PLA 
Vegetable substances. Plants contain va- 
rious saline matters; such as the vegetable 
acids, and the three alkalies, ammonia, 
potass, and soda; also gum, sugar, fat oils, 
essential oils, balsams, camphor, resin, tar, 
farina, narcotic, and colouring matters ; all 
which the reader will find treated of under 
their respective heads. But different kinds of 
plants contain matters peculiar to themselves, 
which an ingenious and profound modern 
chemist (Dr. Thomson) has classed under the 
general term extractive principle, and to 
which he ascribes the following general pro- 
perties: 1st. Soluble in water, and the solu- 
tion is always coloured. When the water 
is slowly evaporated, the extractive matter is 
obtained in a solid state, and transparent ; but 
when the evaporation is rapid the matter is 
opaque. 2. The taste of extractive is al- 
ways strong; but it is very different, accord- 
ing to the plant from which it is obtained. 
3. Soluble in alcohol, but insoluble in ether. 
4. By repeated solutions and evaporations, 
the extractive matter acquires a deeper co- 
lour, and becomes insoluble in water. This 
change is considered as the consequence of 
the absorption of the oxygen of the atmo- 
sphere, for which the extractive principle has 
a strong affinity: but if the solution is left to 
itself, exposed to the atmosphere, the extract 
is totally destroyed in consequence of a kind 
of putrefaction which speedily commences. 
5. When oxymuriatic acid is poured into a 
solution containing extractive, a very copious 
dark-yellow precipitate is thrown down, and 
the liquid retains but a light lemon-colour. 
These flakes are the oxygenized extractive. 
It is now insoluble in water ; but hot alcohol 
Still dissolves it. 6. The extractive principle 
unites with alumina, and forms with it an in- 
soluble compound. Accordingly, if sulphat 
or muriat of alumina is mixed with a solution 
of extractive, a flaky insoluble precipitate ap- 
peal's, at least when the liquid is boiled; but 
if an excess of acid is present, the precipitate 
does not always appear. 7. It is precipitated 
from water by concentrated sulphuric acid, 
muriatic acid, and probably by several other 
acids. When the experiment is made with 
sulphuric acid, the fumes of vinegar generally 
become sensible. 8. Alkalies readily unite 
with extractive, and form compounds which 
are soluble in water. 9. The greater number 
of metallic oxides (form insoluble compounds 
with extractive. Hence many of them, when 
thrown into its solution, are capable of sepa- 
rating it from water. H ence also the me- 
tallic salts mostly precipitate extractive. 
Muriat of tin possesses this property in an 
eminent degeee. It throws down a brown 
powder, perfectly insoluble, composed of the 
oxide of tin and vegetable matter. 10. If 
wool, cotton, or thread, is impregnated with 
alum, and then plunged into a solution of 
extractive, they are dyed of a fawn-brown 
colour, and the liquid loses much of its ex- 
tractive matter. This colour is permanent. 
The same effect is produced if muriat of tin 
is employed instead of alum. This effect is 
•still more complete if the cloth is soaked in 
nxymuriatic acid, and then dipt into the in- 
fusion of extractive. Hence we see that the 
extractive matter requires no other mordant 
than oxygen to fix it on cloth. 11. When 
distilled, extractive yields an acid liquid im- 
pregnated with ammonia. 
It cannot be doubted that there are many 
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different species of extractive matter; though 
the difficulty of obtaining each separately 
has prevented chemists from ascertaining its 
nature with precision. Extracts in phar- 
macy are usually obtained by treating the 
vegetable substance from which they are to 
be procured with water, and then evaporat- 
ing the watery solution slowly to dryness. 
All extracts obtained by this method have an 
acid taste, and redden the infusion of litmus. 
They all yield a precipitate while liquid, if 
they are mixed with ammonia. This preci- 
pitate is a compound of lime and insoluble 
extractive. Lime always causes them to 
exhale the odour of ammonia. It has been 
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4-fy 
4. Tire infusion of senna contains a matter 
of a very peculiar nature, but which may be 
considered as a species Of extractive. I he 
senna of commerce consists of the dried leaves 
of the cassia senna, a shrublike annual, culti- 
vated in Egypt. Water, at the ordinary 
temperature of the atmosphere, dissolves 
nearly one-half of the substance of these 
leaves. The infusion obtained has a brown 
co ! our, a bitter taste, and a peculiar aromatic 
odour. It contains a considerable propor- 
tion of carbonat of lime, sulphat of potass, 
andcarbouat of magnesia, besides a little sili- 
ca. But the most curious of its constituents 
is the extractive. When common air is made 
ascertained that the extractive principle is ; to pass for some hours through the inffision, 
a yellow-coloured precipitate appears: the 
same substance is thrown down immediately 
by muriatic acid and oxymuriatic acid. It 
appears also when a current of oxygen gas is 
made to traverse the infusion. This substance' 
is the extractive altered by its combination 
- It 
more abundant in plants that have grown to 
maturity than in young plants. 
As the extracts of vegetables prepared by 
apothecaries for medical purposes, besides 
the extractive principle, always contain other 
bodies, frequently to the number of eight or , 
more, and as the greater number of them are ! Wlth ox yg en - I( has a slight bitter taste 
still but imperfectly examined, we shall sa- 1 1S no lon S er soluble ,n water. Alcohol dis- 
tisfy ourselves at present with pointing out 1 solveSlt ’ but lets fall when diluted. r ihe 
some of those vegetable substances which a!ka! . lss dl - ssolve it,, and form a deep-brown 
have been ascertained to contain extractive • solution. On burning coals, it emits a thick 
principle, and stating the constituents of such , sm °ke, exhales an aromatic odour, and 
as have been analysed. j ! eaves a spongy charcoal. 1 hese properties 
, T-, . , ! indicate a very decided approach to the resin- 
1. Extractive principle is not an uncom- : Qus state> J 11 
5. The infusion of Peruvian bark likewise 
yields an extractive matter of a peculiar na- 
mon ingredient in the sap of trees. Indeed, 
Deyeux and Vauquelin found it in almost all 
those which they examined. It is usually , - ... - 
thrown down when the sap is mixed with j which assumes a line red colour when 
oxymuriatic acid, and it precipitates in brown i llnited to ox yS ei1 5 and like the extractive of 
flakes while the sap is evaporating on a sand- 
bath 
senna, acquires nearly the properties of a 
It was obtained by Eourcroy from 
n t. f ... , c ,, , , r ,. 1 the cinchona of St. Domingo. Water, boiled 
2. It forms a constituent of the bark of all •. -. e , , , b ’ 
... „ i n , -i rru- . , , on it till it refused to dissolve any flung more, 
trees hitherto examined. This was evidently i , , , , , , J . 5 . , 
,i „ „ ,, , , i • i a r A a was slowly evapofated, and the extract ob- 
l „ • f Wh , !cl V) tained was dissolved in alcohol. The alcohol, 
sub ectedtoexper ment namely those of he by evaporation, deposited the peculiar 
oah, Leicester willow, Spanish chesnu elm, ; tl 4 cti .,‘ Its c< ’ j!ollI ! „ as taste 
rnmniAll wmrYOr. ann nur nnhfpr Ur KnrL'e . . ... ... 7 ... . 
common willow, and undoubtedly all barks 
which have an astringent taste ; for tan and 
extractive seem scarcely ever to be found 
separate. 
3. The infusion of catechu contains an ex- 
tractive principle, united chiefly to tan. It 
may be obtained in a state of purity by wash- 
ing the catechu in powder repeatedly with 
water till the fluids obtained cease to preci- 
pitate gelatine. What remains is extractive. 
It is of a pale reddish-brown colour, and a 
slightly astringent taste, leaving in the mouth 
a sensation of sweetness. It lias no smell. 
Its solution in water is at first yellow-brown ; 
but it acquires a tint of red when left exposed 
to the air. The solution in alcohol is of a 
dirty brown. It does not affect vegetable 
blues. Alkalies brighten its colour; but nei- 
ther these bodies nor the alkaline earths pre- 
cipitate it from water. Nitrat of alumina 
and muriat of tin render the solution slightly 
turbid. Nitrat of lead throws down a dense 
light-brown precipitate. It renders the oxysul- 
phat of iron green, and throws down a green 
precipitate, becoming black by exposure to 
the air. Linen, when boiled in the solution, 
takes away almost the whole of the extractive, 
and acquires a light red-brown colour. When 
this extractive is exposed to heat, it softens, 
and its colour becomes darker, but it does 
not melt. When distilled, it yields carbonic 
and carbureted hydrogen gas, weak acetic 
acid, and a little unaltered extractive. A 
porous charcoal remains. 
3 L 
ex- 
bib* 
ter, insoluble in cold, but very soluble in hot, 
water. It was soluble in alcohol. When dry 
it was black, brittle, and broke with a polish- 
ed fracture. Lime-water precipitated it in the 
state of a red powder ; oxymuriatic acid threw 
it down in the state of a fine red powder, 
neither soluble in water nor alcohol, but ca- 
pable of uniting with alkalies. A stronger doses 
of oxymuriatic acid renders it yellow. 
6. Saffron yields extractive matter in great 
abundance. This substance consists of the 
summits of the pistils of the crocus sativus. 
Almost the whole of it is soluble in water. 
The resemblance between extractive bo- 
dies and the colouring matter of p’ants is suf- 
ficiently striking. It is more than probable, 
that when this last set of bodies have been 
examined with more precision by chemists, 
they wall be found to belong to the same 
cla^-s. 
PLASHING of quickset hedges, an ope- 
ration very necessary to promote the growth 
and continuance of old hedges. 
It is performed in this manner : The old 
stubs must be cut off, &c. within two or three 
inches of the ground, and the best and longest 
of the middle-sized shoots must be left to lay 
down. Some of the strongest of these must 
also be left to answer the purpose of stakes. 
These are to be cutoff to the height at which 
the hedge is intended to be left ; and they 
are to stand at ten feet distance one from 
another; whpii there are not proper shoots 
