“If that ain’t the beatingest I ever heard on,” he remarked, gazing at the professor, and then he tapped his head significantly and looked at the boys in a knowing way.

“Nobody home, eh?” he said with a grin.  Fortunately the professor did not hear him; but the boys could hardly keep from laughing outright as they set to work with the spade.  A few minutes of brisk digging set the professor at liberty and he was able to stand upright and triumphantly exhibit a small black rock which looked in no way remarkable, but which, it was evident, he esteemed highly.

“Ah, my little gem,” he said, gazing at it fondly.  “You thought you’d escape me; but you didn’t.  A wonderfully fine specimen, boys.”

“Tell yer what,” said the yokel, from whom they had borrowed the spade, “I’ll pay you fifty cents a day to clean up my back pasture yonder.  It’s chock full of them black rocks.”

“It is?” exclaimed the professor eagerly.  “I must visit it some day.  It would be worth writing a paper about.  Most remarkable.  A whole field of these stones.  Well, well, this is a great day for science.  But how did you boys happen to come along so opportunely?”

Jack explained, and then, suddenly, he thought of the tube of queer-looking black sand.  Possibly the professor would know what it was.  He drew it out and briefly narrated how he came in possession of it.  The professor took the little glass vial out of its protecting lead and flannel.  He adjusted his glasses and held it up to the light.  Then he uncorked it and sprinkled a few grains on the palm of his hand.

He regarded it carefully for a few minutes and then drew out a huge magnifying glass.  The next instant he dropped his scientific calm and uttered a sharp exclamation of astonishment.

“Where is the man who owns this?” he exclaimed.  “We must see him at once.”

CHAPTER XIV.

IN THE HOSPITAL

“We are on our way to see him now,” said Jack.  “He is in the Nestorville hospital.”

“May I go with you?” asked the professor, with astonishing eagerness for him.

“Why, of course.  But that black sand,” said Jack.  “What is it—­gold-bearing material of some kind?”

“Gold!” exclaimed the professor with fine scorn, “gold would be dross beside it.  Of course I haven’t analyzed it yet, but if it is what I think it is, it is the most valuable stuff in the world.”

The boys exchanged bewildered glances.  Clearly their discovery of the injured man, Zeb Cummings, had an aspect they had not hitherto suspected.  But the professor refused to tell them what the sand was, or what he thought it was, till he had seen Zeb Cummings himself.

Leaving the potato-digger under the firm impression that they were all crazy, they hurried back to the road, the professor’s bicycle was placed in the tonneau, and Jack drove just within the speed law to the hospital.