Mrs. (Jane Haldimand) Marcet.

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Emily, It is so. And yet how is it that pictures or dritwings, var-
nished 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 rennt, whichj^re
precisely whtU their name denotes, that is to say, resins, combined
with mucilage^ Myrrh and assafcetida are of this description.

CuroUne, mt possible that a substance of so disagpreeaDleasmell
as assafcetida can be formed from a Tolatile oil f

Mrt, B. The odour of volatile oils is by no means always grateful.
Onions and garlic derive their smell from volatile oils, as well as ro-
ses and lavender.

There is still another form under which volatile oils present them-
selves, which is that of bcUsams, These consist of resinoi^ juices
combined with a peculiar acid, called the benzoic acid. iBalsams
api>ear to have been originally volatile oils,'" the oxygenSLtion 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 modifica-
tions, and proceed to the ^es:t vegetable substance which \seaoutck'
oue. This is a white, milky, glutinous fluid, which acquires consis-
tence and blackens in drying, in which state it forms the substance
with which you are so well acquainted, under the name of gum-

Caroline, I am surprised to he^r that gum -elastic was ever white,
or ever fluidi And from what vegetable is it procured ?

JIrs, B, (It is obtained from two or three different species of trees
in the East Indies, and South America, by making incisions in the
stenk The juice is collected as it trickles from these incisions, and
momdsof clay, in the form of little bottles of gum-elastic, are dipped

* This is an erroneous idea. ^Balsams are original and peculiar
substances, and consist chiefly of resinous matter in a semifluid
•state. The beuEoic acid is most probably formed during the pro-
cess by which it is obtained.-^C

.... -. ' ^ £ — : ,..-_•,. — :

1 1 13. What are the most common resins ?

1114. Of what are mastic and copal varnishes made

11 1 5. What will be the consequences if water be poured into a
vessel containing mastic varnish ?

1116. What are gum resins ?

1117. What are balsams ?

1118. From what is caoutchouc obtained? ^ .

1119. What are iU uses? Digitized by C^OOgle


into it. A layer of this juice adheres to the clay and dries bnit ;
and sereral layers ^re successiFely added by repeating^ this till the
bottle is of su(5cient thickness. It is tjaen beaten to break down
the clay which is easily shaken out. ^he natives of the countries
where this substance is produced, som^imes make shoes and boots
of irby a similar process, and they are said to be extremely pleasant
and'serviceable, both from their elasticity, and their bein^. water-

The substance which comes next in our enumeration ot4he im-
mediate ingredients of vegetables, is extractive matter, ^h'w is a
term which, in a general sense, may be applied to any substance ex-
tracted from vegetables ; but it is more particulatly understood to
relate to the extractive colqring matter of plant^ A great variety
of colors are prepared from the vegetable Kingdom, boA for the
purposes of pamting and of dying ; all the colors called^aXr^are
of this description ; but they are less durable th m mineral celbrs,
for by long exposure to the atmosphere, they either darken or turn

Emily, I know that in painting, the lakes are reckoned far less
durable colors than the ochres ; but what is the reason of it ?

Mn. B, The change which takes place in vegetable colorsCis
owing chiefly to the oxygen of the atmosphere slowly burning tbekr
hydrogen, and leaving, in some measure, the blackness of the car-
bon exposed^ Such change cannot take place in ochre, which is
altogether arrnineral substance.

Vegetable colors have a stronger affinity for animal than for veg-
etable substances ; and^tis is suppo^ to be owing to a small ouao-
tity of nitrogen, which they contai^ Thus, silk and worsted will
take a much finer vegetable dye th^Tn linen and cotton.

Caroline, Dying, then, is>quite a chemical process ?

Mrs, B, Undoubtedly. The condition required to form a good
dyQ is, that the coloring matter should be precipitated, or fixed,
on the substance to be dyed, and should form a compound not solu-
ble in the liquids to which it would 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 sub-
ject in washing; and woollens and silks should withstand the action
of grease and acids, to which they may accidentally be exposed^

Caroline, B ut if linen and cotton have not a sufficient affinity^or
coloring matter; how are they made to resist the action of washing,
which they a ways do when they are well printed ?

Mrs. B. f^hen the substance to be dyed has either no affinity
for the coloring matter, or not sufficient power to retain it, the com-
bination is effected or strengthened, by the intervention of a third
substance, called a mordant^ or basis. The mordant must have a

J 120. Wtat is the extractive matter of vegetables ?

1 121. What are the colors called prepared from vegetables ?

1122. To what is the change which takes place in vegetable col-
ors owin^ ?

1123. Why have vegetable colors a stronger affinity for animal
than for vegetable substances ?

1 124. What is necessary that vegetable colors be durable f

1125. What are mordants and their uses ? r^^^^U

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strong affinity both for the coloring matter and the substance 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. ffhe most common mordant is sulphat of alumine, or
alum. Oxyds of tin and iron in the state of compound palts, are
likewise used for that purpos^.

Tannin is another vegetableingredient of great importance in the
arts. It is obtained chiefly ^om the bark of ^ees ; but it is fouqd
also in nut-galls, and in some other vegetables^ ^

Emih/. Is that the substance commonly called iauy whichiis used
in hot-housesV ^

Mrs. B. Tan is the prepared bark in which the peculiar substance,
tatinin is contained. But the use of tan in hot-houses is of much
less importance than the operation of tanning, by which skin is
converted into leather.

Emily. Pray how is this operation performed ?

Mrs. B. Various methoda are employed for this purpose* which
all consist in exposing skin to the action of ^annin, or of substances
containing this principle in sufficient quantities, and disposed to
yield it to the skin, ffhe most usual way is to infuse coarsely pow-
dered oak bark in wai&r, and to keep the skin immersed in this in-
fusioii for a certain length of time. During this process, which is
Blow and gradual, the skin is found to have increased in weight, and
to have acquired a considerable tenacity and impermeability to wa-
ter. This effect may be much accelerated by using strong satura-
tioi/s of the tanning principle, (which can be extracted from bark,)
instead of employing the bark itself. But this quick mode of pre-
paration does not appear to make equally good leathen

^annin is contained in a great variety of astringeid vegetable
substances, as galls, the rosetree, and wine ; but it is no where 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 insohible compound. Hence
its valuable property of combining with skin (which con^sts chiefly
of glue^ ana of enabling it to resist the action of watera

Emily. Might we not see that efiect by pouring a Hflle melted
isinglass into a glass of wine, which you say contains tannin i

Mrs. B. Yes. . 1 have prepared a solution of isinglass- for that
▼erv purpose^ Do jou observe the thick, muddy precipitate ? That
is the tannin combmed with the isinglass.

Caroline, This precipitate must then be of the same nature as
the leather?

Mrs. B, It is composed of the same ingredients ; but the organ-
ization and texture of the skip being wanting, it has iteitfaer the
consistence nor the tenacity of leather.^

1 136. What substances are commonly used as mordants ?

1127. From what is tannin obtained ?

1128. What are its uses?

1129. What is the process of converting skins into leather, by
the use of tanning ?

1 130. Wb^r does tanning cause skins on being changed to leather,
to be impervious to water?

1131. How does a solation of isinglass in water differ from leatlier ?

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Caroline, One niight suppose (hat men who drink hige quanti-
ties of red wfne^ stand a cnance of hayin? the coats of their stom-
achs coverted into leather, since' tannin has so strong an affinity
for skin.

Jlfrt, B. It is not impossible but that the coats of their stomachs
may be, in some measure, tanned or hardened by the constant ose
of this liquor ; but you must remember that where a number of
other chemical agents are concerned, and above all, where life ex-
ists, tio certain chemical inference can be drawn.

I must not dismiss this subject, without menj^oning a recent dis-
covery of Mr. Hatchett, which relates to it. ^his gentleman found
that a substance very similar to tannin, posselTsing all its leading
properties, and actually capable of tanning leather, may be produc-
ed by exposing carbon, or any substance cootainiog carbonaceous
matter, wheUier vegetable, animal, or mineral, to the action of ni-
tric acid^ •

Caroline. And is not this discovery very Ukely to be of use to
manufactures ?

Mrs. B, That is very doubtful, because tannin, thus artificially

narcd, must probably always be more expensive than that which
tained from bark. But the ftict i? extremely curious, as it af-
fbrds one of those very rare instances of chemistry being able to im-
itate the proximate principles of organized bodies. .

The last of the vegetable materials is woody JHfre-^ is the hard-
est part of plants!) The chief source from which thmubstance is
derived, is wood, but it is also Contained, more or less, in every solid
part of the plant. It forms a kind of skeleton of the part to which
it belongs ami retains its shape after all the other materials have dti-
appeared, (ft consists chiefly of carbon united with a small |K>rtioD
. of salts, and the other constituents conimon to all vegetables^

Emily. O^i is of woody fibre 4km^ that the common charcoal is

J\£rs. B. Tes. Charcoal, as you may recollect, is obtained from
wood by the separation of all its evaporable parts.

Before we take leave of the vegetable materials, it will be proper
at least to enumerate the several vegetable acids which we eitner
bave had or majr have occasion to mention. I bdieye I formerly
told you that their basis or radical, was uniformly composed by^-
dro|^n and carbon, and th^t their difference consisti^only in Ine
various proportions of oxygen which they containe^A-.^

*(To make artificial tannin, Mr. Hatchett used 100 grains of char-
coaiWith 500 of nitric acid, diluted with twice its weight of water,
^bis mixture was heated, and then suffered to digest for two days ;
more acid was then ^ded, and the digestion continued until the
charcoal was dissolved.) This solution facing evaporated to dryness,
leaves a dark 'brown ifiass. This is tiie tannin in question. Its taste
is bitter and highly astring^ent. — C.

} J32. What discovery was made by Mr, Hatchett ?

1 :2S* S^ ^^ ^ prepare artificial tannin ?

1134. What is woody Hbre?

JJ35. Of what does it chiefly consist ?

1136. From what is charcoal made ? Dig,,ed by Google



^e fbKowiog^ a^e the ndmes of the vegetable acids :
~ ifhe Mucpus add obtamed from gum or mucilage ; .

Suberic, * from, cork;

Camphoric J - ' from Camphor ;

Benzoic . - from balsams : ' V^

Oallic ' - from galls, bark, '&c. ^ /• /6 <} : H/ .

Make - from ripe fruits ;

Citric ' •fromlemon juice ;

Oxalic . - from sorrel ;

St^ecifnic - from amber ;

Tartarous - from tartrit of potash ;

jfficetic - from vinegar) ' .

mtey are all decomposable. by heat; soluble ia water,. and^
gefeble blue colours re3J The succinic^ the tartarow, and the ace-
iow adds'^ are thie produttibns of the decomposition of vegetables ;
we^alf; therefore, reserve their examination for a^future period.

rThe oxalic aci(/. distilled from sorrel, is the highest tefm^of vege-
table acidification\ for, if more oxygen bp added to it, it loses its
vegetable tiatiire,''and is resolved ihto carbonic acid and watfer ;
therefor^, thoughall theother acids may be convepted into the ox-
alic by an addition of oxygen, the oxalic Itself is not susceptible of
ja further 4egi!ee of oxygebation : Bor can it be made by any .che-
mical processes, to return to a state of lower acidification.'"
^To conclude thts subject, 1 have only to add a few words on the
fgallic acidX

^^Carolint, Is not this tbe same acid before mentioned, which fpums
ink, by precipitating sulphat, of iron from its solution ?

Mrs, Bi Ye^. Thoagh^ff is usually extracted from galls, on ac-
count of its being most srtMindani in that vegetaibJe siibstanccl it
may also t^e obtained from a great variety of plants, it constitules
what isxalled thO attringmt principle of vegetables ; it is geiierallv
combified with tannin, and ypu will ^(fiod that an i&fusion oftea,.cof-
feei bar^, red wine, or any vegetable substance that contains the
astringent principle, will make a black precipitate with a solution
of sulphat of iron.
<7afdttnc. Put.pray What are galls ?

Jlfr#. B./They are excrescences which grow on the bark of young

* Oxalic acid may be formed artificially. Put one ouoce of white
sugar, powdered, into a retort, and pour on three ounces of nitric
acid^ When the solution is over, make the liquor boil, and when it
ae($uireis a reddish brown .bolqur, ad|4 three ounces more of nitric
acid. Continue the boiling uiitil the fumes ccasc.and the colour of
the liqnor vanishes. Then let the liq^uor bp poured into a wide ves-
sel, and on cooling, white slender crystals will be formed. These
are oxalic acid. — C.

1 137. What are the names of the vegetable acids ?

1138. What is the composition of the basis of these acids ?

1 139. What general qualify iiave all vegetable acids ?

1 I4d. What is the highest term of vegetable acidification ?

1141. What acid is galled the astringent prinqjple of vegetables ?

1 142. From what is4t usualjy extracted ?

^1 1 43. VV'jnat arc the galls that yield this acidt?

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254 c(mfo^moTf

oaks, and are oocaftiondd by an insect ^h'%eh wouods the bark of
trees, and lays its ^Qgs in ttie aperture. The lacerated vessels of
the tree then discharge their contents, and form an excrescence,
which affords a defensive coveriog^for lhe«eeggs.. The insect, when
come to life, first feeds on this excfesceece, and sometime after-
wards eats its way out, as it appears from a hole which is formed in
all gall-nuts that no longer contain an insect} /tt is in hot* climates
only that strongly astringent gall nuts are fottnd*^ those which are
used for the purpose of making ink are brought from AleppoTV

Emily. But are not the oak apples which grow' on* the lein^es of
the oak in this country of a sitniiar nature ?

Mrs, B. Yes; only they are an inferior species of galls, contain-
ing less of the astringent principle, and therefore less applicable to
useful purposes. .

Caroline^. Are the vegetable acids never found but in their pv re
uncom biped state? , .

Mrs. i.lBy no raean&.; on the contrary, they are frequently met
with in the state of con^pound salts ; these, however, are in gwae-
ral not fully saturated with the salifiable bases, so that the acid pre;,
idominates ; and in this state, they are called acidulous s^lts. Of
this kind is the salt called cream of tartarl

Caroline* Is not the salt, of lemMi commoiily used 4o take out
i nk- spots and stains, of this nature ?

Mrs. B. ^o\ that salt consists of tbe oxaHcaeid, combined with
a*^little potash. It is found in that state in sorrel.

Carolinsl And pray bow does tt^ takie out ink-spots ? ^

J/r«. B. ^y uniting with the iron, and rendering it. sdttbleiq

Bendes the vegetable materials which we have enumerated, a ▼%•
riety of other substasciesf common to the three kingdoms, ar&foutid
ip vegetables, silcba^otash\ which w%8 formerly supposed tobe-
'long exclusively to plams, ana was, iii consequence, calli^ the ve-
vg^ble alkali?

Bulphur, phospbotus, earths, and a variety ofmetallk oxyds^are
also found in vegetables, but only in small quantities. And we ineet
sometimes wfth neutril salts, formed by tW combination of'tliese



Car otitis, Tb» account which you have ^^iven us« Mr9. B.,of tbe
■■ materials of vegetables, is doubtless, very mstructive ; but it does

1144. In what climates are strongly astringent ^ll-nut» found ?

1145. Are the vegetable aciijls never found but in their pure un-
corobined stale ?

I !46. How does theaxaUc apid remo^ ink-spots ?

1147. What«ub8tanteacoromoD^to the three kingdoms are found


ilot completely satisfy my curiosity. I wish to kpovr how plants ob-
tain the principles from which their ^arioqs materials are formed ;
by what means these are conrerted into yeg«tal>le matter, and how
they are connected with the lile of the piant.

Mrt, ^j This iroi>lies nothing less than a complete history of the
CheHiistry and physiology of vegetation v subjects on which we have
yet but very imperfect notions. Still 1 hope that I shall be able, in
some measure, to satisfy yourcuriosity. But, in order to render the
snbject more intelligible, I must first make you acquainted mih the
various changes which vegetables undergo, when the vital power
no longer enables (hem to redfst the common ]^w8 of chemical at-
traction. .

rThe composition of vegetables being mcure complicated than that
orfniner^s, the former more readily undargo chemical changes than
the latt^er; for the greater the variety of attractions, the more easi-
ly is thenquilibriam destroyed, and a new order of combinations in-
troduc^ « ,

Emilp( I am surprised that ve^tables should be so easily suscep-
tible of decomposition : for the preservation of the vegetable king-
dom is certainly far more important than that of minerals.

Mrs. B. You must consider, on fhe other hapd^ how much more
easily the former is renewed than the latter. .<^he decomposition of
the vegetable takes place only after the deaQ^of the plant, which,
in the common course of nature Jbappens when it has yielded fruit
and seeds to propagate its species]) If, instead of thus finishing its
career, each plant was to retain-' its form and vegetable state, it
wouid become an useless burden to tb,e earth and its inhabitants.
When vegetables, therefore, cease to be productive, they cease to
live, and nature then begins her process ofdecopiposition, in orde
to resolve them into ther chemical constituents/h^'drogen, ^arbon
and oxygen\ thqse simple and primitive ingredients, which she
keeps in store for all her combinations.

Emily. But since no system of combinaticna can be destroyed ex-
cept by the establishment of another order of attractions, bow can
the decomposition of vegetables redude them to their simple ele-
ments V /^

Mrs, jB.nt is a very long process, during which a variety of new
combinations are successively established, and successively destroy-
ed ; but, in each' of these changes, the ingredients of vegetable
patter tend to unite in a more simple order of compounds, till
they are at length brc^ught to their elenmntary state, or, at least, to
their most simple order of com binationg) Thus you will find that
vegetables are in the end almost entirely reduced to water and car-
bonic acid ; the hydrogen and carbon dividing the oxygen between
them so as to form with it these two substances* ' But the variety of
intermediate combinations that take place during the several stages

1148. Why do vegetables more readily undergo chemical chan
ges than minerals ?

1 149. When do vegetables become decomposed ? '

1150. Into what are vegetables reduced by decomposition •'

1151. Since no system of combination can be destroyed, except
by the establishment of another order of attractions, how can the de-
composition of vegetables' reduce them to their simple elements ?

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of IhedecomposHion of vegetables^ present us with a new set of
compounds, well wor]lby of our examiaation.

Caroline. How is it possible tbat vegetables, while putrefying-,
should produce any thing worthy of observation ?

Mrs. B. They are suscceptible of undergoing certain changes
before they arrive at the state of putrefaction, whicl[) is (he final
term of decomposition ; and of these changes we avail ourselves for
patticular and important purposes. But; in order to make you un-
derstand this subject, which is of considerable importance, Imu'st
explain^it more in detail.

The decomposition of vegetables is always attended by a violent
internal motion, produced by the disunion of one order of particles,
and the combination of anotner. This is called ^^rmektatt 6n4
There are several periods at which this process jstops, so that a state
of rest appears to be restored, and the new order of compounds fair-
ly established. But, unless meand be. used to secure these new
combinations in their actual state, their duration will be but tran-
sient, and a new fermentation will. take place, by which the coin-
pound I a^t formed will be destroyed ; and anotlier, and less Complex,
will succeed.

Emily. The fermentations, thdn, appear to beonly^e successive
steps by which a vegetable descendf[ to its final dissolution)

Jlrs. B. JPrecisely so. Your definition is perfectly coirect.-

Caroline' And how many fermentations, or new arrangements,
does a vegetable undergo before it is reduced to its simple ingri^-
dients? ^

Mrs. B. Chemists do not exactly agree in *this point ; but tbere
are, I think/foundistinct fermentations, or periods, at which the
decomposition or vegetable matter stops and changes its .course.
Hut every kind of vegetable matter is not eqyally susceptible of un-
det*going all these fermentations.

There are lik^ise several circumstances required to produce
fermentation, /water, and a certain degree of heat are both essen-
tial to this process, in order to separate the particles, and thus weak-
en their force of cohesion, that the new chemical affinities may be
brought into act io'^' .

Caroline, In frSzen climates, then, how can the spontaneous de-
composition of vegetables take place ?

Mrs. B. It certainly cannot ; and, accordingly, we find scarcely
any vesliges of vegetation where a constant frost prevails.

Caroline: One would imagine that, on the gontrary, such spots
wtfuld be covered with vegetables ; for since they cannot be decom-
posed, their number must iEil^y-ays increase.

Mrs, B. But, my dCar. heat and water are quite as essential to
the formation of vegetables, as they are to their decomposition. Be-
sides, It is from the dead vegetables, reduced to their elementary
principles, that the rising generation is supplied with sustenance.
No young plant, therefore, can grow, udless its predecessors contri-
bute both to its formation and support ; and these not only famish

1152. • What is the process called, that disunites and decomposes
he elements of vegetables .''

1153. What ai^e the fermentations ?

1 154. How many kinds of fermentation arc there?

1 1 55. What is necessary to produce fermentation^ t

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the seed from which the new plant springs, but likewise the food by
which it is nourished.

Caroline, Under the torrid zone, therefore, where water is never
frozen, and the heat is very great, both the processes of vegetation
and of fenneotatibn must, f suppose, be extremely rapid?

Mrs. B. Not so much ad you imagine ; fonlfn such climates great
part of the water which is required for these processes is in an aeri-

Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 31 of 43)