The young inventor could scarcely believe the good luck that had so unexpectedly come to him and his companions. No sooner was Tom able to move freely about than Bill Renshaw performed the same service for Mr. Jenks and the others, cautioning them to be quiet as he awakened them, and cut the ropes.

"Bless my circulation!" exclaimed Mr. Damon, in a hoarse whisper. "How did you ever get here. I'd given ourselves up for lost."

"Oh, I came in off the mountain, as there's a big storm due," explained the man. "There was no need of me playing the haunt in daytime, anyhow. I went to the cave, found you and your things gone, and I surmised that you might have walked into some trap."

"We did," admitted Mr. Jenks, grimly.

"Well, I hunted around until I found you," went on Bill. "This mountain is honeycombed with caves, all opening from the large one, I know them better than these fellows do, so I could explore freely, and keep out of their sight. They didn't know that there was a second entrance to this place, but I did, and I made for it, when I couldn't find you in some of the other caves where I looked. And, sure enough, here you were."

"Well, we can't thank you enough," said Mr. Parker. "But you say there is a big storm coming?"

"One of the biggest that's been around these parts in some time," replied Bill.

"Then perhaps the mountain will be destroyed," went on the scientist, as calmly as if he had remarked that it might rain.

"I hope nothing like that happens until we get away," spoke Mr. Damon, fervently.

"What had we better do?" inquired Tom.

"Get away, unless you want to discover some more of their secrets," advised Bill. "Those fellows are planning something, but I can't find out what it is. They are suspicious of me, I think. But they are up to something, and I believe, it would be best for you to leave while you have the chance. It may not be healthy to stay. That's why I did my best to untie you."

"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!"

"Well, it can't do any damage in here," said Munson. "We are well protected. Get on one of the plates," and he motioned to one of the hard-rubber platforms that was not occupied. The roar and rumble of the storm outside had given place to short terrific crashes. In their small cave the adventurers could feel the solid ground shake.

A bluish light began dancing about the electrical wires. There was a smell of sulphur in the air. Crash after crash resounded outside. A flash of flame lit up the whole interior of the cave. It came from the copper switch.

"Something's wrong with the insulation!" cried Munson.

"Don't go near it!" yelled Folwell. "If you value your life, stand still!"

Hardly had he spoken than inside the cavern there sounded a report like that of a small cannon. A big ball of fire danced about the middle of the cave and then leaped on top of the steel box.

"This is a fearful storm," cried Munson.

The adventurers in the cave did not know what to say or do. They were in deadly peril.

Suddenly there came a crash louder than any that had preceded it. The whole side of the cave where the switches were was a mass of bluish flame. Then came a ripping, tearing sound, and a tangle of wires and copper connections were thrown to the floor. At the same time the steel box, containing the materials from which diamonds were made, turned blue, and flames shot from it.

"It's all up with us!" cried Munson. "Run for it, everybody! The wires are down, and this place will be an electric furnace in another minute!"

He leaped toward the exit from the cave.

"What about those fellows?" asked Folwell, indicating the place where Tom and the others had been tied.

"They'll have to do the best they can! It's every man for himself, now!" yelled Munson. There was a wild scramble from the cavern.

"Come on!" cried Tom. "We must escape! It's our only chance!"

He leaped into the big cave, followed by the others. Already long tongues of electrical fire were shooting out from the walls and roof as Tom Swift and his companions, evading them as best they could, sought safety in flight.