"Oh, here's the tube," said Mark as he passed it over. "But please be careful, Jack." Jack measured out some black chemical that resembled gunpowder, and poured it into the test tube which Mark handed him. Then he inserted in the opening a cork, from which extended a glass tube, to the outer end of which was fastened a rubber pipe. He paused in his experiment to laugh again. "What are you making-laughing gas?" asked Mark. "No. But-excuse me-ha, ha! I can't help laughing when I think of the way you ducked under the table the other day."