Watching with eager eyes, the adventurers saw sparks flash from the steel box.  Instantly it became red hot, and then glowed white and incandescent.  It was almost at the melting point.

Then came comparative quiet, as the echoes of the thunder died away amid the mountain peaks.

“I guess that did the trick!” cried Folwell.  “It was a terrific crash all right!”

He and the others ran forward.  The steel box was now a cherry red, for it was cooling.  Folwell threw back the lever, and another man disconnected the switch.  There was a period of waiting until the box was cool enough to open.  Then the heavy door was swung back.

With a long iron rod Folwell drew something from the retort.  It was the tray which had held the white balls.  But they were white no longer, for they had been turned into diamonds.  From their hiding-place Tom and the others could see the flashing gems, for, in spite of the fact that the diamonds were uncut, some of them sparkled most brilliantly, due to the peculiar manner in which they were made.

“We have the secret of the diamonds!” whispered Mr. Jenks.  “There must be a quart of the gems there!”

The men gathered about Folwell, uttering exclamations of delight.  The diamonds were too hot to handle yet.

“That’s going some!” exclaimed the chief of the diamond makers.  “We have a small fortune here.”

The was a sudden commotion at one end of the cave.  A man rushed in.  At the sight of him Tom stared and uttered an exclamation.

“Munson—­the stowaway!” he whispered.

“Hello!” cried Folwell, as he saw his confederate.  “I thought you were East, keeping Jenks away from here.”

“He got the best of me!” cried Munson, “he and that Tom Swift!  I stowed away on their airship, but they found me out by a wireless message, and marooned me in the woods.  I’ve been trying to get here ever since!  Didn’t you get my messages of warning?”

“No—­what warnings ?” cried Folwell.

“About Jenks, Tom Swift and the others.  They’re here—­they must be on Phantom Mountain now.  In fact, I shouldn’t be surprised if they were in this cave.  I traced them to their camp, but they’re gone.  They may be among us now—­in some of the secret recesses!”

For an instant Folwell stared at the bearer of these tidings.  Then he cried out: 

“Scatter men, and find these fellows!  We must get them before they discover our secret!”

“It’s too late—­we know it!” exulted Tom Swift.  Then he whispered to the others to hurry to the part of the cave where Bill Renshaw had first hidden them.

CHAPTER XXII—­PRISONERS

“Do you think there is any danger of them finding us?” asked Mr. Damon, as he hurried along beside Tom.

“I’m afraid so,” was the answer.  “I’ve been worried ever since we saw Munson heading this way.  But we couldn’t do any differently.”