"Yes, of course. People who smelt metals found that out long enough ago, and it is the same with making glass. If you expose some minerals separately to great heat they merely become powder; but if you combine them--say flinty sand with soda or potash--they run together and become like molten metal. I believe if ironstone and limestone are mixed, the ironstone becomes fluid, so that it can be cast like a metal--in fact becomes the metal itself." "Then that pool down there, if emptied out, would run like the volcanic glass we have found below?" "Most likely." "Let's go down this slope so as to see the pool from nearer." "Rather a risky proceeding, my boy," said Sir John; "suppose we were to break through." "Break through? Why, you don't think it is hollow under here?" "I should rather believe that there was a stony crust hardened by cooling, and that a very short distance beneath us the rocks are all molten." "But all these great stones lying about don't break through. Let's go a little way down." "Don't be rash then. Will you come, Instow?" "Oh yes, if it's safe. Let's go cautiously." Just then the sailors, who had had to pack up and carry the camping-out necessaries, appeared at the edge, and waited there watching the little party as they slowly descended toward the shimmering pool, threading their way in and out among the blocks of lava and pumice which lay in their road. Sir John led, with Jack close beside him, and the doctor and Ned followed a little way behind, to their right. But they had not descended a hundred yards before Sir John stopped short. "No farther!" he said. "The heat is getting intense, and overpowering gases are escaping from the ashes. We must go back, Jack." "I suppose so," said the lad unwillingly. "We don't see the pool any the better for being here either. Oh, look at that!" There was no need to call attention, for all were startled by a sudden report, and a glow of heat swept past their faces as a huge fountain of fire suddenly played up some sixty or seventy feet like a geyser, and fell back with a heavy splash, lower and lower, still playing till there was only a slight eminence, as if bubbling in the middle of the pool. Then it was perfectly level again, and a cloud of white smoke floated away. "That would have been grand by night," shouted the doctor. "It was grand now," replied Sir John. "Well, I think we had better turn back," said the doctor. "There is no doubt about its being molten fire below here, for the heat gets fiercer. Look." He had been resting on the climbing pole he brought up with him, and found that the end had gone down a couple of feet, while as he drew it out the point was charred and smoking. This induced Sir John and Jack to do the same, and theirs were burnt as well. "Yes, get back at once!" shouted Sir John in startled tones. "Quick, all of you; our weight is acting upon the ashes, and they are gliding down with us."