C H E M 
naturally from vegetables like refin, but extracted from 
their ruptured veffels, in the form of a white or coloured 
juice, of a fetid fmell, more or lets refembling garlic. 
Fecula : A pulverulent, dry, white, infi.pid, combuf- 
tible, matter; affording much pyromucous acid on dis¬ 
tillation; folubie in boiling water, and forming a jelly 
with this liquid; convertible into the oxalic and malic 
acids by means of nitric acid ; exifting in all the white 
and brittle parts of vegetables, particularly in tuberofe 
roots and gramineous feeds ; constituting the bale of the 
houiifhment of animals, and difpofed to become readily 
a principle of their bodies. 
Gluten : An elaftic ductile body, as if fibrous or 
membraneous; infoluble in water; flightly folubie in al¬ 
cohol ; affording a confiderable quantity of ammoniac 
on diftillation; putrefcible like animal matter; turning 
yellow like it on the contact of nitric acid ; convertible 
by this acid into oxalic acid; occafioning the difference 
between the farina of wheat and other farinaceous fub- 
llances, and bellowing on it the capability of being made 
into a pafte. 
Colouring matter : Always attached to one or 
other of the preceding materials ; appearing variable in 
its nature; fbmetiines folubie in water; at others, at¬ 
tackable only by alkalis, oils, or alcohol; indebted for 
the diverfity of its properties to the different quantities 
of oxygen fixed in it; poffeffing an affinity of attraction 
for alumin, oxyd of tin, &c. and capable of combining, 
more or lefs intimately, with the textures of vegetables 
and animals. 
Elastic gum: Analogous to gum-refin; appearing 
to exift in feveral vegetables; remarkable for the exten¬ 
sibility and elafticity it retains after deficcation ; afford¬ 
ing ammoniac when diltilled ; diffufing a fetid fmell when 
burned ; having been at firft in the form of a white milky 
fluid, and converted from that ftate to an elaftic folid, 
by the abforption of atmofipheric oxygen. 
The ligneous part, o.r wood : A matter too much 
negleCted hitherto by chemifts ; conftituting the folid 
bafts of all vegetables, but far more abundant in thofe 
which are hard ; erroneoufly confidered as an earth ; in¬ 
foluble in water; affording, on diftillation, the peculiar 
acid called pyroligneous; containing a large quantity of 
carbon ; pafiing to the ftate of three or four acids by the 
aCtion of the nitric ; and appearing to be the laft produCl 
of vegetation. 
From what has been thus exhibited refpeCting the im¬ 
mediate and known materials of vegetables, it follows, 
that they are all reducible, on an ultimate analyfis, to 
three or four principles, which are their primitive com¬ 
ponent parts ; namely, hydrogen, carbon, oxygen, and, 
in many, azot; and that they differ from each other 
only in the various proportions of the elements from 
which they are formed. Now, if we invelligate, by fim- 
ple calculation, the numbej of different compounds that 
may refult from the union of three or four principles in 
every poffible proportion, we ftiall find, that a much 
larger number might exift. But as each of the ternary 
or quaternary compofitions which conftitute the imme-' 
diate materials of vegetables, admits, as far as it appears, 
a certain latitude of proportions, while retaining its ge¬ 
neral nature of extract, mucilage, oil, acid, reftn, &c. it 
is eafy to conceive, that the different proportions of the 
principles included within thefe feveral latitudes, fet 
bounds to the vaft immeafurable variet)*- of colour, fmell, 
tafte, and confiftency, which are obfervabie in all the ma¬ 
terials of vegetables, and which men difcern in fuch of 
them as they employ in their food, garments, habitations, 
&c. On the, fame confideration, it will not be more diffi¬ 
cult to conceive, that vegetables muff vary in the nature 
and fpecific properties of their materials, according to the 
feveral periods of their vegetation ; that they can never 
remain in the fame ftate; and that the different fcenes 
exhibited in the periods of germination, leafing, bloffom- 
ing, fructification, and maturity, which ponltitcte ve- 
Vol. XV. No. 187. 
I S T R Y. 169 
getable life, muft be accompanied and marked by internal 
change, as they are by external appearance. Of this the 
varioufly modified tafte, inceffantiy changing colour, fmell 
not more liable, and difference of texture, which charac¬ 
terize the feveral epochs of vegetation, afford incontefti- 
ble proofs. 
A new advantage arifing from the modern chemiftry, 
is the having thus diftinguifhed the nature of the mate¬ 
rials in plants, far more complex than that of mineral 
fubftances. The acquifition of this knowledge, leads to 
an appreciation of the changes wrought in vegetable mat¬ 
ters by different chemical agents. Thus we can no longer 
profels ourfelves ignorant of the aCtion of the deftruc- 
tive agent fire on vegetable fubftances. From the pre¬ 
ceding confiderations, are underitood hew, when a com¬ 
plete vegetable, or any one of its different produCts, is 
fubjeCted to the aCtion of fire, caloric tends to reduce 
thefe complicated compounds to more Ample ones, by 
occafioning the union of their principles, two and two 
together, in proportions very different from thofe which 
before obtained. By gently heating them, the hydrogen 
is extricated, which burns aione, and much carbon re¬ 
mains : if they be ftrongly heated, the carbon is difen- 
gaged at the fame time with the hydrogen, they both 
burn in the air, and the only refiduum left, conillts of 
that fmall quantity of earth and falts, which conftitutes 
vegetable allies. 
All the immediate materials of vegetables being re¬ 
ducible in their ultimate analyfis to three or four origi¬ 
nal principles; namely, hydrogen, carbon, oxygen, and 
a little azot in fome of them, this analyfis, moreover, 
anfwering with the utmoft precifion to the manner in 
which vegetables are nourished, grow, fpread, and per¬ 
petuate their fpecies, fince we know that vegetation, to 
take place, requires only thefe Ample principles ; nothing- 
more remains, but to find how plants appropriate thefe 
forts of elements, and combine them in their organic 
flrainers, to compole the different fubftances, the pro¬ 
perties of which have been announced. It appears be¬ 
yond all doubt, that water is the fource whence vege¬ 
tables derive their hydrogen; that they decompofe this 
fluid in their leaves, by the help of the foiar light, ab- 
forb its hydrogen, which becomes fixed in them in the 
ftate of oil, or extract, or mucilage, See. and feparate its 
oxygen, a great part of which, being diffolved in light 
and caloric, flies off in the ftate of vital air. But a por¬ 
tion of the oxygen of the water is fixed at the fame time 
in the texture of the vegetable, in which it is retained 
chiefly by the carbon. 
It is not fo eafy to account for the carbon that exilts 
in vegetables. Some natural philofophers fuppofe, that 
vegetables decompofe carbonic acid at the fame time 
with water, and abforb its carbon: but this fuppofition 
is not proved, though it has acquired flrength, fince the 
decompofition of carbonic acid, combined'with foda, by- 
means of phofphorus, lias been dilcovered. Other che¬ 
mifts are of opinion, that vegetable earths, mould, dung, 
and particularly the water of dunghills, furniffi the car¬ 
bon, attenuate, and even diffolved in water ; that plants 
abforb this "principle by their roots; and that they do 
not extraff it from carbonic acid. According to this 
hypothefis, manure affords only carbon, and the water 
of it is nothing more than a faturated foiution of this 
principle. To thefe data we muft reft rain at prelent the 
theory of vegetation. 
The applications of the fa6ts above-ftated, are ex¬ 
tremely multifarious; they relate to agriculture, rural 
economy, pharmacy, materia medica, and all the arts 
in which vegetable fubftances are employed. They alfo 
point out the true nature of germination : the develope- 
ment of leaves: bloffoming: fruftification : the matu¬ 
ration of fruits and feeds: the fuccefiive formation of 
gum, extract, oil, relin, falts, fugar, the colouring mat¬ 
ter, and wood, in the different periods of vegetable life : 
the growth of the woody fubftance, bark, &c. the phar- 
X x maceutical 
