a porcelain tube, containing charcoal,
passes. To one end of the tube is adapted a glass retort with water in
it; and the other end communicates with a receiver placed on the
water-bath. A lamp being applied to the retort, and the water made to
boil, the vapour is gradually conveyed through the red-hot charcoal, by
which it is decomposed; and the hydrogen gas which results from this
decomposition is collected in the receiver. But the hydrogen thus
obtained is far from being pure; it retains in solution a minute portion
of carbon, and contains also a quantity of carbonic acid. This renders
it heavier than pure hydrogen gas, and gives it some peculiar
properties; it is distinguished by the name of _carbonated hydrogen
gas_.

CAROLINE.

And whence does it obtain the carbonic acid that is mixed with it?

EMILY.

I believe I can answer that question, Caroline. --From the union of the
oxygen (proceeding from the decomposed water) with the carbon, which,
you know, makes carbonic acid.

CAROLINE.

True; I should have recollected that. --The product of the decomposition
of water by red-hot charcoal, therefore, is carbonated hydrogen gas, and
carbonic acid gas.

MRS. B.

You are perfectly right now.

Carbon is frequently found combined with hydrogen in a state of
solidity, especially in coals, which owe their combustible nature to
these two principles.

EMILY.

Is it the hydrogen, then, that produces the flame of coals?

MRS. B.

It is so; and when all the hydrogen is consumed, the carbon continues to
burn without flame. But again, as I mentioned when speaking of the
gas-lights, the hydrogen gas produced by the burning of coals is not
pure; for, during the combustion, particles of carbon are successively
volatilized with the hydrogen, with which they form what is called a
_hydro-carbonat_, which is the principal product of this combustion.

Carbon is a very bad conductor of heat; for this reason, it is employed
(in conjunction with other ingredients) for coating furnaces and other
chemical apparatus.

EMILY.

Pray what is the use of coating furnaces?

MRS. B.

In most cases, in which a furnace is used, it is necessary to produce
and preserve a great degree of heat, for which purpose every possible
means are used to prevent the heat from escaping by communicating with
other bodies, and this object is attained by coating over the inside of
the furnace with a kind of plaster, composed of materials that ar