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 tr.i.m.m.i.n.g lamps, applies also to candles, which so often require snuffing?
MRS. B.
I believe it does; at least, in some degree. But besides the circ.u.mstance just explained, the common sorts of oils are not very highly combustible, so that the heat produced by a candle, which is a coa.r.s.e kind of animal oil, being insufficient to volatilise them completely, a quant.i.ty of soot is gradually deposited on the wick, which dims the light, and r.e.t.a.r.ds 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 absorb oxygen at the common temperature of the atmosphere. Hence arises a variety of changes in oils which modify their properties and uses in the arts.
If oil simply absorbs, and combines with oxygen, it thickens and changes to a kind of wax. This change is observed to take place on the external parts of certain vegetables, even during their life. But it happens in many instances that the oil does not retain all the oxygen which it attracts, but that part of it combines with, or burns, the hydrogen of the oil, thus forming a quant.i.ty of water, which gradually goes off by evaporation. In this case the alteration of the oil consists not only in the addition of a certain quant.i.ty of oxygen, but in the diminution of the hydrogen. These oils are distinguished by the name of _drying oils_.
Linseed, poppy, and nut-oils, are of this description.
EMILY.
I am well acquainted with drying oils, as I continually use them in painting. But I do not understand why the acquisition of oxygen on one hand, and a loss of hydrogen on the other, should render them drying?
MRS. B.
This, I conceive, may arise from two reasons; either from the oxygen which is added being less favourable to the state of fluidity than the hydrogen, which is subtracted; or from this additional quant.i.ty of oxygen giving rise to new combinations, in consequence of which the most fluid parts of the oil are liberated and volatilised.
For the purpose of painting, the drying quality of oil is further increased by adding a quant.i.ty of oxyd of lead to it, by which means it is more rapidly oxygenated.
The rancidity of oil is likewise owing to their oxygenation. In this case a new order of attraction takes place, from which a peculiar acid is formed, called the _sebacic acid_.
CAROLINE.
Since the nature and composition of oil is so well known, pray could not oil be actually _made_, by combining its principles?
MRS. B.
That is by no means a necessary consequence; for there are innumerable varieties of compound bodies which we can decompose, although we are unable to reunite their ingredients. This, however, is not the case with oil, as it has very lately been discovered, that it is possible to form oil, by a peculiar process, from the action of oxygenated muriatic acid gas on hydro-carbonate.
We now pa.s.s to the _volatile_ or _essential oils_. These form the basis of all the vegetable perfumes, and are contained, more or less, in every part of the plant excepting the seed; they are, at least, never found in that part of the seed which contains the embrio plant.
EMILY.
The smell of flowers, then, proceeds from volatile oil?
MRS. B.
Certainly; but this oil is often most abundant in the rind of fruits, as in oranges, lemons, &c. from which it may be extracted by the slightest pressure; it is found also in the leaves of plants, and even in the wood.
CAROLINE.
Is it not very plentiful in the leaves of mint, and of thyme, and all the sweet-smelling herbs?
MRS. B.
Yes, remarkably so; and in geranium leaves also, which have a much more powerful odour than the flowers.
The perfume of sandal fans is an instance of its existence in wood. In short, all vegetable odours or perfumes are produced by the evaporation of particles of these volatile oils.
EMILY.
They are, I suppose, very light, and of very thin consistence, since they are so volatile?
MRS. B.
They vary very much in this respect, some of them being as thick as b.u.t.ter, whilst others are as fluid as water. In order to be prepared for perfumes, or essences, these oils are first properly purified, and then either distilled with spirit of wine, as in the case with lavender water, or simply mixed with a large proportion of water, as is often done with regard to peppermint. Frequently, also, these odoriferous waters are prepared merely by soaking the plants in water, and distilling. The water then comes over impregnated with the volatile oil.
CAROLINE.
Such waters are frequently used to take spots of grease out of cloth, or silk; how do they produce that effect?
MRS. B.
By combining with the substance that forms these stains; for volatile oils, and likewise the spirit in which they are distilled, will dissolve wax, tallow, spermaceti, and resins; if, therefore, the spot proceeds from any of these substances, it will remove it. Insects of every kind have a great aversion to perfumes, so that volatile oils are employed with success in museums for the preservation of stuffed birds and other species of animals.
CAROLINE.
Pray does not the powerful smell of camphor proceed from a volatile oil?
MRS. B.