n readily be made
out, and at the corner here, where a portion has been chipped away, it
has the appearance of something else besides calcium."

"Why, it looks like wood."

"That is what it is. I should not have noticed the wood if the peculiar
lettering had not shown up through the coating."

"What are the letters, and do you know what they stand for?"

"We had better not pass judgment on that until we have removed all the
calcium."

At this moment Harry came in to view the slab. It was the slab he had
carelessly picked up in the cave, and therefore it had a great
fascination for him. The calcium was carefully chipped off, and it was
found to be a piece of oak board, with a smooth cut-off end, parallel
sides, nine inches wide, nearly two inches thick, and about eleven
inches long, the opposite end having the appearance of being broken. The
only letters which could be made out were "HI," and a portion of another
letter which could not be determined.

[Illustration: _Fig. 14. THE SLAB FOUND IN THE CAVE_]

"What do you think the letters were intended to indicate?"

"They might be the name of a ship, or some sign. I do not think it was
part of a ship. I tried to find something in the cave, on the day I went
in while you went after the team, which would afford some clue, but so
far nothing confirms me in any view which I may have."

"Isn't it curious that these letters should show through only after the
slab was exposed to the light?"

"Why is that any more curious than photography is?"

"Because in photography something is put on the glass or the sheet that
the negatives are made of, and it turns and makes a mark under certain
conditions."

"Well, here is something put on this slab that turns also. Photography
is a wonderful thing. Dr. Draper, the first great photographer, and who
was also a scientist, says that every wall, or other object, which you
stand before, has your photograph imprinted on it. The only question is
to find some chemical which will develop the picture."

"What is meant by developing the picture?"

"You remember some time ago we talked about reagents, and the properties
of certain chemicals to act on others, and in doing so, to make a
change. Sometimes the change is a complete one, and makes a new product;
in other cases the result is a complete change of color. Now, in
photography, if a certain chemical is placed on a glass or a film, and
the film is exposed, the light and dark portio