water, but dissolve
readily in alcohol, in naphtha, in turpentine, or in other essential
oils; these are designated as "gum-resins." Still others yield oils or
pitchy substances on distillation; these are known as "oleo-resins."
There are many other dried vegetable juices, however, that in commerce
are not classified among the gums and resins, and of these the most
important is the substance commonly known as india-rubber.

=Rubber and Rubber Products.=--"Caoutchouc" is approximately the name
given by Indians of the Amazon forests to a substance that had also been
found in India. Some of it was brought to Europe from the Amazon region
as early as 1736, and for nearly one hundred years no general purpose
was discovered for which it could be used, except to erase lead-pencil
marks--hence the name india-rubber, which has held ever since.

Common rubber is the prepared juice of a dozen or more shrubs and trees,
all of which grow in tropical regions.[37] The belt of rubber-producing
plants extends around the world and includes such well-known species as
the fig, the manihot (or manioc), and the oleander; indeed, it is a
condition of sap rather than a definite species of plant that produces
rubber, and the latter is a manufactured rather than a natural product.
The process of preparing the juice is practically the same in every part
of the world.

The rubber-gatherer of the Amazon, who is practically a slave, wades
into the swamp, makes several incisions in the bark of the tree,
fashions a rough trough of clay under it, and waits till the sap fills
the clay vessel. When the sap has been gathered he makes a fire of the
nuts of the urucuri palm and places an inverted funnel over it to
concentrate the smoke. He first dips the end of a wooden spindle into
the juice and then holds it in the smoke until the juice coagulates;
this process is repeated until there has formed a ball of rubber
weighing from five to ten pounds. The smoke of the palm-nuts is a
chemical agent that converts the juice into the crude rubber of
commerce.

Crude gum, however, is lacking both in strength and elasticity. The
process that makes it a finished product is known as _vulcanization_.
The crude rubber, having been exported to the manufacturer in the United
States or Europe, is shredded, washed, and cleansed, and partly fused
with varying proportions of sulphur. For a very soft product, such as
the inner surface of tires, only a small proportion is u