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.

"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 qu