arnish. _Copal_ is a term applied to
oleo-resins soluble in turpentine, and used almost universally as
varnishes. They come from the tropical regions of South America, Africa,
and from the East Indies. _Kauri_ is the fossil gum of a cone-bearing
tree dug from the ground in northern New Zealand. _Amber_ is the fossil
gum of extinct cone-bearing trees found mainly along the Baltic coast of
Prussia. It is used chiefly for the mouth-pieces of tobacco-pipes and
cigar-holders; the inferior product is made into varnish. It is sold
wherever tobacco is used. _Sandarach_, found on the north African coast,
is used principally in Europe, being employed as a varnish. The United
States and Great Britain consume most of the foregoing products.

QUESTIONS FOR DISCUSSION

Name any elastic substance you know about that is in every way a
substitute for rubber.

What has been the relation between rubber and good roads?

Describe the structure of a bicycle tire.

Why are tar, pitch, and turpentine called naval stores?--and what
determines the locality in which they are made?

What is varnish, and for what purposes is it used?

FOR STUDY AND REFERENCE

Obtain specimens of crude rubber, vulcanized rubber, and hard rubber;
note carefully the characteristics of each.

Burn a very small piece of cheap white rubber-tubing in an iron spoon or
a fire-shovel; note the character of the residue.

Obtain specimens of gutta-percha, resin, pitch, turpentine, shellac,
copal, dammar, and creosote for study and inspection.

CHAPTER XII

COAL AND PETROLEUM

The economic history of nearly every country that has achieved eminence
in modern times dates from its use of coal and iron; and indeed the
presence of these substances in workable deposits means almost unlimited
power. The present era is sometimes called the Age of Steel, but the
possibilities of producing steel in enormous quantities, at less than
one-fifth its price at the beginning of the nineteenth century, depended
mainly upon the use of mineral coal instead of charcoal in its
manufacture.

Coal consists of accumulations of vegetable matter that were formed in
prior geological ages. Under the action of heat and moisture, and also
the tremendous pressure of the rock layers that afterward covered them,
the vegetable matter was converted to mineral coal.

The aggregate coal-fields of the United States are not far from two
hundred thousand square miles in extent, but of th