Page 64 - My FlipBook

52 MECHANICAL DENTISTRY AND METALLURGY.
eliminated from the wood, the illuminating gas, which
escapes, and three substances a watery pyroligneous acid
(wood vinegar), a thick, viscid liquid (wood tar), and a
solid mass (charcoal).
"The wood tar is, like rubber, of a resinous natiu'e ; it
consists of the oil of wood tar and a licjuid substance, burnt
resin. Both become hard on cooling; the former is the
well-known paraffin, the latter the equally well-known pitch.
" The rubber undergoes similar changes. If it is heated
while excluded from air, as is the case in vulcanizing, there
escapes (as in the case of wood, illuminating gas), the
hydrogen sulphid, and there remains a plastic, which
hardens on cooling, as in the case of pitch or paraffin, and
we have our hard rubber.
" If we think over this subject, it becomes clear to us
why sulphur is added to the rubber. By dry distillation
one or more equivalents of hydrogen separate from the
mass and remain gaseous, or unite with other substances
present and form a liquid, thus leaving behind a hard sub-
stance, which consists mainly of C. It is well known that
the hardest substance, the diamond, is pure C. The more
equivalents of H that remain, the softer is the substance,
as in the following scale : coal, resin, pitch, axle grease,
oil, ethereal oils, gases. The same is the case in the re-
versed order. If, from the soft rubber, hard rubber is to
be made, it is necessary to remove from it one or more
equivalents of H. This is the case in dry distillation. If
there is no dry distillation, if the rubber was heated under
free admission of air, the C would immediately unite with
the O of the air, forming carbonic acid, combustion would
take place, even though it would be slow and difficult. This
cannot take place when the air is excluded; the carbon
remains unchanged, whereas the H finds a substance with
which it is more willing to unite at a high temperature.