streaks are simply the juice rendered more aqueous, owing to the
contact of the acid having deprived it of the albumen. At the same
time, the centre of the pellicles becomes opaque, and of the colour of
the yolk of an egg; they enlarge as if by the prolongation of
divergent fibres. The whole liquid assumes at first the appearance of
an agate with milky clouds; and it seems as if organic membranes were
forming under the eye of the observer. When the coagulum extends to
the whole mass, the yellow spots again disappear. By agitation it
becomes granulous like soft cheese.* (* The substance which falls down
in grumous and filamentous clots is not pure caoutchouc, but perhaps a
mixture of this substance with caseum and albumen. Acids precipitate
the caoutchouc from the milky juice of the euphorbiums, fig-trees, and
hevea; they precipitate the caseum from the milk of animals. A white
coagulum was formed in phials closely stopped, containing the milk of
the hevea, and preserved among our collections, during our journey to
the Orinoco. It is perhaps the development of a vegetable acid which
then furnishes oxygen to the albumen. The formation of the coagulum of
the hevea, or of real caoutchouc, is nevertheless much more rapid in
contact with the air. The absorption of atmospheric oxygen is not in
the least necessary to the production of butter which exists already
formed in the milk of animals; but I believe it cannot be doubted
that, in the milk of plants, this absorption produces the pellicles of
caoutchouc, of coagulated albumen, and of caseum, which are
successively formed in vessels exposed to the open air.) The yellow
colour reappears on adding a few more drops of nitric acid. The acid
acts in this instance as the oxygen of the atmosphere at a temperature
from 27 to 35 degrees; for the white coagulum grows yellow in two or
three minutes, when exposed to the sun. After a few hours the yellow
colour turns to brown, no doubt because the carbon is set more free
progressively as the hydrogen, with which it was combined, is burnt.