There was a little crackling sound as the heat expanded the powder, and the end of the test tube became quite red from the flame. "That tube'll melt!" exclaimed Mark, peering over the pile of books. "It's too near the flame." "Guess you're right," admitted Jack. "I'll raise it up a bit." He turned down the flame and elevated the tube slightly. Then he took a position where he could watch the process of making what he hoped would be a new kind of gas. He wanted to be where he could see the vapor beginning to collect in the top of the tube, pass off through the glass in the cork, and then through the little rubber hose to the bell glass held by Mark. If the gas was generated too quickly, Jack knew he would have to turn down the heat slightly. The crackling sound continued. Then, as Jack watched, he saw a thick, yellowish vapor collecting in the top of the test tube near the cork. "It's coming!" he cried. "There's my new gas!" "What's the name of it?" asked Mark. "I haven't named it yet. I want to collect it in the jar and show it to Professor Lenton. He said he didn't believe I could make it." The boys resumed their careful watching of the experiment. It was a nervous moment, for, from experience, Mark knew you never could tell what would happen when Jack began to try new combinations of chemicals. He was ready to drop down on an instant's warning, out of the way of flying missiles. "See any bubbles in that pan of water yet?" cried Jack. "No, not yet." "That's queer. The test tube is full of the yellow gas, and some ought to be over to where you are now. I'm going to turn on some more heat." He increased the Bunsen flame. The crackling noise was louder. The test tube became a fiery red. "It's bubbling now!" suddenly called Mark. "That's good! The experiment is a success! I knew I could make it. Is any of the gas coming up in the glass jar?" Mark bent over to make a closer examination. There were a few seconds of silence, broken only by the roaring of the burner and the crackling of the black powder. "Yes, there is vapor in the jar," he said. "Good! That's the stuff!" cried Jack. "Now I guess Professor Lenton will admit that I'm right." He turned the Bunsen flame up higher. A moment later he uttered a cry, for he saw the cork being forced from the test tube. The pressure of the new gas was too much for it. "Lookout!" cried Jack. "She's going up!" Then followed a sharp explosion, and the laboratory seemed filled with fragments of broken glass and torn books. CHAPTER II JACK MAKES OXYGEN "There it goes! There it goes!" cried Mark, making a dive for the laboratory door, but slipping and sprawling on the floor. "There it goes, Jack!" "No; it's gone already!" cried Jack, who, even in the midst of danger and excitement, seemed to remain calm and still to have his appreciation of it joke. "Come on!" cried Mark as he scrambled to his feet. "We must get out of here, Jack!"