e bad
conductors of heat.

Carbon, combined with a small quantity of iron, forms a compound called
plumbago, or black-lead, of which pencils are made. This substance,
agreeably to the nomenclature, is _a carburet of iron_.

EMILY.

Why, then, is it called black-lead?

MRS. B.

It is an ancient name given to it by ignorant people, from its shining
metallic appearance; but it is certainly a most improper name for it, as
there is not a particle of lead in the composition. There is only one
mine of this mineral, which is in Cumberland. It is supposed to approach
as nearly to pure carbon as the best prepared charcoal does, as it
contains only five parts of iron, unadulterated by any other foreign
ingredients. There is another carburet of iron, in which the iron,
though united only to an extremely small proportion of carbon, acquires
very remarkable properties; this is steel.

CAROLINE.

Really; and yet steel is much harder than iron?

MRS. B.

But carbon is not ductile like iron, and therefore may render the steel
more brittle, and prevent its bending so easily. Whether it is that the
carbon, by introducing itself into the pores of the iron, and, by
filling them, makes the metal both harder and heavier; or whether this
change depends upon some chemical cause, I cannot pretend to decide. But
there is a subsequent operation, by which the hardness of steel is very
much increased, which simply consists in heating the steel till it is
red-hot, and then plunging it into cold water.

Carbon, besides the combination just mentioned, enters into the
composition of a vast number of natural productions, such, for instance,
as all the various kinds of oils, which result from the combination of
carbon, hydrogen, and caloric, in various proportions.

EMILY.

I thought that carbon, hydrogen, and caloric, formed carbonated hydrogen
gas?

MRS. B.

That is the case when a small portion of carbonic acid gas is held in
solution by hydrogen gas. Different proportions of the same principles,
together with the circumstances of their union, produce very different
combinations; of this you will see innumerable examples. Besides, we are
not now talking of gases, but of carbon and hydrogen, combined only with
a quantity of caloric sufficient to bring them to the consistency of oil
or fat.

CAROLINE.

But oil and fat are not of the same consistence?

MRS. B.

Fat is only congealed oil; or oil, melted fat. The one requires a li