MRS. B.
Animal oils most commonly; but the preference given 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.