Jack nodded.

“I know all that,” he said.

“And yet you are prepared to get it back?”

“I said I thought there was a possibility,” was Jack’s quiet reply.

“And what may that be?”

“Did you ever hear of Z.2.X., doctor?” was the entirely unexpected question.

CHAPTER XXII.

“Z.2.X.”

“Z.2.X.?  Well, such things are rather out of my line, but I have heard of it—­yes,” replied the doctor, looking more puzzled than ever.  “But what do you know about it?”

“Till two days ago—­nothing,” replied Jack, “but now I believe that I know where there is a trainload of it.”

“Good heavens, boy, you don’t know what you’re talking about.  Why, the stuff is as valuable—­as valuable as radium.  Possibly it is worth more.”

“Then even a small quantity would restore my father’s fortune and his health?” asked Jack, persisting in his line of inquiry.

“Undoubtedly it would restore his fortune, and in my belief his health, which he is unlikely to gain otherwise.”

“Then I’ll do it,” said Jack, speaking for himself and Tom, for the two lads had discussed the idea the night before.  “Those dividends from our share of the hydroaeroplane plant will fit out an expedition, and if we fail—­well, we can still sell out our interest and help dad get on his feet again.”

The telephone bell jangled.  Jack answered it.  The voice that came over the wire was that of Professor Jenks.  His tones trembled with excitement as he spoke to the boy.

“I have analyzed that sample from the Colorado River,” he said.

“Well, what is your verdict?” asked Jack, with a painfully beating heart.

“That when all the expenses of reduction and refining and transportation and digging are deducted that it will be worth at least $100 an ounce,” was the reply.  “It would bring an even higher price, for the placing of a large amount on the market will probably have the effect of lowering it.”

“Great Scott!” breathed Jack, “and there’s a whole island of it there for the taking.”

“Yes; but how are yow going to get it?  The cliffs are unscalable, the river unnavigable.  It might as well be in Mars for all the good it does anyone,” objected the professor.

Jack’s next words were direct, to say the least.

“I’ve figured out all that,” he said.  “We can get it, if it’s there to be got.  I’ve a reason now for going out there if it’s possible to come to some arrangement with Zeb Cummings.  Can you meet me at the hospital this afternoon to talk over the matter?”

“Are you serious?” gasped the professor.

“Perfectly,” Jack assured him.  “If we can’t get at it by earth or water we can reach it from the air, can’t we?”

“Heaven bless my soul, I never thought of that,” choked out the professor.  “I—­Melissa’s calling me.  I’ll meet you at the hospital this afternoon.”