“Couldn’t come from water, sir,” said Ned. “I didn’t expect to see a pond up here; but I suppose it’s hot, and that’s steam.”
“Oh yes, that’s hot enough,” said the doctor, who was panting with his exertions. “Liquid fire, eh, Jack?”
“Wouldn’t it be molten metal of some kind, father?” cried the boy.
“No, my lad, it is molten stone—rock. Lava.”
“But it puzzles me,” cried Jack, “how stone can melt. You said something to me one day about a flux.”
“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.”