"What's the use now? It's all over." There was a tinkling sound, as fragments of the broken test tube, the bell-jar and other things began falling about the room. Mark was fumbling at the door of the laboratory, seeking to escape. "Come on back," said Jack. "It's all over. There's no more danger. We'll try it again." Just then one of the pile of books, that had been blown on an upper shelf, came down, landing on Mark's head. "No danger?" cried Mark, trembling from excitement. "No danger? What do you call that?" and he pointed to the books at his feet, while he rubbed his head ruefully. "Well, there aren't any more," observed Jack, with a look upward. Just then the door opened, and an elderly gentleman, wearing spectacles, entered the laboratory. He seemed much excited. "What happened? Is any one hurt? Was there an explosion here?" he asked. Then he saw the devastation on all sides--the broken glass, the scattered and torn books--and he noticed Mark rubbing his head. "There was--er--a slight explosion," replied Jack, a faint smile spreading over his face. "Are you hurt?" the professor asked quickly, stepping over to Mark. "Shall I get a doctor?" "A book hit him," explained Jack. "A book! Did a book explode?" "No, sir. You see, I was making a new kind of gas, and Mark was helping me. He was afraid the test tube would explode, so I piled books around it, and--" "And it did blow up!" cried Mark, still rubbing his head. "The test tube, and the other tube, and the rubber hose, and the bell-jar. I told you it would, Jack." "Then you weren't disappointed," retorted Jack, this time with a broad smile. "I don't like to disappoint people," he added. "What kind of gas was it, Darrow?" asked Professor Lenton. "Well, I hadn't exactly named it yet," answered the young inventor. "I was going to show it to you, and see what you thought of it. It's the kind you said I couldn't make." "And did you make it?" asked the instructor grimly. "Yes, sir--some." "Where is it?" "It's--er--well, you can smell it," replied Jack. Sure enough, there was a strong, unpleasant odor in the laboratory, but that was usual in the college where all sorts of experiments were constantly going on. "Hum--yes," admitted the professor. "I do perceive a new odor. But I'm glad neither of you was hurt, and the damage doesn't seem to be great." "No, sir. It was my own apparatus I was using," explained Jack. "I'll be more careful next time. I'll not put in so much of the chemical." "I don't believe there had better be a 'next time' right away," declared Mr. Lenton. "The next attempt you make to invent a powerful gas, you had better generate it in something stronger than a glass test tube. Use an iron retort." "Yes, sir," replied Jack.