lso thus prepared for the cheaper
grades.

[37] The following are the chief rubber-producing trees: _Siphonia
elastica_, or _Hevea brasiliensis_, Amazon forests, yields Para rubber;
_Manihot Glaziovii_, also a tapioca-producing shrub, Ceara province,
Brazil, furnishes Ceara rubber; _Castilloa elastica_, Central American
States, Nicaragua rubber; _Ficus elastica_, British India, and _Urceola
elastica_, Borneo, Indian rubber. There are rubber-producing trees in
Florida, but they have little commercial value at the present time.
African rubber is taken from a variety of plants.

[38] The process of vulcanizing was made practicable during the ten
years ending in 1850. It was invented and perfected by Goodyear in the
United States and by Hancock in England; for ordinary purposes, where
both strength and elasticity are required, about five per cent. of
sulphur is added. The addition of about fifty per cent. changes the
rubber to a hard black substance known as "ebonite," or "hard rubber."

[39] In 1823 a Scotchman, Mackintosh, applied the discovery, that rubber
gum was soluble in benzine, to the water-proofing of the cloth that
bears his name. This invention was about the first extensive commercial
use to which rubber had been put.

[40] From the fact that most of the dwellings in the United States are
built of wood, the United States is a very heavy consumer of turpentine.

[41] A slender strip of metallic lead was used instead of graphite in
the first pencils made. The use of graphite did not become general until
about 1850. The hardness of a pencil is regulated by mixing clay with
the powdered graphite.

[42] These percentages are on the supposition that the ores are
chemically pure; the percentage of metal actually obtained is somewhat
less.

[43] These percentages are on the supposition that the ores are
chemically pure; the percentage of metal actually obtained is somewhat
less.

[44] These percentages are on the supposition that the ores are
chemically pure; the percentage of metal actually obtained is somewhat
less.

[45] These percentages are on the supposition that the ores are
chemically pure; the percentage of metal actually obtained is somewhat
less.

[46] The limestone has no essential part in the smelting of the ore
except to produce an easily-flowing, liquid slag; hence it is called a
_flux_. Some ores smelt and flow so easily that a flux is not required.

[47] Under ordinary circumstances about