower row of lenses on the flier are search beams."

"Search beams?" Hilary echoed inquiringly.

"Yes. They act like X-rays, more powerful though, and with the further
property of rendering everything they touch transparently crystal for
depths of ten to fifteen feet. Lead is the only element they can not
penetrate. Another secret our scientists can not fathom, so they talk
learnedly about the stream of rays polarizing the structure of matter
along a uniaxis."

"Can't those lenses be duplicated, and turned as weapons against the
Mercutians?"

"No. They are made of a peculiar vitreous material native to Mercury."

"And no one has found out the principle on which they work?"

"Well, there have been theories. We haven't many scientists left, you
know. But the most popular one is that these lenses have the power of
concentrating the rays of the sun to an almost infinite degree, and
then spreading them out again, each individual beam with the
concentrated energy of the whole. Some new way of rearranging quanta
of energy."

"Hmm!" H