n to them is owing to
their being less expensive; for vegetable oils burn equally well, and
are more pleasant, as their smell is not offensive.

EMILY.

Since oil is so good a combustible, what is the reason that lamps so
frequently require trimming?

MRS. B.

This sometimes proceeds from the construction of the lamp, which may not
be sufficiently favourable to a perfect combustion; but there is
certainly a defect in the nature of oil itself, which renders it
necessary for the best-constructed lamps to be occasionally trimmed.
This defect arises from a portion of mucilage which it is extremely
difficult to separate from the oil, and which being a bad combustible,
gathers round the wick, and thus impedes its combustion, and
consequently dims the light.

CAROLINE.

But will not oils burn without a wick?

MRS. B.

Not unless their temperature be elevated to five or six hundred degrees;
the wick answers this purpose, as I think I once before explained to
you. The oil rises between the fibres of the cotton by capillary
attraction, and the heat of the burning wick volatilises it, and brings
it successively to the temperature at which it is combustible.

EMILY.

I suppose the explanation which you have given with regard to the
necessity of trimming lamps, applies also to candles, which so often
require snuffing?

MRS. B.

I believe it does; at least, in some degree. But besides the
circumstance just explained, the common sorts of oils are not very
highly combustible, so that the heat produced by a candle, which is a
coarse kind of animal oil, being insufficient to volatilise them
completely, a quantity of soot is gradually deposited on the wick, which
dims the light, and retards the combustion.

CAROLINE.

Wax candles then contain no incombustible matter, since they do not
require snuffing?

MRS. B.

Wax is a much better combustible than tallow, but still not perfectly
so, since it likewise contains some particles that are unfit for
burning; but when these gather round the wick, (which in a wax light is
comparatively small,) they weigh it down on one side, and fall off
together with the burnt part of the wick.

CAROLINE.

As oils are such good combustibles, I wonder that they should require so
great an elevation of temperature before they begin to burn?

MRS. B.

Though fixed oils will not enter into actual combustion below the
temperature of about four hundred degrees, yet they will slowly absor