ttle
more heat to maintain it in a fluid state than the other. Have you never
observed the fat of meat turned to oil by the caloric it has imbibed
from the fire?

EMILY.

Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat when
cold?

MRS. B.

Not at the common temperature of the atmosphere, because they retain too
much caloric to congeal at that temperature; but if exposed to a
sufficient degree of cold, their latent heat is extricated, and they
become solid fat substances. Have you never seen salad oil frozen in
winter?

EMILY.

Yes; but it appears to me in that state very different from animal fat.

MRS. B.

The essential constituent parts of either vegetable or animal oils are
the same, carbon and hydrogen; their variety arises from the different
proportions of these substances, and from other accessory ingredients
that may be mixed with them. The oil of a whale, and the oil of roses,
are, in their essential constituent parts, the same; but the one is
impregnated with the offensive particles of animal matter, the other
with the delicate perfume of a flower.

The difference of _fixed oils_, and _volatile_ or _essential oils_,
consists also in the various proportions of carbon and hydrogen. Fixed
oils are those which will not evaporate without being decomposed; this
is the case with all common oils, which contain a greater proportion of
carbon than the essential oils. The essential oils (which comprehend the
whole class of essences and perfumes) are lighter; they contain more
equal proportions of carbon and hydrogen, and are volatilized or
evaporated without being decomposed.

EMILY.

When you say that one kind of oil will evaporate, and the other be
decomposed, you mean, I suppose, by the application of heat?

MRS. B.

Not necessarily; for there are oils that will evaporate slowly at the
common temperature of the atmosphere; but for a more rapid
volatilization, or for their decomposition, the assistance of heat is
required.

CAROLINE.

I shall now remember, I think, that fat and oil are really the same
substances, both consisting of carbon and hydrogen; that in fixed oils
the carbon preponderates, and heat produces a decomposition; while, in
essential oils, the proportion of hydrogen is greater, and heat produces
a volatilization only.

EMILY.

I suppose the reason why oil burns so well in lamps is because its two
constituents are so combustible?

MRS. B.

Certainly; the