Mrs. (Jane Haldimand) Marcet.

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J^r^, B. That is true, my dear. But I widhed to give yoU a gen-^
eral idea of the nature of vegetation, before we entered into par^
ticnlars. Besides, it is not so irrelevant as you suppose to talk of ve^

fetables in their dead state, since we cannot analyze them without
estroying life ; and it is only by hastening to suiimit them to ex-
amination, immediatelv after thejr have ceased to Ijye, that we can
anticipate their natural decomposition. Thpre are two kinds^of afla-
Ivsis of which ve^tables are susceptible ; /first, that which separates
tnetn into their immediate roatenals, siidh as sap, resin, mucilage,
&cj>^ondly, that which decomposes them into their primitive
elemots, as carbon, hydrogen, and oxygen)

Emily, Is there not a third kind ef analyril of plants, which con-
sists in separating th^ir various parts, as the stem, the leaves, and
the several organs of the flower ?

Mrs, B, That, my dean i^ rather the department of the botanist;
we shall consider these different parts of plants only, as the organs
by which the various secretions or separations are performed ; but
we^nst first eiamine the nature of tnese secretions.

^e sap is the principal material of vegetables; since it contains
the ingredients that nourish every part of the plsCnt. l^e basis of
this juice, which the roots suck up from the soil, i^aterp this holds
in solution the various other ingredirats required bvlhe several
parts of the plant, which are gradually sccretcMi frpm the sap by the
different organs appropriated to that purpose, as it passes them in
circfflating through the plant.

mucus, or mucilage^ is a vegetable substance, which, like all the

1066. Why should death destroy ve^table combinations ?

1067. What is an admirably wise dispensmtion of Providence in
regard to the nature of plants ?

1068. Of how many kinds of analysis are vegetables susceptible ?

1069. What is the first ?

1070. What is the second ?

1071. What is. the principal material of vegetables '

1072. What is the basis of this juice? .^ i

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242 COMPOSITION ^

others, is secreted from the sap ; when in excess, it exudes from the
trees in the form of g^unft

Caroline* Is that the 'gum so^requently used instead of paste or
gluel V

Jm's. R, It is ; almost all fruit trees yield some sort of gum, but
that most commonly used in the arts is^ obtained from a species of
acacia-tree, in Arabia, and is called gum arable; it forms the chief
nourishment of the natires of those parts, who obtain it in great'
quantities from incisions which they make in the trees.
Caroline, Idid not know that gum was eatable.
Mrs. B, There is an account of a whole ship's company being
saved from starving by feeding on the cargo, which was gum sen-
ega!. I should not, however, imagine, that it would be either a plea-
sant or a particularly eligible diet to those who have not, from their
birth, been accustomed to it. It is, however, frequently taken me-
dicinally, and considered as very nourishing. Several kinds of ye-
getable acids may be obtained, by particular processes, from g^m
or .noucilage, the principal of which is called the mucous acid.

iSugor is not found in its simple state in plants, but is always mix-
ed with gum, sap, or other ingredients : this saccharine' matter is to
be met with in every vegeta^ble^sj^t abounds most in roots, fruits,
and particularly in the sugar cade^

Emily. If all regetables contftm sug^r, why is it extracted ex-
clusively from the sugar-cane ?

Jlfr«. B. /Because it is both most abundant in that plant, and
most easily Vbtained from it* Besides, the sugars produced by oth^
yegetables differ a little in their nature^

During the late troubles in the West Indies,. when Europe was
but imperfectly supplied with sugar, several attempts were made to
extract it from other yegetables, and very good sugar was obtained
from parsnips and froni 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, and it would not be exactly like com-
mon sugar.^

Emily. Pray, in what manner, is sugar obtained from the sugar-
cane?



* Some foreign chemists (MM. Ktrfcoff, Braconnot, &c.) have
found that if starch be boiled for a long time in water containing
one fortieth part of sulphuric acid, and evaporated down to a certain
consistence, the solution of starch concretes, in cooling, into a solid
brownish mass, which has the taste and other general properties of
sugar. During this process, no gas is disengaged, and the acid is
not decomposed.

1073. What is the mucilage of vegetables ?

1074. What uses are made of mucilage or gum f

1075. In what state does sugar naturally exist ?

1076. In what does it mostly abound ?

1077. If it is found in all vegetables, why is it extracted from su-
garcaneonly?

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OF VEGETABLES. 243

%Wr«. B. The juice of ihitf plant is first eipressed by passing it
between t?^o cylinders of iron. It is then boiled with liine-waler,
which makes a thick scum nse to the surface. The clarified Mquor
is let off below, and evaporated to a very small quantity, after
which it is suffered to crystallize 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 su^r. For this purpose it is dissolved in water, and after-
wards purified by an animal fluid called albumen. White of eggs
chiefly consists.of this fluid, which is also one of the constituent parts
of blood : and consequently eggs, or bullock's blood, are commonly
used for this purpose.

The albuminous fluid being diffused through the s^rup, combines
with all the solid impurities contained in it, ajjd inses with them to
the surface where it forms a thick bcuim; /ne clear liqupr is then
again evaporated to a proper consisteno?, ahd poured mto moulds,
in which, by a confused crystallization, 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, throug"h w^hich
water is made to pass ; the water slowly tricklingjthrough the sugar
combines with and carries off the t;olouring mater)

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 IVIr. Howard in the
process of refining sugar. Fray what are they ?

Mrs, B. It would bejnuch too long to give you an account of the
process in detail. But^e 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; oyer a strong
fire, Mri Howard carries 6ff the water by means of a lar^e 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 toa vacuum,
the water evaporates quickly, with n6 greater heat than that of a lit-
tle steftm, which is introducea round the hoiler. The air-pumnis of
course of large dimensions, and is worked by a steam engines A
great saving is thus obtained, and a striking instance afforded m the
power of science in suggesting useful economical improvements.

Emily, And pray bow are sug^r-candy and barley-sugar pre-
pared ?

Mrs, B, jCandied sug^r is nothing more than the regular crystals,
obtained uj slow evaporation from a soltition of sugar. Barley-
sugar is sugar melted by heat, and afterwards cooled in inoulds of
a sj^iral form^
^ugar may be deeoro posed by a red heat, and, like all other ve-
getable substances, i'esolved into carbonic acid and hydrogenj The
formation and the decomposition of sugar afford many verynnter-
esting particulars, which we shall fully examine after having gone

1078. In what manner is sugar obtained from su^rar^cane ?

1079. How is sugar refined or couverted into loaf sugar ?

1080. What is Mr. Howard's improvement for refining sugar ^

1081. How is sugar-candy and barley-sugar prepared ?

108^. How may sugar be decomposed, and what is the product ^

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244 coMPOsindN

through the other materials of rc^etables. We shall find that there
isreasoo to suppose that sugar is not like the other materials, se-
creted from the sap by appropriate organs ; but that it is formed by
a peculiar process with which you^ere not yet acquainted.

C(iro/tn«.\.Pray, is not honey of the same nature as sugar ?

Mrs, B. (Uon^y is a mixture of saocharine matter -and giim^

Emily, rthought that honey was in some measure an animafl sub-
stance, as it is prepared by the bees*

Jlfri. B, It is rather collected by them from flowers, and convey-
ed to their store-houses, the hives. It is the wax only that under-
goes a real alteration in the body of the bee, and is Whence convert-
edjuito an animal substance.*

Alanna is another kind of sugar, which is miited with a nauseous
extractive matter, to which it owes its peculiar taste and colour.^ It
exudes like gum from various trees in hot Qlimates, some of which
have their leaves glazed by it/

The n^t of the ve^table materials w/ecula; Inis is the general
name g^v^n to the £atrinaceous substance contained in all se^s, and
in some roots, as the potatoe, parsnip, &c^ It is intended by nature
for the first aliment of the young veretaole ; but that of one parti-
cular 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
flonr?-

Jlfr#. B, Tes. The fecula of wheat contains also another Tege-
table substance which seems p^uliar to that seed, or at least has not
as yet been obtained from any other. ThisMs gluten^ which is of a
sticky, ropy, elastic. nature)( and it is suppobed lo be owing to the
viscious qualities of this stfostance, that wheat-flour forms a much
better paste than any other.

Emily. Gluten by^^our description, must be very like gum ?

J(fr«. jB.In their sticky nature they certainly have some resem- .
blance ; but gluten is essentially diflerent from gum in other points,
and especially in^ bein? insoluble in watej, whilst gum, you
know, is extremely'solubl^

The oi/«^ contained in vi^etables albconsist of hydrogen and car-
bon in various proportions^ They areVof two kiods,jlia?e^ and volo'
Hle^ both of which. we formerly mentioned. Do you remember in
what the difference between fixed and volatile oil consists ?

Emily, If 1 recollect rightly ,^Ce former w decomposed by heat,
whilst the latter are merely volalilized by iDt

* It was the opinion of Ho her, that the bees prepared the. wax from
hon^ and sugar. There is, however, found on the l^ves of some
plants a substance, having all the properties of wax ; and that bees-
wax itself is notan animal substance, is clear from its analysis^C*

1083. Of what does honey consist ?
10R4. What is said of the wax of bees ?

1085. What is manna ?

1086. What is the fecula of vegetables ?

1087. What is gluten ?

1088. How does gluten differ from gum ?
iS^' S^^^^^'^^^'^^efiretable oils consist?

1090. What is the difference between fixed and volatile oils ?

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OF VEGETABLES. 245

Mrs, B, Very well. »Mxed oil is contaioed only in the seeds of
plants, excepting in the olive, in which it is produced in, and ex-
press^ from, the fruit!> We have already observed that seeds con-
tain also fecula ; these^o 8ubstances,united with a little mucilage,
form the white snbstance contained in the seeds or kernels of plants,
and is destined for the nourishment of the yooog plant, to which the
seed gives birth. The milk of almonds, which is expressed from
the seed of that name, is composed of those three substances.

Emily, Pray, of what nature is the linseed oil which is used in
paiLtiftg?

'Jdrs, B, It is a fixed oil, obtainedflrom (he seed of fla^ Nut
oil, which is frequently used for the same purpose, is eitpressed
from walnuts.^

Olive oil is that which is best adapted to culinary purposes,

Caroline, And what are the oils used for burning?

Mrt. B* ^nnnal oils, most commonly ; but the preference given
to them is Win^ to their being less expensive ; for vegetable oils
bum e(maily well, and are more pleasant, as their smell is not of-
fensive^

Emiiy, ^nce oil is so good a combustible, what is the reason
that lamps sgi frequently re<[uire trimming ?

Mrs, B, j^is sometimes proceeds from the construction of the
lamps, which may not be sufficiently favourable to a perfect com-
bustion ; but there is certainly a defect in the nature of oil itself,
which renders it necessary for the best constructed lamps to be oc-
casionally trimmed. This defect arises from a portion of mucilage
which 4t is extr£»nely difficult to separate from the oil, and which
being a bad combustible, gathers round the wick, and thus im-
pedes its combustion, and consequently dims the lighfl^

Caroline, But will not oils burn without a wick ? '^

Jdrs, B, Not unless their temperature be elevated to five or six
hundi*ed degrees ; the wick apswers this purpose, as I think I once
before explained to yov. /The oil rises between the fibres of the
cotton by capillary attractren, and the heat of the burning wick
volatilizes it, and brings it successively to the temperature at which
it is combustiblA

Emily, I suppf^e the explanation which you have given with re-
gard to the necessity of ttimming 4amp^, applies also to candles,
which so often require snuffing ?

Mtt, B, I believe it does ; at least in some degree. But besides
'^be circumstances just explained, tlve common sorts of oils are not
very highly combustible, *80 that ^he heat produced by a candle,
which is a coarse kind of animal oil, being insufficient to volatilize
them completely, a quantity of soot is gradually deposited on the
'^wick, which dims the ti^ht, and retards the'cdmbYkstionA

Caroline, Wax candles, then, contain d6 incombustible matter,
since they do iiot Yeqiiire snuffing ?



1091. From what part of filants tire fixed oils obtained ?

1092. From what is linseed eil obtained ?

1093. What oils best for "burning ?

1094. Since oil is a goodoombustible, what is the reason thai:
^krops require so frequent trimming ?

1095. What is the use of wicks in lamps ? ^ ^^^^ ^ GooqIc
•1096. Why do candle^ more than lamps require trimmiG^ r



246 OOMPOSITIOIT

Jtfrt. B.(^ax is t moch better combostiSle fban talloiribiit .
still not pemctly 8o, since it likewise contains some particlerthat
are unfit for burning ; but when these gather round the wick,
(which in a wax light is comparatiTely small,) they weigh it down
OQ one side, and fall off together with the burnt part of the wick.

Caroiine* As oils are such good combustibles, 1 wonder that Ibey
should require so great an eleraUon of temperature before they be-
gin to bum ?

Jirt. B. Though fixed oils will not enter into actual combustion,
below the temperature of about four hundred degrees,'^ yet they
will slowly absorb oxygen^ the common temperature of the at-
mosphere. Hence arises a variety of change 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
OD the external parts of certain yegetables, 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
bums the hydrogen of the oil, thus forming a quantity of water,
which gradually goes off by evaporation, in this case', the altera-
tion of the oil consists not onljr in thesMidition of a certain quantity
v«f oxygeu, but in the diminution of the h^rogen. These oils are
disUnguished by the name of drying oiU. ^inseed, poppy, and nut
oils^are of this description. ' V

Emily, I am well acquainted with drying oils, as I continually
'MBLse them in painting. But I do not uaderstand why the acquisi-
tion of oxygen on one band, and the loss of hydrogen on the other,
should reo&r them drying.

Jlrt, B.[ This, I ^en oe ive, -may arise from two reasons ; either
from the ojbygen which is added bein^f less favourable to the state
of fluidity than the hydrogen, which is subtracted ; or from this
additional quantity of oxygen giving rise to new combinattona, in
consequence of which the most fluid parts of the oil are liberated
and volatilized.

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 move rapidly oxygenated^ ^

The TCuicidity of oils is likewise <mingito their oxygenating In
this case, «i new order of attraction take^place, from whicn pe-
culiar acid is formefl, ' called the tebaeie add,

Caroline, Since the nature and composition of oil is soweM
known, pray, could not oil be actually nuufe, by combining' its prin-
ciples ?

* This statement is too low. ^one of the fixed oils boil at & lest
^temperature than 600 decrees, nor wiU they bum until ^onrerted
into vapour ; consequently they cannot bum at a lower tempers*
ture than 600.— Cft

1097. Why are wax better than tallow candles ^

1098. What Elevation of temperature is neoessanr in order to
bum oil? ^

J099. What are the principal drying oils?
•ef oiU? ^ ^^ ^^ ^*^^ ^^ '®*^ increase the drying qualttj
1101. To what it the rancidity of oil owii^fN by Google



^inf tlFBOBTABUCS* '247

Mrs. B. Tiat is by no mitens a necessair conseiitieiUi^ ; for

there are inDameralile Tarieties of conpound bodies which we can

decompose, altboagb we are unable to reunite their ingredients.

^rhis, however, is not the 6ase with oil, as it has very lately been dis-

^"CoVered ^t it is possible to foite oil, by a peculiar process, from

the action of oxygenated muriatic acid gas on ^dro-carbbnai«*

We now pass tq^be^o^tfe or eiicrUvaiiUM Tbese fonti tite^iasis
of all the vegetabm perfumes, andyfiTre'contaiined, more or less, in
every part of the plant excepting tlm see^ they are, at least, n^er
found in that part df the s^ which'contnns tM embryo plant.

Eihilp, The smeU offlowers, then, proceedsptom yolatile tmh

Mrs. B, Certainly ; but AEis oil is often dkiBt abundant iirthe
rind of fruits, as in 6ranges,)eroon8, &c., from which it may be ex-
tracted by the slightest pressure; it is found also in the leaves of
plants, and even m the wood a

Caroline* Is it not very pfentiful in the leaves of mini) . and of
thyme, and all the sweet smelling herbs ?

Mrs. B, Yes: rtimarkably so; and in geranium leaves also,
which have a much more powerful odour than the flowdrs.

The perfume of sandal rans is an instance of its existence in wood.
In short, all vegetable odours or perfhmes 'are produced by the
evaporation of particles of these volatile oils.

EmiJjf. They are, I suppose, very light, t|nd of very thin consist-
ence, since they are volatile ?

JIfri. B. They vary very much in this'resp^t, someof them be-
ing as thick as butter, whilst others are as fluid at water. In order
to be prepared for perfames« or essences, these oils are $rst proper-
ly purified, and then, either distilled wHh spirit of wine, as is the
case with Iftvender water, or simply mixed with a large proportion
of waj^, as is oftto done with reganl to peffpermint 'Frequently,
also/these odoriferous Waters are prepared nierely by soaking the
planV in water, and distilling The water then c^mes over im-
pregnated with tbie vohitile'oil.^

CaroHne* Sucb waters are frequently used to take spots of
' grrease out of cloth, or silk : how do they produce that effect ?

Jtfff . B. (Sycombining with'the substance that fomUs these stains ;
for volatile^oiis, ttnd likewise the spirit in which they tire distilled
will dissolve wax,'tidlow, speitoaceti, and resinA if^ therefore, the

* Hydro-carbonate, is also called olffiant qr mimiakif^ gtm^ on
account of the supposed property here mentioned. But later ex-
periments have shown ^at tne inbstance it forms with chlorine, is
not an oU, but a kind of ether, hence it is now known under the
name of dibnie e^^er. — C.



1 103. Is there any known method of making oil by combining
'its principles ?

1 103. What forms ihe basis of vegetable perfumes ?

1104. In what part'of the plant alte the volatile or eMaitiid oils
*eontained?

1 105. From what proceeds ihe smell of floWers ?

1 106. How are volatile oils obtained ?

nor. Why will water mixed with "vegetable oils assist in je-
imoving spots of grease from cloth ? ^ ^ ,,^^ ,^ Google



248 COMPOSITION

spot proceeds from any of those substances, it will remove it. In-
sects 'of every kind have a gpreat aversion to p^rfames, so that vola-
tile oils ^re employed with success in maseums for (he preserva-
tion of stuffed birds and other species of animals.

Caroline. Pray, does not the powerfo! smell of Camphor proceed
from a volatile oil ?

Mrs, B, Ctmphor seems to be a substance of its owo kind, re-
markable by many pecuUarities. But if not exactly ofjtlie same
nature as volatile oil, it b at least very analagous to it. Qt is ob-
tained chiefly from the camphor- tree, a species oi laurel which
grows in China, and in the Indian isles, from the stem and roots of
which it is extracted!^ ^mall quantities have also been distilled
from thyme, sage, aiifl other aromatic plants^ and it is deposited in
pretty large quantities by some volatile oils«fter long standing. It
is extremely volatile and inflatnmable. It is insoluble in water, but
id soluble in oils, in which state, vs well as in its solid form, it is fre-
quently applied to medicinal purposes. Amongst the particular
properties oLcamphor, there is one too singular to be passed over
m silence, m yo<i take a small piece of campher, and place it on
the surface oT a basin of pure water, it will immediately begin to
move rotmd and round with great rapidity ; i>ut jf yom pour into
the basin a single drop of atiy odiferous fluid, it wHl instantly put a
stop to this motionT^ You can at any time trv so simple an experi-
ment ; but you m«^t not expect that I shall be able to account for
the phenomenon, as nothing satisfectory has as yet been advanced
for^its explanation,

Caroline. It is very singular indeed ; and I will certainly make
the experiment Pray, what are resinM, which you just now men-
tiOEiedf »«

Mrs. B. (They are volatile oils, that have been acted on, and pe-
culiarly modified, by oxygen^ ,
Caroline. They are, therefore, oxygenated Volatile oils ?
JIfri. £. Not exactly ; for the process does not appear to consist
so much in the oxygenation of the oil, as in the combustion of a por-
tion of its hydrog^en, and -a -small -portion of its carbon. For when
resins are artificially made by the cotnbinotion of volatile oils with
oxjTgen, the vessel m which the process is performed is bedewed
with water, and the air included within it is loaded with carbonic
acid.

Emili/. This process must be, in some inspects, simlar to that for
preparing drying oils ?

Mrs. B' Yes ; and it is by this ooeration that both of them acquire
a great degree of consistence, ^itch, tar, and turpentine, are the
most common resins ; they exiide from the pine ana fir treeM Co-

* Camphor coroes chiefly from Japan, (it is obtained by distilling*
the wood of the laitrus camphora^ oV camphor tree, with water, in
large iron potS, with earthen caps stuffed with straw. The cam*
pho|r sublimes and concretes upon the straw.^-C.

1 108. From what is camphor obtained ?
- 1109. Is camphor contained in other plants ?
m^- ^at u the ra'etkod of obtaining it !^
HU* Y!,^^^ remarkable peculiarity has camphor?
1112. What are resinsf oigLbyG^Ogle



Bd by wjt

you. in

h, it wm



OF TEOETABLES. 249

pal, mastic and frankincense, are also of tbid class of r^etable sub-
stances.

Emily. Is it^ these resins that the mastic and copal varnishes
so much used ra^ painting are mad§°)

Mrs, B, Tes. Dissolved either in oil or in alcohol, resins form
varnishes. ■ From these solutions they may be precipitated by
ter, in ivbich they are insoluble. This I can easily show —
you will pour some water into this g^lass of mastic varnish.,
combine with the alcohol in which the resin is dissol"^, and the
latter will be precipitated in the form of a white clou^



Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 30 of 43)