d. Now carbon is
one of the minerals whose exchanges are peculiarly interesting. Chemists
say that the diamond is the only instance in Nature of pure carbon:
it burns in oxygen under the influence of intense heat, and leaves no
ashes. Next to this--strange gradation!--is charcoal, which comes within
a very little of being a diamond. But just that little interval is
apparently so great, that none but a chemist would suspect there was
any relationship between them. Then come all those immense beds of coal
which compose one of the geological strata of the earth's crust, a
stratum that was formed before the appearance of the animated creation,
when the earth was clothed with a gigantic forest, whose mighty trunks
buried themselves with their fallen leaves, and became, in time, a
continuous bed of carbonaceous stone.

If we look at the vegetable and animal kingdoms, we find carbon entering
into the composition of every tissue. But there are certain tissues and
anatomical elements (as physicians say) which are formed largely of
carbon and have no nitrogen whatever. These are oils and fats and
everything related to them. What will be chiefly interesting, however,
to our readers, is the power of transformation of one of these
substances into another. Starch, gum, and sugar can all be changed into
fat. The explanation of it is in the fact, that these substances are all
chemically alike,--that is, they all have nearly the same proportions of
carbon, oxygen, and hydrogen, and no nitrogen; but by slight differences
in the combination of these elements, they exist in Nature as so many
distinct substances. Their approach to identity is further confirmed
by the fact, that starch can be made into gum, and either of them into
sugar, in the laboratory. The transformation of starch and gum into
sugar is also constantly going on in the ripening of fruits. When
country-dames make currant-jellies and currant-wine, they know very
well, that, if they allow the berries to get dead-ripe, their jelly will
not be so firm as when they seize an early opportunity and gather them
when first fully red. They may also have observed that jelly made late,
besides being less firm, is much more likely to candy. At first, the
currants contain hardly any sugar, but more gum and vegetable jelly
(glue); when dead-ripe, they have twelve times as much sugar as at
first, and the gum and glue are much diminished. The gummy and gluey
materials have been transformed i