imperfectly supplied with sugar, several attempts were made to extract
it from other vegetables, and very good sugar was obtained from parsnips
and from carrots; but the process was too expensive to carry this
enterprize to any extent.
CAROLINE.
I should think that sugar might be more easily obtained from sweet
fruits, such as figs, dates, &c.
MRS. B.
Probably; but it would be still more expensive, from the high price of
those fruits.
EMILY.
Pray, in what manner is sugar obtained from the sugar-cane?
MRS. B.
The juice of this plant is first expressed by passing it between two
cylinders of iron. It is then boiled with lime-water, which makes a
thick scum rise to the surface. The clarified liquor is let off below
and evaporated to a very small quantity, after which it is suffered to
crystallise by standing in a vessel, the bottom of which is perforated
with holes, that are imperfectly stopped, in order that the syrup may
drain off. The sugar obtained by this process is a coarse brown powder,
commonly called raw or moist sugar; it undergoes another operation to be
refined and converted into loaf sugar. For this purpose it is dissolved
in water, and afterwards purified by an animal fluid called albumen.
White of eggs chiefly consist of this fluid, which is also one of the
constituent parts of blood; and consequently eggs, or bullocks’ blood,
are commonly used for this purpose.
The albuminous fluid being diffused through the syrup, combines with all
the solid impurities contained in it, and rises with them to the
surface, where it forms a thick scum; the clear liquor is then again
evaporated to a proper consistence, and poured into moulds, in which, by
a confused crystallisation, it forms loaf-sugar. But an additional
process is required to whiten it; to this effect the mould is inverted,
and its open base is covered with clay, through which water is made to
pass; the water slowly trickling through the sugar, combines with and
carries off the colouring matter.
CAROLINE.
I am very glad to hear that the blood that is used to purify sugar does
not remain in it; it would be a disgusting idea. I have heard of some
improvements by the late Mr. Howard, in the process of refining sugar.
Pray what are they?
MRS. B.
It would be much too long to give you an account of the process in
detail. But the principal improvement relates to the mode of evaporating
the syrup, in order to bring it to the consistency of sugar. Instead of
boiling the syrup in a large copper, over a strong fire, Mr. Howard
carries off the water by means of a large air-pump, in a way similar to
that used in Mr. Leslie’s experiment for freezing water by evaporation;
that is, the syrup being exposed to a vacuum, the water evaporates
quickly, with no greater heat than that of a little steam, which is
introduced round the boiler. The air-pump is of course of large
dimensions, and is worked by a steam engine. A great saving is thus
obtained, and a striking instance afforded of the power of science in
suggesting useful economical improvements.
EMILY.
And pray how is sugar-candy and barley-sugar prepared?
MRS. B.
Candied sugar is nothing more than the regular crystals, obtained by
slow evaporation from a solution of sugar. Barley-sugar is sugar melted
by heat, and afterwards cooled in moulds of a spiral form.
Sugar may be decomposed by a red heat, and, like all other vegetable
substances, resolved into carbonic acid and hydrogen. The formation and
the decomposition of sugar afford many very interesting particulars,
which we shall fully examine, after having gone through the other
materials of vegetables. We shall find that there is reason to suppose
that sugar is not, like the other materials, secreted from the sap by
appropriate organs; but that it is formed by a peculiar process with
which you are not yet acquainted.
CAROLINE.
Pray, is not honey of the same nature as sugar?
MRS. B.
Honey is a mixture of saccharine matter and gum.
EMILY.
I thought that honey was in some measure an animal substance, as it is
prepared by the bees.
MRS. B.
It is rather collected by them from flowers, and conveyed to their
store-houses, the hives. It is the wax only that undergoes a real
alteration in the body of the bee, and is thence converted into an
animal substance.
Manna is another kind of sugar, which is united with a nauseous
extractive matter, to which it owes its peculiar taste and colour. It
exudes like gum from various trees in hot climates, some of which have
their leaves glazed by it.
The next of the vegetable materials is _fecula_; this is the general
name given to the farinaceous substance contained in all seeds, and in
some roots, as the potatoe, parsnip, &c. It is intended by nature for
the first aliment of the young vegetable; but that of one particular
grain is become a favourite and most common food of a large part of
mankind.
EMILY.
You allude, I suppose, to bread, which is made of wheat-flower?
MRS. B.
Yes. The fecula of wheat contains also another vegetable substance which
seems peculiar to that seed, or at least has not as yet been obtained
from any other. This is _gluten_, which is of a sticky, ropy, elastic
nature; and it is supposed to be owing to the viscous qualities of this
substance, that wheat-flour forms a much better paste than any other.
EMILY.
Gluten, by your description, must be very like gum?
MRS. B.
In their sticky nature they certainly have some resemblance; but gluten
is essentially different from gum in other points, and especially in its
being insoluble in water, whilst gum, you know, is extremely soluble.
The _oils_ contained in vegetables all consist of hydrogen and carbon in
various proportions. They are of two kinds, _fixed_ and _volatile_, both
of which we formerly mentioned. Do you remember in what the difference
between fixed and volatile oil consists?
EMILY.
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?
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.
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 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 quantity 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 quantity 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 quantity 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 quantity 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 pass 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
butter, 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.
_Camphor_ seems to be a substance of its own kind, remarkable by many
peculiarities. But if not exactly of the same nature as volatile oil, it
is at least very analogous to it. It is obtained chiefly from the
camphor-tree, a species of laurel which grows in China, and in the
Indian isles, from the stem and roots of which it is extracted. Small
quantities have also been distilled from thyme, sage, and other aromatic
plants; and it is deposited in pretty large quantities by some volatile
oils after long standing. It is extremely volatile and inflammable. It
is insoluble in water, but is soluble in oils, in which state, as well
as in its solid form, it is frequently applied to medicinal purposes.
Amongst the particular properties of camphor, there is one too singular
to be passed over in silence. If you take a small piece of camphor, and
place it on the surface of a bason of pure water, it will immediately
begin to move round and round with great rapidity; but if you pour into
the bason a single drop of any odoriferous fluid, it will instantly put
a stop to this motion. You can at any time try this very simple
experiment; but you must not expect that I shall be able to account for
this phenomenon, as nothing satisfactory has yet been advanced for its
explanation.
CAROLINE.
It is very singular indeed; and I will certainly try the experiment.
Pray what are _resins_, which you just now mentioned?
MRS. B.
They are volatile oils, that have been acted on, and peculiarly
modified, by oxygen.
CAROLINE.
They are, therefore, oxygenated volatile oils?
MRS. B.
Not exactly; for the process does not appear to consist so much in the
oxygenation of the oil, as in the combustion of a portion of its
hydrogen, and a small portion of its carbon. For when resins are
artificially made by the combination of volatile oils with oxygen, the
vessel in which the process is performed is bedewed with water, and the
air included within is loaded with carbonic acid.
EMILY.
This process must be, in some respects, similar to that for preparing
drying oils?
MRS. B.
Yes; and it is by this operation that both of them acquire a greater
degree of consistence. Pitch, tar, and turpentine, are the most common
resins; they exude from the pine and fir trees. Copal, mastic, and
frankincense, are also of this class of vegetable substances.
EMILY.
Is it of these resins that the mastic and copal varnishes, so much used
in painting, are made?
MRS. B.
Yes. Dissolved either in oil, or in alcohol, resins form varnishes. From
these solutions they may be precipitated by water, in which they are
insoluble. This I can easily show you. --If you will pour some water
into this glass of mastic varnish, it will combine with the alcohol in
which the resin is dissolved, and the latter will be precipitated in the
form of a white cloud--
EMILY.
It is so. And yet how is it that pictures or drawings, varnished with
this solution, may safely be washed with water?
MRS. B.
As the varnish dries, the alcohol evaporates, and the dry varnish or
resin which remains, not being soluble in water, will not be acted on
by it.
There is a class of compound resins called _gum-resins_, which are
precisely what their name denotes, that is to say, resins combined with
mucilage. Myrrh and assafœtida are of this description.
CAROLINE.
Is it possible that a substance of so disagreeable a smell as assafœtida
can be formed from a volatile oil?
MRS. B.
The odour of volatile oils is by no means always grateful. Onions and
garlic derive their smell from volatile oils, as well as roses and
lavender.
There is still another form under which volatile oils present
themselves, which is that of _balsams_. These consist of resinous juices
combined with a peculiar acid, called the benzoic acid. Balsams appear
to have been originally volatile oils, the oxygenation of which has
converted one part into a resin, and the other part into an acid, which,
combined together, form a balsam; such are the balsams of Peru, Tolu,
&c.
We shall now take leave of the oils and their various modifications, and
proceed to the next vegetable substance, which is _caoutchouc_. This is
a white milky glutinous fluid, which acquires consistence, and blackens
in drying, in which state it forms the substance with which you are so
well acquainted, under the name of gum-elastic.
CAROLINE.
I am surprised to hear that gum-elastic was ever white, or ever fluid!
And from what vegetable is it procured?
MRS. B.
It is obtained from two or three different species of trees, in the
East-Indies, and South-America, by making incisions in the stem. The
juice is collected as it trickles from these incisions, and moulds of
clay, in the form of little bottles of gum-elastic, are dipped into it.
A layer of this juice adheres to the clay and dries on it; and several
layers are successively added by repeating this till the bottle is of
sufficient thickness. It is then beaten to break down the clay, which is
easily shaken out. The natives of the countries where this substance is
produced sometimes make shoes and boots of it by a similar process, and
they are said to be extremely pleasant and serviceable, both from their
elasticity, and their being water-proof.
The substance which comes next in our enumeration of the immediate
ingredients of vegetables, is _extractive matter_. This is a term,
which, in a general sense, may be applied to any substance extracted
from vegetables; but it is more particularly understood to relate to the
extractive _colouring matter_ of plants. A great variety of colours are
prepared from the vegetable kingdom, both for the purposes of painting
and of dying; all the colours called _lakes_ are of this description;
but they are less durable than mineral colours, for, by long exposure to
the atmosphere, they either darken or turn yellow.
EMILY.
I know that in painting, the lakes are reckoned far less durable colours
than the ochres; but what is the reason of it?
MRS. B.
The change which takes place in vegetable colours is owing chiefly to
the oxygen of the atmosphere slowly burning their hydrogen, and leaving,
in some measure, the blackness of the carbon exposed. Such change cannot
take place in ochre, which is altogether a mineral substance.
Vegetable colours have a stronger affinity for animal than for vegetable
substances, and this is supposed to be owing to a small quantity of
nitrogen which they contain. Thus, silk and worsted will take a much
finer vegetable dye than linen and cotton.
CAROLINE.
Dying, then, is quite a chemical process?
MRS. B.
Undoubtedly. The condition required to form a good dye is, that the
colouring matter should be precipitated, or fixed, on the substance to
be dyed, and should form a compound not soluble in the liquids to which
it will probably be exposed. Thus, for instance, printed or dyed linens
or cottons must be able to resist the action of soap and water, to which
they must necessarily be subject in washing; and woollens and silks
should withstand the action of grease and acids, to which they may
accidentally be exposed.
CAROLINE.
But if linen and cotton have not a sufficient affinity for colouring
matter, how are they made to resist the action of washing, which they
always do when they are well printed?
MRS. B.
When the substance to be dyed has either no affinity for the colouring
matter, or not sufficient power to retain it, the combination is
effected, or strengthened, by the intervention of a third substance,
called a _mordant_, or basis. The mordant must have a strong affinity
both for the colouring matter and the substance to be dyed, by which
means it causes them to combine and adhere together.
CAROLINE.
And what are the substances that perform the office of thus reconciling
the two adverse parties?
MRS. B.
The most common mordant is sulphat of alumine, or alum. Oxyds of tin and
iron, in the state of compound salts, are likewise used for that
purpose.
_Tannin_ is another vegetable ingredient of great importance in the
arts. It is obtained chiefly from the bark of trees; but it is found
also in nut-galls, and in some other vegetables.
EMILY.
Is that the substance commonly called _tan_, which is used in
hot-houses?
MRS. B.
Tan is the prepared bark in which the peculiar substance, tannin, is
contained. But the use of tan in hot-houses is of much less importance
than in the operation of _tanning_, by which skin is converted into
leather.
EMILY.
Pray, how is this operation performed?
MRS. B.
Various methods are employed for this purpose, which all consist in
exposing skin to the action of tannin, or of substances containing this
principle, in sufficient quantities, and disposed to yield it to the
skin. The most usual way is to infuse coarsely powdered oak bark in
water, and to keep the skin immersed in this infusion for a certain
length of time. During this process, which is slow and gradual, the skin
is found to have increased in weight, and to have acquired a
considerable tenacity and impermeability to water. This effect may be
much accelerated by using strong saturations of the tanning principle
(which can be extracted from bark), instead of employing the bark
itself. But this quick mode of preparation does not appear to make
equally good leather.
Tannin is contained in a great variety of astringent vegetable
substances, as galls, the rose-tree, and wine; but it is nowhere so
plentiful as in bark. All these substances yield it to water, from which
it may be precipitated by a solution of isinglass, or glue, with which
it strongly unites and forms an insoluble compound. Hence its valuable
property of combining with skin (which consists chiefly of glue), and of
enabling it to resist the action of water.
EMILY.
Might we not see that effect by pouring a little melted isinglass into a
glass of wine, which you say contains tannin?
MRS. B.
Yes. I have prepared a solution of isinglass for that very purpose. --Do
you observe the thick muddy precipitate? --That is the tannin combined
with the isinglass.
CAROLINE.
This precipitate must then be of the same nature as leather?
MRS. B.
It is composed of the same ingredients; but the organisation and texture
of the skin being wanting, it has neither the consistence nor the
tenacity of leather.
CAROLINE.
One might suppose that men who drink large quantities of red wine stand
a chance of having the coats of their stomachs converted into leather,
since tannin has so strong an affinity for skin.
MRS. B.