Mrs. (Jane Haldimand) Marcet.

Conversations on chemistry .. online

. (page 35 of 43)
Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 35 of 43)
Font size
QR-code for this ebook

oxygen enters into tbe formation of mucilage, of fecula, of sugar,
and of vegetable acids. But the greater part of the oxygen pro-
ceeding from the decomposition of the water is converted into a gase-
ous state by the caloric disengaged from the hydrogen during its
condensation in the formation of the vegetable materials. In this
state the oxygen is transpired by tjie leaved of plants when exposed
to the sun's rays. Thus you find that the decomposition of water*
by the organs of the plant, is not only a means of supplying it with
its chief ingredient, hydrogen, but at the same time of replenishinr
the atmosphere with oxygen, >a principle wbich requires continusu
renovation, to make up for the great consumption of it occasioned

. 1'263. How does the under side of leaves differ from the upper

1-264. Of what use is li|fht in the growth of vegetables ?

15^65. Of what use is air in vegetation ^ .

1266. How are the various salts and other ingredients required for
the formation and support of the vegetable system carried into

P'^^- Digitized by Google


by the numerous oxygenations, combustions, and respirations, that
are constantly taking place on the surface of the globe.*

Entity: What a striking instance of the harmony of nature !

Mrs, B' And how admirable the design of Providence, wha
makes every different part of the creation thus contribute to the
support and renovation of each other !

But the intercourse of the vegetable and animal kingdoms,
through the medium of the atmosphere extends still further, jjni-
mals, in hreathmg, not only, consume the oxygen of the air, bdnoad
it with carbonic acid, which, Jf accumulated in the atmosphere,
would, in a short time, render it totally unfit for respiration. Here
the vegetable kingdom again interferes ; it attracts and decomposes
the carbonic acid, retains the carbon for, its own piirposes, and re-
turns the oxygen for our^

Caroline. How interesting this is ! I do not know a more beauti-
ful illustration of the wisdom which is displayed in the laws of na-

Mrs. B. Faint and imperfect as are the ideas which our limited
perceptions enable us to form of divine wisdom, still they cannot
fail to inspire us with awe and admii'ation. What then, would be
our feelings, were the complete system of nature at once displaj^d
before us ! So magnificent a scene wo.uld probably be too great for
our limited comprehension ; and it is; no doubt, among the wisedis-

* The foregoing paragraph might mislead the student. Indeed it
seems to have been written without regard to proper authorities*
For instance, there is no proof that water is decomposed by the or-
gans of plants ; nor is it in the least degree probable that the oxy-
gen emitted by them owes its gaseous state, to the calbric set free
by the condensation of hydrogen. Authors on this subject agree
that the thickest veil covers the processes by which the sap is con-
verted into the several parts of the plant. But it has been de-
monstrated, that most, if not all the oxygen emitted by the leaves, is
obtained by th6 defcom position of air, instead of water, as here stated. •
If leaves are exposed to the rays of the .sun, whilie under common
water, they emit oxygen. But if the water is first deprived of iti
airv by an air pump, or b^ boiling, not a particle of oxygen is emit-
ted. Now atmospheric air, always contains a quantity of carbonic
acid g^s, and experiments show, that plants give out oxygen in
some proportion to the quantity of this gas contained in the water.
^^^le fact then seems to be, that plants absorb carbonic acid, that
this is decomposed by sOme unknown process ; the plant retaining
the carbon, while the oxygen is given out.-^C.

f(Tt is a curious fact, demonstrated by experiments' that the
leaves of plants. perform different offices at different periods of the
24 hours. During the day they give out water, absorb carbonic
acid, and emit oxygen gas ; but during the night they absorb wa-
ter, and oxj'gen gas, and give out cafbonic acid) — C.

1^67. How do animal and vegetable life mutoall/ support each
other ?

1268. WJiat curious fact is stated of the leaves of vegetables in the
note ?

1269. What in the organization of nature is particuUrly suited to
the rational powers of man ? V /^ Digitized byC^OOgle




pensations of Providence, to veil th|B splendor of a glory with which
we should be overpowered. Bututis well suited to a rational beibg
to explore, step by step, (he worljf of the creation,, to endeavor to
CDunecttbem into harnnionidud systems ; and, in a word, to trace, in
the chain of beings, the kindred ties and benevolent design which
unites itS' various links, and secures its preservation)

Caroling, But of what nature ate the organs of ^anis which are
endued with such wonderful powers ?

Mrs, B. They are. so minute that their structure, as well as the
mode rn which they perform their functions, generally elude our ex-
amination ; but we may consider them as so many vessels or appa-
ratus appropriated to perform, with the assistance of the principle
of life, certain chemical processes, by means of which these vegeta-
ble compounds are generated. We may, however, trace the tannin,
resins,^ gums, mucilage, and some other vegetabje materials, in the
organized arrangement of plants, in which they form the bark, the
w^tfxl, the leaves, flowers, and seeds.
nThe hark is composed of the epidermisy the parenchyma^ and the

^he epidermis is the external covering of the plant. It is a thin
transparent membrane, consisting of a number of ^lender fibres,
crossing e^ch other, and forming a kind of n^t worn When of a
white glossy nature, as in several species of trees, in the stems of
corn and of seeds, it is composed of a thin coating of siliceous earth,
which accounts for the strength 9nd hardness of those long and slen-
der stems. Sir H. Davy was led to the jdiscovery of the siliceous
oatdre t)f the epidermic of such plants/by observing the singular
phenomenon of sparks of fire emitted byHhe collision of ratan canes
with which two boys were fighting in a dark roomt , On analysing the
epidermis of the cane, he found it. to be almost entirely siiiceoii^

Gdroiine, With iron, then, a cane I suppose, will strike fire v&j

Jlrs. Bi I understand that it will. — In evergreens the epidermis
is mosjjy resinous, and in some few plants is formed of wax. The
resin/irom its want of afenity for water, tends to preserre the plant
from The destructive effects of violent rains, severe ' climates, or in-
clement^easons, to which this species of vegetables is peculiarly

Emity. Resin must preserve wood just like a varnish, as it is the
essential infirredient of varnishes. ,

Mrs. B, Ye9; af\d hy this means it prevents, likewise, all unne-
depsary expenditure of moistui:e. ^

The parenchyma is immediately beneath the epidermis ;nt is that

* In the scouring rush, {Equiseium hyemale)\.he siliceous epider-
mis is still more obvious. If drawn across a piece of soft metal, at
silver or copper, it cuts it lijie a file. It even makes an impressioD
on the hardest steel.— C.

1270. Of what is bark composed?
127^. What i^ the epidermis ?

1272. In what manner vras Sir H. Davy led to discover the sili-
ceous nature of the epidermis of particular plants ?

1273. How does resin tend to preserve the plant ?

1274. What is the parenchyma?

Digitized by LjOOQ IC


green rind which appears when vou strip a branch of any tree or
shrub of its external coat of bafK^ The parenchyma is not confin«
ed to the stem ortranches^ but extends over fevery part of the plant
It forms the green matter of the leaves, and is. composed of tubes
filled with a peculiar juice.

The cortical layers are immediately in contact with the wood ;
they abound with tannin and gallic acid, and consist of small vessels
through which the sap descends after being elaborated in the leaves.
The cortical layers are annually renewed, the old bark being con-
verted into wood.

Mrs. B. That function is performed by the tubes of the al^r-
num or wood, which is immediately beneath ^e cortical la^rs^/rhe
wood is composed of woody fibre, mucilage and resin|i ^be fiores
are disposed in two ways : some of them longitudinallyVand these
form what is called the silver grain of the wood. The others which
are concentric, are palled the spurious grain. Thede last are dispos-
ed in layers, from tiie numl^er of which the age of the tree may be
computed,, a new one being produced annuaUy by the conversion of
the bark into wood) The oldest,and consequently most internal part
of the albernum. Is called heart wood ; it^pears to be dead,at least
no vital functions are discernible in it* Xlt is through the tubes of
the living albernum that the sap rises^ These therefbrie, spread into
the leaves, and there communicate ffith the extreipities of the ves-
sels of the cortical layers, idto which they pour their contents.

Caroline, Of what use, then, are the tnbes of the parenchyma^
since neith^the ascending nor descending sap passes through them ?

Mrsi ^.(They are supposed to perform the important function of
secreting from the sap the peculiar juices from which the plant
more immediately derives its nOurishmenhj' These juices are very .
conspicuous, as the vessels which contajZ them are miich larger
than those through which the sap circulates. The peculiar jliices
of plants difiier much in their nature, not only in difierent species
of vegetables, but frequently in different parts of the same individual
plant; they are sometimes saccharine, as in the sug^ar- cane, some-
times resinous as in firs and evergreens, sometimes of a milky ap-
pearance, as in the laurel.

Emily, I have oflen observed, that in breaking a young shoot, or
in bruising a leaf of laurel, a milky juice will ooze out in great

Mrs, B. And ^ is by making incisions in the bark, that pitchy tar,
and turpentine* are obtained from fir-treesi The durability of this
species of wood is chiefly owing to the resinous nature of its peculiar

* Turpentine is obtained as described m the text. But tar and

Sitch are obtained by a very different method. A conical cavity is
ug in the earth, at the bottom of which is placed a reservoir. Over
this is piled billets of fir-wood, forming a large pile. The pile is
covered with turf to smother the fire which is kindled at the top. As
the wood is heated, and gradually converted into charcoal, the tar is

1275. Through what does the sap ascend ?
1976. Of what is'the wood compdsed ?

1277. How are the fibres disposed ?

1278. Of what use are the tubes of the parenchyma ?

1279. How may pitch, tar, and turpentine be obtained ^i



juices. Yrke volatile oils have, id a great measure, the same preserv-
ative efl^ts, as they defend the parts with which they are connect-
ed, from the attack of insects. This tribe seems to have as great an
aversion to perfumes, as the human species have delight in them.
They scarcely ever attack any odoriferous parts of plants, and it is
not uncommon to see every leaf of ^tree destroyed by a blight,
whilst the blossoms remain untouched^ . Cedar, sandal, and all ar-
omatic woods, are, on this account, oi great durability.

Emily- But the wood of the oak, which is so much esteemed for
its duraljility, has, I believe, no smell. Does it derive this quality
from its hardness alone ?

Jlrs. B. Not entirely ; for the chesnut, though cposiderably
harder and firmer than the oak, is not so lasting. The durability
of the oak, is, I believe, in a great measure/owing to its having
very Kttle heart -wood, tl\e albernum preserviog its vital functions
longer than in other tree^

Caroline. If incisions are made into the albernum and cortical
layers, may not the ascending and descending sap be procured in
the same manner as the peculiar juice is from . the vessel of the

JUTS. B» Yes ; but in order to obtain specunens of these fluids, in
any quantity, the experiment must be madenn the spring) when the
sap circulates with the greatest energy. For this purpose- a small
bent glass tube should be introduced into the incision, throug^j
which the sap may flow without mixing with any of the other juices
of the tree. From the bark the sap will flow much noore plentifully
than from the woodi^s the ascending sap ismuch more liquid, mojce
abundant and more rftpid in its motion, than that which descends^
for the latter having been deprived by the operation of the leaVds
of a considerable part of its moisture, contains a much greater pro-
portion of solid matter, which retards its motion. It does not ap-
pear that there is.any excess of descending sap, as none ever exudes
from the roots of plahts ; this process, therefore, seems to be car^
ried on only in proportion to the wants of the plant, and the sap de-
scends no further, and in po great quantity, than is required to
nourish the several organs. Therefore, though the sap rises and
descends in the plant, it does not appear to undergo a real circula-

The last of the organs of plants, i^he flower^ or blouom^ which

fiven out and runs Mnto the cavity, and finally into the reservoir,
ar is a mixture of resin^ empyreumatic oil, charcoal^ and aeetit aci j|
he color is derived )frora the charcoal. Pitch is made by boiling
tar, by which its more volatile parts are driven off.— C.

1280. On what are the durability of cedar, sandal, and all aromat-
ic woods depending?

1281. On what is the durability of oak depending ?
128^. Of what 18 tar said in the note to contist ?

1283. At what time in the year does the sap circulate with roost
energy ?

1284. Why will sap flow more plentifully from the bark than
from the wood ?

1285. What is the ultimate purpose of nature iMbe xegetable

qreation ? Digitized by v^OOg i

Vegetation. 285

produces the/rui/« an^ seedj These may be considered as tbe ulli-
mat6 purpose of nature in u\e ve^table creation. From fruits and
seeds animals derive both a plentiful source of immediate nourish-
ment, and an ample provision for the reproduction of the same
mean? of subsistence.

The seed which forms the final .product of mature plants, we
have already examined, as constituting the first rudiments of fu-
ture vegetation.

These are the principal organs ofvegetation, by means of which,
the several chemical processes which are carried on during the life
of the plant are performed.

Emily. But h6w are the several principles which enter* into the
composition of vegetables, so combined by the organs of the plant,
as to be converted into vegetab^ matter?

Mrs. B. /SJy chemical processed no douht ; but the apparatus in
which tbey^are performed, is so Extremely minute as .completely to
elude Cur examination. We can form an opinion, therefore, only
by the result of these operations.

The sap is evidently composed of water, absorbed by ,the roots
and holding in solution the v.arious principles which it derives from
tbe soil. From the roots the sap ascends through the tubes of the
alburnum into the stem, and thence branc^hes put to every extrem-
ity of- the plant. Together with the sap circulates a certain quan-
tity of carbonic acid, Which is gradually disengaged from the for-
mer by the internal heat of the plant.

Caroline. What? have vegetables a peculiar heat, analogous ta ,
animal heat ?

J^fr'8. S, It is a circumstance thathas lone: been suspected'; but
late experiments have decided beyond a doubt that^fegetable heat
is considerably abotre that ot Unorganized matter in winter, and
below it in summers The wood of a tree in its interior, is abouC
sixty degrees wheif^the thermometer is at seventy or eighty de-
grees in the air* And the bark, though so much exposed, is seldom
beli)w forty in winter.

tit is from the sap after it has been elaborated by the leaves, that
vegetables derive their nourishment ; iq its progress through the
plant from the leaves to the roots, it deposits in the several sets of
tessek with which it communicates, the materials on which the .
growth and nourishntent of each plant depend^ It is thus that the '
various peculiar juices, saccharine, oily, mucolis, acid, and colour-
ing, are formed ; as also the more solid parts, fecula, woody-fibre,
tannin, resins^ concrete salts ; in a word all the immediate materi-
als of vegetables, as well as the organized parts of plants, which
latter, besides the power of secreting these from the sap, for the
general purpose of the plant, have afeo that of applying thera io
their own particiilarnourishment.

Emily. But why should the process of vegetation take place only

1286. How are the several principles which enter into the com-
position of vegetables so corpbined by tbe organs of the plant as to
be converted into vegetable matter ?

1*287. How does the temperature of vegetables compare with
that of unorganized matter ?

1^288. How are the several pieces as well as more solid parts o»
vegetables formed ? Digitized by Google


at one season of the year, whilst a total inactioa prevails during
the other ? ^^

Mrs. B.^eat is such an important chemical a^ent, (hat its ef- such, migiit perhaps alone, account (or the impulse which
the Spring- g'ives jLo ve^etatio^ But, in order to explain the me-
chanism of that operation, it Has been supposed that the warmth of
spring dilates the vessels. of plants, and produces a kind of vacuum,
into which the sap (which had remained in a state of inaction in the
trunk during the winter) rises ; this is followed by the ascent of the
sap contained in the roots, and room is thus- made for fresh sap,
which the roots in their turn pump up" from the soil This process
goes on till the plant blossoms and bears fruit, which tei*minates its
summer career ; but when the cold weather sets in,fthe fibres and
yess'els contract, the leaves wither, and are no long-er able to per-
form their office of transpiration ; and as this secretion stops^ the
roots cease to absorb sap from the soil. If the plant be an annual,
its life then terminates r if not, it remains in a state of torpid inac-
tion during- the winter, or the only internal motion that takes place
is that of a small quantity of resinous juice, which slowly rises from
the stem into the branches, and enlarges their buds during the wia-

Caroline, YetCn evergreens vegetation must cootidue through-
out the year.

Mrs- B. Yes ; but in wmter itgDeson in a very imperfect man-
ner, compared to the vegtJtation of spring antf summen

We have dwelt much longer on the history of vegetable chemis-
try ihnn 1 liad intended ; but we have at length, I think, brought
the silbiect to a conclusion.

Caroline. I rather wonder that you did not reserve! the account
of the fermentations for the conclusion; for the decomposition of
vegetables naturally follows their death, and can hardly, it seems,
be introduced with so much propriety at any other period.

•Mrs, B. It is difficult to determine at what point precisely it may
be most eligible to entei: on the history of vegetation ; ^very part
of the subject is so closely connected, and forms such an uninter-
rupted chimin, that it is by .no means easy to divide it. Had I begun
with the gei^mination of the seed, which, at first view seems to be
the most proper arrangement, I could not h^ve ex plained the na-
ture and fermentation of the seed, or have Hp«'cr*frbed the changes
which manure must undergo, in order to yield the vegetable ele-
ment^ To understand the nitore of germination, it is necessary, I
<hin If; previously to decompose the parent plant* in order to be-
come acquainted with the materials required for that purpose. I
hope, therefore, that, upon second consideration you will find that
the order which I have adopted, though apparently less correct, is,
in fact, the best calculated for the elucidation of the subject.

I2fl9. Why should the process of vegetation take place only in
warm weather ^

1290. What is the condition of vegetables called evergreens, in
the season of winter .''

1291. Why was not the fernnentation of vegetables reserved for
the concluding part of what is said on this subject ?

Digitized by LjOOQ IC




•Mr*. B. We have now come to the last branch of chemistry,
which comprehends the most complicated order of compound beings.
This is the animal creation, the history of which, cannot but excite
thehighestdegree.of curiosity and interest, thoUg-h we often fail in
attempting to explain the laws by which it is governed.

Emily, But since all animal ultimately derive their nourishment
from vegetables, the chemistry of this order of beings must^onsist
merely in the conversion of vegetable into Animal mattei^

Mrs. B, Very true ; but the" manner in which this is effected is,
in a great measure, concealed from, our observation. This pro-
cess is called anitnalization, and is performed by peculiar organs.
The difference of the animal and vegetable kingdoms does not,
however, depend merely on a different arrangement of combina-
tions. A new principle abounds in the animal kingdom, which is
but rarely and in very small quantities found in vegetables; this is
nitrogen. There is likewise in animal substances a greater and
more constant proportion of phosphoric acid, and other saline mat-
ters. But these are not essential to the formation of animal matter.

Caroline. Animal compounds contain, then, four fundamental
principles ;/^xygen. hydrogen, carbon and nitroge^ .

Jtfr*. ^. Tes; and these form the immediate rflaterials of ani-
malsy which are^cZfl(/mc, albumen, and^ftnnSf*

Emily. Are tftose all f I am surprised thatanimals should be com-
posed of fewer kinds of materials- than vegetables ; inr they appear
much more complicated in their organization.

Mrs.B. Their organization is certainly more perfect and intri-
cate, and thd ingredients that occasionally enter into their compo-
sition are more numerous. But notwithstanding the wonderful va-
riety observable in the texture of the animal organs, we find that the
original compounds, from which dll the varieties of animal matter
are derived, may be reduced to the three heads jufet mentioned. An-
imal substances being the most complicated of all natural com-
pounds, are most easily susceptible of decomposition, as the scale of
attractions increases in proportioii to the number of constituent
principles. Their analysis is," however, both difficult and imper-
fect ; for a^ney cannot be examined in iheir living state, and are
liable to alteration immedifttely after deatMit is probable that when
submitted to the investigation of a chemisff they are always more or

* These are the principal ingredients of the-soft' parts. But in
addition to these, animal substances contain ^cohmring matter of
bloody mucous, sulphur, phosphoAis, earths, alkalies, oils, aeids, resins,
and.«everal others, whicferit is unnecessary to specify.— C

J 292. What forms the subject of the 23d conversation ?

1293. What is animalization ?

1294. What do animal compounds contain ?

1295. What are the immediate materials of animals ?

1296. On what account is the analjtsis of animal compounds. diffi*
^uU andvimperfect .? r^ i

Digitized by LjOOQIC


less altered io their combinations and pro^rties, from what thev
were, whilst they made.partof the lining animal.

Emily. The mere diminution of terapfsrature, which they experi-
ence by the privation of animal, heat, roust, 1 should suppose, be
sufficient to deranj^e the order of attractions, that existed during life.

Mrs. B, That is one pf the causes, no dpubt; biit there are many
other circumstances which prevent us from studying* the nature of
liviug animal substances. We must, therefore, in a considerable
degree, confine oar researches to the phenomena of these compounds
in their inanimate «(ate*

These three kinds of animal matter, gelatine, albumen, ^od
fibrine, form the basis of all the yai^ious parts of the animal system :
either solid, as the akin, Jlesh, nerves, membranes, carlilascs, and
hones ; or fluid, bloody chyle, milk, mucous, the gastric 2Lndpancre-
aticjuices, bile, perspiration, Saliva, tears, 8fc, ,

Caroline, Is it not surprising that so great a variety of. sAbst^n-
ces, and so different in their nature^ should yet all arise from so few
materials, and from the same original. elements ?

Mrs. B. The difference in the nature of various bodiesrfepenas,
as I have often observed to you, rather on their st^te of commnatioD,

Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 35 of 43)