“We appreciate what you have done,” declared Mr. Jenks, “but I want my rights.  I must learn a few more facts about how to make diamonds from lightning flashes, and then I will have the same secret they cheated me out of.  I think if we wait a while we may be able to see the parts of the process that are not quite clear to us.  What do you say, Tom Swift?”

“Well, I would like to learn the secret,” replied the lad, “and if Bill thinks it’s safe to stay here a while longer—­”

“Oh, I guess it will be safe enough,” was the reply.  “Those fellows won’t bother about you now that they are about to make some more diamonds.  Besides, they think you’re all tied up.  Yes, you can stay here and watch, I reckon.  I’ve got a couple of guns, and—­”

“Then we’ll stay,” decided Tom.  “We can put up a better fight now.”

Silently, in their prison, but which they could now leave whenever they pleased, the adventurers watched the diamond makers once more.  The same process they had witnessed before was gone through with.  The white balls were put inside the steel box and sealed up.  Then they waited for the storm to reach its height.

That this would not be long was evidenced by the mutterings of thunder which every moment grew louder.  The outburst of electrical fury was likely to take place momentarily, and that it would be unusually severe was shown by the precautions taken by the diamond makers.  They attached a number of extra wires, and brought out some insulated, hard rubber platforms, on which they themselves stood.  Tom and Mr. Jenks were much interested in watching this detail of the work, and sought to learn how each part of the process was done.

“I almost think we can make diamonds, Tom, when we get back to civilization,” whispered Mr. Jenks.

“I hope we can,” answered Tom, “and we can’t get back any too soon to suit me.  I want to be in my airship again.”

“I don’t blame you.  But look, they are getting ready to adjust the switch.”

The adventurers ceased their whispered talk, and eagerly watched the diamond makers.  Folwell and Munson were hurrying to and fro in the big cave, attending to the adjustments of the machinery.

“On your insulated plates—­all of you,” Folwell gave the order.  “This is going to be a terrific storm.  The gage shows twice the power we have ever used, and it’s creeping up every minute!  We’ll have more diamonds than ever had before!”

“Yes, if the mountain isn’t destroyed,” added Mr. Parker, in a low voice.  “I predict that it will be split from top to bottom!”

“Comforting,” thought Tom, grimly.

“I guess we’re all ready,” said Folwell, in a low tone to Munson.  “We’d better get insulated ourselves.  I’m going to throw the switch.”

He did so.  A moment later the man who had before given warning of the storm came dashing in.  He was very much excited.

“It’s awful!” he cried.  “The lightning is striking all over!  Big rocks are being split like logs of wood!”