“But, bless my watch-charm!” exclaimed Mr. Damon, “I didn’t know lightning made diamonds.”

“It does not—­always,” went on the scientist.  “But great heat and pressure are necessary to create the gems.  In nature this was probably obtained by prehistoric volcanic fires, and by the terrific pressure of immense rocks.  It is possible to make diamonds in the laboratory of the chemist, but they are so minute as to be practically valueless.

“However, these men seem to have hit upon a new plan.  They utilize the terrific heat of lightning, and the pressure which is instantaneously obtained when the bolt strikes.  I am anxious to see how it is done.  Look, I think they are getting ready to make the gems.”

Indeed there seemed to be an air of expectancy among the diamond makers.  The mixing machine had now been stopped, and, as it was more quiet in the cave, our friends, in their hiding-place, had to speak in mere whispers.  All the men were now gathered about the great steel box.

This receptacle had been closed by a solid metal door, which was screwed and clamped tight.  Then one of the men examined a number of heavily insulated electric wires that extended from the box off into the darkness where Tom and his companions could not discern them.

“That’s Folwell—­the man I befriended, and who got me into this game,” whispered Mr. Jenks.  “He was also one of the first to turn against me.  I think he’s one of the leaders.”

Folwell came back, after having gone into a dark part of the cave.  He went over to an electrical switch on one of the stone walls.

“It’s almost time,” Tom heard him say to his confederates.  “The storm is coming up rapidly.”

“Will it be severe enough?” asked one of the helpers.  “We had all our work for nothing last time.  The flashes weren’t heavy enough.”

“These will be,” asserted Folwell.  “The indicator shows nearly a million volts now, and it’s increasing.”

“A million volts!” exclaimed Tom.  “I hope it doesn’t strike anywhere around here.”

“Oh, it will probably be harmlessly conducted down on the heavy wires,” said Mr. Parker.  “We are in no danger, at present, though ultimately I expect to see the whole mountain shattered by a lightning bolt.”

“Cheerful prospect,” murmured Tom.

There was a terrific crash outside.  The rocky floor of the cave trembled.

“Here she comes!” cried Folwell.  “Get back, everybody!  I’m going to throw over the switch now!”

The men retreated well away from the steel box.  Folwell threw over the lever—­the same one Mr. Jenks remembered pulling.  Then the man ran to the electric switch on the wall, and snapped that into place, establishing a connection.

There was a moment’s pause, as Folwell ran to join the others in their place of safety.  Then from without there came a most nerve-racking and terrifying crash.  It seemed as if the very mountain would be rent into fragments.