s blazing on them,
wondering whether the finger of death would reach in from the swamp to
touch them. The fog had not yet made an appearance on the main post
and Dr. Bird had no fear of it when he entered his car and drove down
to pick up his assistant.

Davis came out to meet him with a curious hood made of vitiolene and
rubber, pulled down well over his head. In his hand he carried a
second one. Dr. Bird adjusted the second mask and the two men loaded
the rear of the car with apparatus designed for collecting samples of
air. The outside of each sample cylinder was heavily coated with black
rubberine paint. At a word from the Doctor, Davis took the wheel and
drove off along the winding ribbon of concrete which led to the upper
end of the Michaelville range.

For a mile they drove through a clear, calm night with no traces of
fog apparent. Dr. Bird's eyes continually searched the swamps on both
sides of the road.

"Stop!" he said suddenly, his voice coming muffled through the
enveloping mask. The car stopped and the Doctor pointed to the west.
Over the swamp a few stray fingers of fog were curling up from the
water.

* * * * *

Leaving Davis in charge of the car, Dr. Bird donned rubber hip boots
and with a gas cylinder in his hand, splashed through the water
toward the fog. He reached the place with no difficulty and spent ten
minutes trying to collect a sample. Finally, with a muttered
exclamation, he removed his mask and inhaled deeply a dozen times.
Carrying the mask in his hand, he made his way back to the car.

"False alarm," he said as he pulled on his mask. "It was so thin that
I couldn't get a sample so I tested it by breathing. There isn't a
trace of cough in that fog. Drive on."

A half mile farther along the road, a curtain of fog swept in on them,
momentarily hiding the road from view. They were through the belt of
fog in a few feet and the car came to a stop. Dr. Bird sprang out, gas
cylinder in hand. He returned to the car shortly.

"We may have what we are looking for, Davis," he said, "but I am not
at all certain. It looked very much like ordinary fog. Let's go down
to the range."

The car drew up between the two main buildings of the Michaelville
front. The air was clear as far as they could see, but from under the
north building, a tiny wisp of fog was coming. As it came under the
glare of the three huge arc-lights which flooded the ground with
light, it grew