notwithstanding, as the diamond. This black-lead
(understand black-lead, as it is called, contains no metallic lead) is
used largely for making lead-pencils. The manufacture of lead-pencils,
by the bye, is a very interesting subject. Formerly they cut little
pieces of black-lead out of lumps of the natural black-lead such as you
see there; but now-a-days they powder the black-lead, and then compress
the very fine powder into a block. There is a block of graphite or black
lead, for instance, prepared by simple pressure (Fig. 18 _b_). The great
pressure to which the powder is subjected brings these fine particles
very close together, when they cohere, and form a substantial block. I
will show you an experiment to illustrate what I mean. Here are two
pieces of common metallic lead. No ordinary pressure would make these
two pieces stick together; but if I push them together very
energetically--boys would call it giving them "a shove" together--that
is to say, employing considerable pressure to bring them into close
contact--I have no doubt that I can make these two pieces of lead stick
together--in other words, make them cohere. To cohere is not to adhere.
Cohesion is the union of similar particles--like to like; adhesion is
the union of dissimilar particles. Now that is exactly what is done in
the preparation of the black-lead for lead-pencils. The black-lead
powder is submitted to great pressure, and then all these fine particles
cohere into one solid lump. The pencil maker now cuts these blocks with
a saw into very thin pieces (Fig. 19 _b_). The next thing is to prepare
the wood to receive the black-lead strips. To do this they take a piece
of flat cedar wood and cut a number of grooves in it, placing one of
these little strips of black-lead into each of the grooves (Fig. 19 _a_,
which represents one of the grooves). Then having glued on the cover
(Fig. 19 _c_), they cut it into strips, and plane each little strip into
a round lead-pencil (Fig. 19 _d_). But what you have there as black-lead
in the pencil (for this is what I more particularly wish you to
remember) is simply carbon, being just the same chemical substance as
the diamond. To a chemist diamond and black-lead have the same
composition, being indeed the same substance. As to their money value,
of course there is some difference; still, so far as chemical
composition is concerned, diamonds and black-lead are both absolutely
true varieties of the element carbon.