n: _Fig. 13. Tanning Vat._]

"The tannic acid or the chemicals act on the skins, or, rather on the
gelatin, glutin and albumen in the skins, and thus harden the texture
and preserve it. Where tannin is not used and only the chemicals are
employed, it is called 'tawing' the leather, instead of 'tanning.'"

"Well, we can get the bark; I know where there are several oak trees,
and also a number of beech trees."

"Then gather the bark by all means, and by the way, if you can find gall
nuts we could use them to advantage."

"We don't know what they are."

"Then, if you can get some sumac, we can use that."

"Yes, I know; the kind with the long, red leaves."

"That is what I mean."

"Do the gall nuts have tannic acid?"

"Yes; but principally gallic acid, but gallic acid will also tan the
skins so as to make leather. The principal use of the gall nut is for
making ink."

"What kind of tree does the gall nut grow on?"

"On any kind of tree or bush."

"That is rather odd."

"It is not the fruit of any tree, but is produced by the gall fly, which
punctures almost any kind of tree or shrub. In this puncture the insect
lays its eggs, and the tree in trying to treat the wound covers up the
egg, and the sap, flowing from the tree, forms a sort of nut which
finally hardens and produces a most bitter substance deposited by the
fly. The nut is about the size of a marble, and must be gathered before
the larva is hatched out. It is the most valuable nut in the world."

The necessary bark for the tanning process was ready within the next two
days, and a tank prepared in which the hides were laid spread out, with
the bark between them and covered with water.

"In our conversations, Professor, you have the habit in describing
plants, and especially the leaves to call them by certain names. It
would help us if we knew just what you meant by the different names you
give them."

"That is a wise suggestion, because it has been said that the basis of
knowledge, or of true science, is correct definitions.[1] What is meant
by that is this: We should both have an understanding of the term used
to describe a thing. In our talks I have tried to avoid the use of what
is called technical terms, but it is difficult to describe some things
without using such terms, and I have for some time thought of making a
list of the things we are talking about, and defining them, so you can
at any time go there and look up the definition."

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