sked Bert. "I presume that's what you've
been looking for?"

"Yes, I did go off hunting for them," said Tom slowly.

"Well, did you find any?" burst out Jack. "Can't you relieve the
suspense?"

"I found this," said Tom, placing an empty bottle on the table.

"Why--why, there's nothing in it!" exclaimed Jack, looking at it. "How
can that form a clew?"

"Not because of what is in it but what _was_ in it," said Tom with a
smile. "Unless I'm mistaken this will help to prove my innocence--that
is, if the experiment I'm going to try works out. We'll soon see. I
wonder if the laboratory is closed," and he went out into the corridor.

CHAPTER XVIII

ON THE TRAIL

"What's he up to now?" asked Bert of Jack, as the two stood in the
room, looking at one another.

"Give up. We'll have to wait and see. It's something important
though, to judge by Tom's actions."

"Yes, but an empty bottle! What can he hope to do with that for a
clew?"

"I don't know. Leave it to Tom."

The latter came back in a little while, carrying several bottles, test
tubes and an alcohol lamp.

"Well, for the class's sake!" cried Jack. "Are you going to give us a
demonstration of the action of liquids on solids?"

"No, I'm going to prove that mind is superior to matter," laughed Tom.

"Say, it sounds good to hear that!" cried Jack. "You haven't laughed
before in two weeks."

"Well, I feel a bit like it now," said Tom. "I'm beginning to get a
glimpse of daylight in this darkness."

He arranged his material on the table in front of him, having removed
the books and papers. Then, taking a bottle of some colorless liquid
which he had brought from the college laboratory, he poured some into
the apparently empty bottle he had first exhibited.

"What's that?" asked Bert.

"Sterilized water."

"Say, where did you find that bottle?" demanded Jack.

"In Farmer Appleby's barn," was the calm rejoinder. "I picked it up
just by chance, but it may mean something big."

"What?" cried Jack. "You don't mean to say you've been around there?"

"Surely. Why not?"

"Why, he might think you wanted to do away with the rest of his horses."

"He didn't see me. I took care of that. Besides that's the only place
where I can consistently look for clews. I was sure whoever poisoned
the horses must have left some trace behind him, and this may be it."

"The empty bottle?" asked Bert incredulously.

"It may not be altogether empty,"