oor. "But why?" he asked himself. "Why in this
particular building?"

He strode out of the room and inched his way across the outer room
toward the front door, again narrowly missing the open trap door.

Once outside, he made his way along the side of the building in the
direction that Morgan had taken. When he reached the corner, he could
see the black bulk of the _Space Knight_ a hundred yards away. He ran
toward the base of the ship and met Morgan coming toward him.

"Find anything, Sergeant?" he called.

"Nothing, sir," replied Morgan. "The ship is ready to blast off and her
cargo holds are full. But that's all."

"Full of what?"

"I couldn't see, sir. The main hatch was locked and I could only see
through the viewport. But it just looked like general cargo to me."

"Couldn't have been crystal?"

"It might have been, sir. It was pretty dark in the hold but it looked
like a lot of boxes to me."

"You don't put crystal blocks in boxes," said Strong.

"Sometimes they do, sir. The more expensive grades are crated, so that
the surfaces won't get scratched. Pieces that are going to be used for
outer facings on a building, for instance."

"All right, Sergeant. But I found something back in that building that
is going to prove very interesting."

"The cadets, sir?"

"No. An illegal use of oxygen!"

Quickly Strong explained his discovery, concluding, "Come on. We're
going back in there for a closer inspection!"

"But we can't, sir," said Morgan.

"Why not?"

"We only have enough oxygen left in our tanks to get us back to the
cleared area."

"Blast it!" growled Strong. "Aren't there any masks aboard the ship?"

"No, sir," replied Morgan.

"Very well, then. The only thing we can do is go back and bring out a
searching party in force." Strong turned and walked rapidly away. "Come
on, Sergeant, I think we're on the way to answering a lot of questions
about the failure of the screens."

Almost running, the two spacemen disappeared into the swirling mist of
deadly gases.

No sooner were they out of sight than Tom Corbett and Astro, faces
covered with oxygen masks, emerged from the warehouse and headed toward
the ship, Miles and Brett close behind them with paralo-ray guns leveled
at their backs.

[Illustration]

CHAPTER 16

Roger Manning opened his eyes, then closed them. He lay perfectly still
and listened. The sound he heard was the unmistakable blasting roar of a
spaceship. But there w