combustion of oil is just the same as that of a candle;
if tallow, it is only oil in a concrete state; if wax, or spermaceti,
its chief chemical ingredients are still hydrogen and carbon.

EMILY.

I wonder, then, there should be so great a difference between tallow and
wax?

MRS. B.

I must again repeat, that the same substances, in different proportions,
produce results that have sometimes scarcely any resemblance to each
other. But this is rather a general remark that I wish to impress upon
your minds, than one which is applicable to the present case; for tallow
and wax are far from being very dissimilar; the chief difference
consists in the wax being a purer compound of carbon and hydrogen than
the tallow, which retains more of the gross particles of animal matter.
The combustion of a candle, and that of a lamp, both produce water and
carbonic acid gas. Can you tell me how these are formed?

EMILY.

Let me reflect . . . . Both the candle and lamp burn by means of fixed
oil--this is decomposed as the combustion goes on; and the constituent
parts of the oil being thus separated, the carbon unites to a portion of
oxygen from the atmosphere to form carbonic acid gas, whilst the
hydrogen combines with another portion of oxygen, and forms with it
water. --The products, therefore, of the combustion of oils are water
and carbonic acid gas.

CAROLINE.

But we see neither water nor carbonic acid produced by the combustion of
a candle.

MRS. B.

The carbonic acid gas, you know, is invisible, and the water being in a
state of vapour, is so likewise. Emily is perfectly correct in her
explanation, and I am very much pleased with it.

All the vegetable acids consist of various proportions of carbon and
hydrogen, acidified by oxygen. Gums, sugar, and starch, are likewise
composed of these ingredients; but, as the oxygen which they contain is
not sufficient to convert them into acids, they are classed with the
oxyds, and called vegetable oxyds.

CAROLINE.

I am very much delighted with all these new ideas; but, at the same
time, I cannot help being apprehensive that I may forget many of them.

MRS. B.

I would advise you to take notes, or, what would answer better still, to
write down, after every lesson, as much of it as you can recollect. And,
in order to give you a little assistance, I shall lend you the heads or
index, which I occasionally consult for the sake of preserving some
method and arrangement in th