"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.