"In the first place," Jack went on, "I wish to exhibit this garment," and he held up to view a long linen coat, commonly called a duster. "You will observe," he went on, "that there are several brown lines on it. I have measured these and they are exactly the shape and size that would be made by the sharp rim of a bell, if it was rested on the garment when some one was wearing it." "You will have to have better evidence than that," sneered Professor Grimm. "I think I will have," announced Jack quietly. "Of course those marks might have been made by any sharp, rusty object. Now the bell metal rusts scarcely at all, but the iron clapper of a bell does. The rust from that runs down inside a bell, and gets on the edges. I took some iron rust from the clapper of the stolen bell and placed it in a test tube. I assumed, for the purpose of experimenting, that I did not know that it was iron rust, but only suspected it. I applied the proper chemical tests, and I got the results that showed me there was iron present in the test tube. Here, I will show you." Jack mixed a few chemicals and soon the brown mixture in the tube turned red. "That is from the bell clapper," the young chemist went on. "Here is a solution made from scraping the lines on the duster. I will apply the test and see what happens." While the others looked on anxiously Jack dropped some of the mixture into the second tube. In an instant it turned red. "There!" exclaimed Jack, holding up the two tubes, side by side. "The same color coming in both mixtures from the same strength of chemicals that I used, shows that the iron rust on the duster and that on the bell clapper are the same." "What does that prove, except that you might have worn the duster?" asked Dr. Mead. "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.