the
young officer. "There's been no volcanic activity on Venus in the last
ten years serious enough to create such a cloud."

Strong waited for the professor's reaction, but the elderly man was
already entering the air lock. Before Strong and Hawks could catch up to
him, the air-lock hatch slammed closed.

"Hey," exclaimed Strong, "what does he think he's doing?"

"Don't worry about it, Steve," replied Hawks. "He probably forgot we
were out here with him, he's so concerned about this dirt. We'll just
have to wait until he's out of the air lock."

The Solar Guard officer nodded, then looked around him at the thick
black cloud that enveloped the ship. "Well," he said, "one of the
professor's theories has been knocked out."

"Yes," replied Hawks. "Which means this stuff is either the remains of a
large asteroid or--"

"The third possibility," finished Strong, "which the professor never
explained."

Suddenly the air-lock hatch opened again and the two spacemen stepped
inside. Closing the hatch behind them, they waited until the pressure
was built up again to equal that of the ship, and then they removed
their helmets and space suits.

[Illustration: _The hatch opened again and the two spacemen entered the
air lock_]

Leaving the air lock and walking down the companionway, Hawks suddenly
caught Strong by the arm.

"Have you considered the possibility of this cloud being radioactive,
Steve?" he asked.

Strong nodded slowly. "That's all I've been thinking about since I first
heard about it, Mike. I think I'd better report this to Commander
Walters at Space Academy."

"Wait, Steve," said Hawks. "If you do that, Walters might close the
exposition. Wait until you get a definite opinion from Professor
Newton."

Strong considered a moment. "I guess a few more minutes won't make a lot
of difference," he said finally. He realized how important the
exposition was to his old friend. But at the same time, he knew what
would happen if a radioactive cloud suddenly settled on the city of
Venusport without warning. "Come on. Let's see what the professor has to
say about this stuff."

They found the professor on the control deck bending over a microscope,
studying samples taken from the flask. He peered intently into the
eyepiece, wrote something on a pad, and then began searching through the
pages of a reference book on chemicals of the solar system.

Lieutenant Claude stepped up to Hawks and saluted sharply. "Power dec