If I recollect rightly, the former are decomposed by heat, whilst the latter are merely volatilised by it.
MRS. B.
Very well. Fixed oil is contained only in the seeds of plants, excepting in the olive, in which it is produced in, and expressed from, the fruit. We have already observed that seeds contain also fecula; these two substances, united with a little mucilage, form the white substance contained in the seeds or kernels of plants, and is destined for the nourishment of the young plant, to which the seed gives birth. The milk of almonds, which is expressed from the seed of that name, is composed of these three substances.
EMILY.
Pray, of what nature is the linseed oil which is used in painting?
MRS. B.
It is a fixed oil, obtained from the seed of flax. Nut oil, which is frequently used for the same purpose, is expressed from walnuts.
Olive oil is that which is best adapted to culinary purposes.
CAROLINE.
And what are the oils used for burning?