10 
There are 35 species, the most remarkable 
of which is the tinctoria, a native of the warm 
parts of Asia, Africa, and America. This 
plant requires a rich soil, well filled, and not 
too dry. The seed of it, which, as to figure 
and colour, resembles gunpowder, is sown in 
little furrows that are about the breadth of 
the hoe, two or three inches deep, at a foot 
distance from each other, and in as straight a 
line as possible. Continual attention is re- 
quired to pluck up the weeds, which would 
soon choke the plant. Though it may be 
sown in all seasons, the spring is commonly 
preferred. Moisture causes this plant to 
shoot above the surface in three or four days. 
It is ripe at the end of two months. When 
it begins to ilower, it is cut with priming- 
knives ; and cut again at the end of every 
six weeks,' if the weather is a little rainy. It 
lasts about two years, after which term it de- 
generates: it is then plucked up, and planted 
afresh. As this plant soon exhausts the soil, 
because it does not absorb a sufficient quan- 
tity of air and dew to moisten the earth, it is 
of advantage to the planter to have a vast 
space which may remain covered with trees, 
till it becomes necessary to fell them in order 
to make room for the indigo. 
The valuable dye-stuff called indigo bears 
some faint resemblance to starch; but its 
properties are sufficiently peculiar to distin- 
guish it from all other substances, and its im- 
portance entitles it to a distinguished place 
among vegetable principles. It is commonly 
procured by the following process : 
When the plant has been cut down, it is 
placed in strata in a large wooden vessel, and 
covered with water. In this situation it can- 
not remain long in these warm climates with- 
out undergoing some change. Putrefaction, 
accordingly, very soon commences, or rather 
a kind of fermentation, which goes on best at 
the temperature of 80°. The water soon be- 
comes opaque, and assumes a green colour ; 
a smell resembling that of volatile alkali is 
exhaled, and bubbles of carbonic acid are 
emitted. When the fermentation has conti- 
nued long enough, which is judged of by the 
paleness of the" leaves, and which requires 
from six to twenty-four hours according to 
the temperature of the air and the state of the 
plant, the liquid is decanted off the plants 
mto large flat vessels, where it is constantly 
agitated till blue floculi begin to make their 
appearance ; water is now poured in, which 
causes the blue flakes to precipitate. The 
yellow liquid is decanted off, and the blue 
sediment poured into linen bags. When 
the water has drained from it sufficiently, it 
is formed mto small lumps, and diitd in die 
shade. In that state it is imported into Eu- 
rope, and sold under the name of indigo. 
The leaves of the indigofera yield a green 
infusion to hot water, and a green powder 
may be precipitated from it; but unless a 
fermentation has taken place, neither the 
colour nor properties of it have any resem- 
blance to indigo. 
Indigo may be obtained from the merium 
tinctorium, and the isatis tinctoriu or woad , 
a plant commonly enough cultivated in Bri- 
tain, and even found wild in England When 
arrived at maturity, this plant is cut down, 
washed, dried hastily in the sun, ground in a 
mill placed in heaps, and allowed to ferment 
for a fortnight. It is then well mixed, and 
made up into balls, which are piled' upon 
INDIGO. 
each other, and exposed to the wind and sun. 
In this state they become hot, and exhale a 
putrid ammoniacal smell. r l he fermentation 
is promoted, if necessary, by sprinkling the 
balls with water. W hen it lias continued for 
a sufficient time, the woad is allowed to fall 
to a coarse powder; in which slate it is sold 
as a dye-stuff By treating woad nearly in 
the same manner with the indigofera, indigo 
has been obtained from it by different che- 
mists. 
Indigo is a fine light friable substance, ot a 
deep-blue colour. Its texture is very com- 
pact, and the shade of its surface varies ac- 
cording to the manner in which it has been 
prepared. The principal tints are copper, 
violet, and blue; the lightest indigo is the 
best : but it is always more or less mixed with 
foreign substances, partly owing, doubtless, 
to the carelessness of the preparation, and 
partly to the bodies which the plant contain- 
ing indigo yields to water. From the analy- 
sis of Bergman, to whom we are indebted tor 
one of the most complete treatises on the 
properties of indigo which has yet appeared, 
the purest indigo which he could procure, 
was composed of the following constituents: 
47 pure indigo 
12 gum 
6 resin 
22 earth 
13 oxide of iron 
100 . 
The earth consisted of, 
10.2 barytes 
10.0 lime 
1.8 silica 
22.0 
But in all probability the earth differs in 
different specimens ; for Proust found mag- 
nesia in considerable quantity in the speci- 
mens which he examined. The forty-seven 
parts of blue pigment are alone entitled to 
the name of indigo ; and to them therefore 
we shall confine our attention. 
Indigo is a soft powder, of a deep blue, 
without either taste or smell. It undergoes 
no change, though kept exposed to the air. 
Water cloes not dissolve any part of it, nor 
produce any change upon it. Bergman, 
however, found that indigo, when kept long 
under water, underwent a kind ot putrefac- 
tion, or at least exhaled a fetid odour. When 
heat is applied to indigo, it emits a bluish 
red smoke, and at last burns away with a 
very faint white flame, leaving behind it the 
earthy parts in the state of ashes. 
Neither oxygen nor the simple combus- 
tibles have any effect upon indigo, except it 
is in a state of solution; and the same _e- 
mark applies to the metallic bodies. 
The fixed alkaline solutions have no action 
on indigo, except it is newly precipitated 
from a state of solution. In that case they 
dissolve it with facility. The solution has at 
first a green colour, which gradually disap- 
pears, and the natural colour of the indigo 
cannot be again restored. Hence we see 
that the alkalies when concentrated decom- 
pose indigo. Pure liquid ammonia acts in 
the -same way. Even carbonat of ammo- 
nia dissolves precipitated indigo, and destroys 
its colour; but the fixed alkaline carbonats 
have no such effect. 
Lime-water has scarcely any effect upon 
indigo in its usual state ; but it readily dis- 
solves precipitated indigo. 1 do solution is at 
first green, but becomes gradually yellow. 
When the solution is exposed to the air, a 
slight green colour returns, as happens to the 
solution of indigo in ammonia , but it soon 
disappears. The effect of.the other alkaline 
earths upon indigo has not hitherto been 
tried ; but it cannot be doubted that they 
would act nearly as lime-water, but \\ ith more 
enere v. I he other eaitns seem to have but 
little action on indigo in any state. 
The action of the a ids upon indigo has 
been examined with most attention, and it 
certainly exhibits the most important pheno- 
When diluted sulphuric acid is digested 
over indigo, it produces no effect, except 
that of dissolving the impurities; but con- 
centrated sulphuric acid dissolves it readily. 
One part of indigo, when mixed with eight 
parts of sulphuric acid, evolves heat, and is 
dissolved in about 24 horns. Atcoiding to 
Haussman, some sulphurous acid and hy- 
drogen gas are evolved during the solution. 
If so, we are to ascribe them to the mucilage 
and resin, which are doubtless destroyed by 
the action ot the concentrated acid. 1 lie 
solution of indigo is well known in this coun- 
try by the name of liquid blue. Bancroft 
calls It sulphat of indigo. While concentrat- 
ed it is opaque and black ; but when diluted 
it assumes a tine deep-blue colour ; and its 
intensity 7 is .such, that a single cuop of the 
concentrated sulphat is sufficient to give a 
blue colour to many pounds of water. Beig- 
man ascertained the effect of different re- 
agents on this solution with great precision. 
Ilis experiments threw light, not .only on fhe 
properties of indigo, but upon the phenome- 
na that take place when it is used as a dye- 
stuff. The following is the sum ot these ex- 
periments : . , „ , 
Dropt into sulphurous acid. Colour at 
first blue, then green, and very speedily de- 
stroyed. — In wc<ik. tartaiic acid. Becomes 
gradually green, and in 1 44 hours had as- 
sumed a very pale yellow colour. Colour 
not restored by alkalies.— In vinegar. Be- 
comes green, and in four weeks the colour 
disappeared. In weak potass. Becomes 
green, and then colourless. 1 Tn weak cai- 
bonat of potass. T he same changes, but more 
slowly. If the solution is very weak, the co- 
lour of the indigo is not destioyed. In am- 
monia and its carbonat. __ Colour becomes 
green, and then disappears. In a weak so- 
lution of sulphat of soda. Colour after some 
weeks becomesgreen. In tartrat of potass. 
Became green, and then colourless. In a 
solution ot sugar. Became green, and at last 
yellowish.- In sulphat of iron. Colour be- 
came green, and in three weeks disappeared. 
—In the sulphurets. Colour destroyed in a 
few hours. — Realgar, white oxide ot arsenic, 
and orpiment, produced no change. Black 
oxide of manganese destroyed the colour 
completely 7 . In the infusion ot madder. 
Colour became green, and at last yellow.— 
In the infusion ot woad, the same changes, 
but more speedily. . . . 
From these experiments it is obvious tiiat 
all those substances which have aveiy T strong 
affinity for oxygen give a green colour to in- 
digo, and at last destroy it. Hence it is ex- 
tremely probable that indigo becomes gieen 
