shoulders sagged visibly.

"_Do something!_" Gloria screamed. "Do something before the others come
back!"

Emmett glanced apprehensively at the air lock. She was right. At the
moment they outnumbered the enemy, but when the others returned the
Agronians could overpower them by sheer weight of number. And they could
return without warning, at any instant.

"Why did one prevent the other from killing us?" George asked.

"He may have been afraid the other would miss and damage the ship,"
Emmett said. "Or possibly--"

"No. They're trained from birth to be soldiers. They're expert marksmen
and their weapons are foolproof. They can adjust the blast from a weapon
to travel any distance."

"Why should one enemy prevent another from killing us?" Emmett repeated
wonderingly. He remembered another question that had nagged at his mind:
_Why had the Agronians totally destroyed Earth?_ Why hadn't they
eliminated Earthmen and preserved the planet for exploitation--as a
colony, a military base, any one of a thousand uses?

There was only one possible answer. A race might destroy a planet if it
was useless. Earthmen had discovered useless planets, planets with
poisonous atmospheres. Was Earth's atmosphere poisonous to the
Agronians?

One Agronian had prevented another from killing them with a viciousness
and an urgency that indicated it had been a life-and-death necessity.

Why? What would happen if they were to die?

Something clicked in his mind and a startling certainty occurred to him.
_Oxygen was poisonous to the Agronians!_

That was why his life had been spared. And the pilot's--and Gloria's.
Their spacesuits would have been punctured and their oxygen supply would
have spread with deadly rapidity throughout the room.

Without hesitation he removed his helmet and adjusted the controls of
his oxygenating machine until it was discharging oxygen at maximum
capacity.

With a shrill outcry the two aliens darted toward him. But a thin,
ghostly vapor of oxygen spread rapidly through the fog-like atmosphere,
and halted them in their tracks.

"You deserve to die," Emmett whispered.

The enemy collapsed at his feet and writhed helplessly on the floor.
Their bodies quivered spasmodically and were still.

* * * * *

Gloria's hysterical, joyous laughter rang in his ears like triumphant
bells, and through the Agronian atmosphere that burned his face and
smarted his eyes he dimly saw George's