He paused to watch the men at work.  They were all busy, some attending to the machines, and others coming and going in and out of the cave.  In one part a man was apparently getting ready a meal.

Suddenly there rushed into the cave a man who seemed much excited.

“Are you nearly ready with that stuff?” he cried.  “There’s a good storm gathering on the mountain!”

“Yes, we’ll be ready in half an hour,” answered one of the men at the mixing machine.

“Good.  It will be flashing lightning bolts then, and we can see what luck we have.  The last batch was a failure.”  The man hurried out again.  Mr. Parker touched Tom and Mr. Jenks on their shoulders.

“What is it?” asked Tom.

“I know the secret of making the diamonds,” said the scientist.

“What?” cried Mr. Jenks.

“It is by the awful power of the lightning bolts!” whispered Mr. Parker.  “Everything is explained now—­the reason why they make diamonds in this lonely place, near the top of the mountain.  They need a place where the lightning is powerful.  I can understand it now—­I suspected it before.  They make diamonds by lightning!”

“Are you sure?” cried Mr. Jenks.

“Positive.”

“I agree with you,” said Tom Swift.  “I was just getting on that track myself, when I saw the electric wires running to the steel box.  That explains the upright rod on the top of the mountain.  The man says a storm is coming—­very well; we’ll stay here and watch them make diamonds!”

As he spoke there came the mutter of thunder, and the mountain vibrated slightly.  The men in the cave redoubled their activity.  Tom and his friends felt that the secret process they had so long sought was about to be demonstrated before their eyes.

CHAPTER XXI—­FLASHING GEMS

Eagerly the adventurers looked through the opening at the end of the passage into the larger cave.  The men opened the small oven in which the balls of white chemicals and carbon mixed, had been baked, and a pile of things, that looked like irregularly-shaped marbles, were placed in the steel box.

This box, which was about the size of a trunk, was of massive metal.  It was placed in a recess in the solid rock, and all about were layers of asbestos and other substances that were nonconductors of heat.

“That box becomes red hot,” exclaimed Mr. Jenks, in a whisper.  “When things are in readiness, that lever is pulled and the diamonds are made.  I pulled it once, but I did not then know the process involved.  I supposed that the lightning had nothing to do with making the diamonds.”

“It has—­a most important part,” said Mr. Parker.  The hidden adventurers could talk in perfect safety now, for the men in the large cave were too excited to pay much attention to them.  The muttering of the thunder grew louder, and at times a particularly loud crash told that a bolt had struck somewhere in the vicinity of the cave.