k in the laboratory, but would be wholly impractical on a
worldwide scale," Ken admitted. "How will we ever rid the atmosphere of
the dust! A colloid will float forever in the air, even after the comet
is gone."

"Exactly," Professor Maddox said, "and, as far as we are concerned, the
whole atmosphere of the Earth is permanently poisoned. Our problem is to
process it in some manner to remove that poison.

"During the past few days we have come to the conclusion that there are
only two alternatives: One is to process the whole atmosphere by passing
it through some device, such as the filter you have suggested. The
second is to put some substance into the air which will counteract and
destroy the dust, precipitate it out of the atmosphere."

"Since the first method is impractical what can be used in carrying out
the second?"

"We've set ourselves the goal of discovering that. We're hoping to
synthesize the necessary chemical compound to accomplish it."

"It would have to be a colloid, too, capable of suspension in the
atmosphere," said Ken.

"Correct."

"If we do find such a substance we still have the problem of
decontaminating existing metals. We couldn't build a moving piece of
machinery out of any metal now in existence without first cleaning the
dust out of its surface."

"That's part of the problem, too," said his father.

* * * * *

Ken resumed his duties in the laboratory the following morning. Dr.
Adams had warned him not to walk up College Hill, so he had borrowed the
horse Dave Whitaker still had on loan from his uncle. He felt
self-conscious about being the only one enjoying such luxury, but he
promised himself he would go back to walking as soon as Dr. Adams gave
permission.

On the third day, the horse slipped and fell as it picked its way
carefully down the hill. Ken was thrown clear, into the deep snow, but
the horse lay where it had fallen, as if unable to move. Ken feared the
animal had broken a leg. He felt cautiously but could find no evidence
of injury.

Gently, he tugged at the reins and urged the horse to its feet. The
animal finally rose, but it stood uncertainly and trembled when it tried
to walk again.

Ken walked rather than rode the rest of the way home. He took the horse
to the improvised stable beside the science shack. There he got out the
ration of hay and water, and put a small amount of oats in the trough.
The animal ignored the food and dri