d; "another vent has opened! See it spout?"

Some hundred yards distant were clouds of green vapor that rolled into
the air. At their base a fountain of mud sputtered and spouted and fell
back to build up a cone. The green cloud whirled sluggishly, then was
caught by the breeze and began its slow, rolling progress across the
flat rock. It was coming their way, rolling down toward the ship, and
Chet gripped suddenly at his companion's arm.

"Come on!" he said! "I'm going away from here, and I'm going now. We'll
get Diane and Kreiss: remember what a whiff of gas did to him this
morning."

He was drawing Harkness toward the face of the rock; he wondered at his
slowness. Walt seemed fascinated by the oncoming cloud.

"Wait!" Harkness paused at the top of the descending slope. Chet turned,
to look where Harkness was watching.

The green cloud moved slowly. As he turned to stare it touched the bow
of their ship; it flowed slowly, sluggishly, along the sides, and then
swept up and over the top. The lookouts of the control room were
obscured, and the port from which they had come!

"Cut off!" breathed Harkness, his voice heavy with hopeless conviction.
"We can't get back! And now we're on our own past any doubt!"

* * * * *

"It may not last," Chet was urging an hour later, when, with Kreiss and
Diane, they stood on high ground to look down on the ship.

The sparkling sheen of the metal cylinder had changed from silver to
pale green. The cloud that enveloped it was not heavy, but it was always
the same. Yet still Chet insisted: "It may not last."

"Sorry to disappoint you," replied Kreiss, "but there is little ground
for such a belief." Again he was the professor instructing a class.
"These fumeroles, in my opinion, are venting a region far below the
surface. It is possible that further seismic disturbances may alter
conditions; a rearrangement of the lower rock strata may close existing
crevices and open others like this you have seen; but, barring that, I
see no reason for thinking that this emission of what appears to be
chlorine with other gases may not continue indefinitely."

Chet looked at Diane. Was it a twinkle that appeared and vanished in her
eyes as Herr Professor Kreiss concluded his remarks. She would laugh in
the very face of death, Chet realized, but her tone was entirely serious
as she offered another suggestion.

"If this wind should change," she said, "and if it ble