"Go on," he said huskily. "What is the stuff? Not talc--that I can tell easily. Not celluloid either--you'd never be such a fool as to build a ship of such a highly inflammable material. That stuff's lighter, also. What is it?" But Joe was not yet to be persuaded into an answer. He spread the thin transparent sheet out, caught the four corners, and taking a jug of water, poured some of the contents into the centre of the sheet. Not a drop penetrated it. Joe demonstrated the fact quietly and without show of haste. Then he stepped to the window and cast the water out. A moment later he was striking a match. "Stop! Stop! How do I know that it isn't like celluloid?" cried Andrew in some alarm. "Supposing it fires. Supposing there's an explosion." Joe smiled. "It won't," he said curtly. "Look there." The flame was licking round one of the corners of this thin sheet of material. It blackened the surface above, while that below, immersed in the flame, gradually changed colour. It became a dull red, then got redder and redder till it was glowing. Slowly it changed its form, the corner curled up into a globule. The latter separated itself from the sheet and tumbled on to the glass-topped table, where it broke into a number of smaller drops. "Glass! No--too light by far. Not celluloid. Not talc. Then what is it?" demanded Andrew impatiently, taking the various articles and examining them. "Why, this angle piece is strong--as strong as aluminium!" he cried. "Stronger--stronger and tougher," asserted Joe. "You can bend it; it's flexible. You can bend it double, and still it comes back to its original formation. Aluminium would crack at once; even steel would. Now, try the tube. See, it kinks when you bend it, though it requires some strength to do that. Now, set it on its end on the floor; we'll put a book on the top end. Sit on the book, Uncle." Andrew did so--gingerly it must be confessed--for this transparent tube with its small diameter and its walls less than an eighth of an inch in thickness looked as if it would at once succumb to his avoirdupois. But it did not. He sat boldly upon the book now. He balanced himself upon the frail support and jerked his feet from the ground. "Jingo!" he cried. "What in thunder is the stuff? It's strong, strong as possible. Surprisingly powerful stuff. It bends if you use sufficient force, yet doesn't break. It's tough; you've shown me that, for a knife edge bites into it with difficulty. Then it softens and melts at a fairly high temperature, proving that it can be easily treated and moulded. Well?"