Caouthouc may be roughly defined as a peculiar substance composed of
carbon and hydrogen, found in suspension in the milky juice of a great many
different families of plants. It has been stated that .all milky vegetable juices
contain it ; but this is not the case, many of these juices yield gum resins free
from caoutchouc.
Properties and Composition of the Juice. — Caoutchouc juice or sap
has been imported from time to time into England in considerable quantities,
but it is found more economical to prepare the crude rubber where the juice
is collected. It resembles ordinary cow's milk in colour and consistence. Its
specific gravity varies from i'oi2 to i"04i. Several circumstances may conduce
to give the commercial juice a grayish brown, milky gray, or pale yellow
colour, but the pure juice as it issues from the tree is white. Dr. Adriani
[Ghem. News II. 277, 289), who made some valuable experiments upon the
fresh juice of the Ficus elastica, tapped by himself, says that, as the general
result of his experiments, the quantity of solid matter contained in the milky
juice decreases according to its being collected from incisions made in the
higher, and consequently younger, parts of the plant. The tree which yielded
the juice for his experiments was a young plant 2'2S metres in height.
INDIARUBBER. 155

Amount of juice Height at which t^ » i -j t.
evaporated. it was taken. Total residue. Percent.
