"That is all, as yet," Jack admitted. "But I will prove that the
duster is not mine, and that I never wore it. I have something else
here," he went on.

From among a pile of things on the laboratory table Jack took a white
object, with brown spots. Walking rapidly across the room he handed
it to Adrian.

"The rag off my sprained wrist!" exclaimed the sporty student. "Where
did--"

Then he stopped, seeming to realize he had said too much.

"I will ask Professor Grimm to smell of that," Jack continued,
thrusting the rag under the teacher's nose.

"Arnica!" exclaimed the instructor. "The same that you used, and
which enabled me to discover it was you who played the trick."

"It is arnica," Jack admitted, "but it happens I was not the only one
who used it that night. I have also to show this article, which was
picked up in your room, Professor Grimm," and Jack extended the spark
plug Budge had given him.

"Ha! What is that?" asked Mr. Grimm.

"It is some part of an automobile," Dr. Mead said. "Who, of our
students, has one. Ah! I begin to see," he added.