s transparent or colorless. In that case it is necessary
to add an antacid such as urea to prevent its getting yellow or opaque.
To make it opaque and less inflammable oxides or chlorides of zinc,
aluminum, magnesium, etc., are mixed in.

Without going into the question of their variations and relative merits
we may consider the advantages of the pyroxylin plastics in general.
Here we have a new substance, the product of the creative genius of man,
and therefore adaptable to his needs. It is hard but light, tough but
elastic, easily made and tolerably cheap. Heated to the boiling point of
water it becomes soft and flexible. It can be turned, carved, ground,
polished, bent, pressed, stamped, molded or blown. To make a block of
any desired size simply pile up the sheets and put them in a hot press.
To get sheets of any desired thickness, simply shave them off the block.
To make a tube of any desired size, shape or thickness squirt out the
mixture through a ring-shaped hole or roll the sheets around a hot bar.
Cut the tube into sections and you have rings to be shaped and stamped
into box bodies or napkin rings. Print words or pictures on a celluloid
sheet, put a thin transparent sheet over it and weld them together, then
you have something like the horn book of our ancestors, but better.

Nowadays such things as celluloid and pyralin can be sold under their
own name, but in the early days the artificial plastics, like every new
thing, had to resort to _camouflage_, a very humiliating expedient since
in some cases they were better than the material they were forced to
imitate. Tortoise shell, for instance, cracks, splits and twists, but a
"tortoise shell" comb of celluloid looks as well and lasts better. Horn
articles are limited to size of the ceratinous appendages that can be
borne on the animal's head, but an imitation of horn can be made of any
thickness by wrapping celluloid sheets about a cone. Ivory, which also
has a laminated structure, may be imitated by rolling together alternate
white opaque and colorless translucent sheets. Some of the sheets are
wrinkled in order to produce the knots and irregularities of the grain
of natural ivory. Man's chief difficulty in all such work is to imitate
the imperfections of nature. His whites are too white, his surfaces are
too smooth, his shapes are too regular, his products are too pure.

The precious red coral of the Mediterranean can be perfectly imitated by
taking a cast