208 
On Indigo. 
[July 
fined entirely to the pellicle of the leaves, and exists in largest quantity at the 
commencement of maturation, while the plant is in flower ; at a somewhat later 
period the indigo product is more beautiful, but less abundant ; afterwards much 
less of it is obtained, and of a worse quality. The plant is remarkable for giving 
a blue 'tinge to the urine and milk of cows that feed upon its leaves; a circum- 
stance which accords with the known permanence of the dye. The statement of 
Mr. Weston, in this Journal, agrees with these observations on the ripening of 
the blue principle. He shews that the developement of this matter in the Indigo - 
feres goes on in the leaves, even after they are separated from the plant and dried. 
When packed up for a few weeks, more or less according to their preceding state 
of ripeness, the leaves assume a light lead colour, which gradually deepens into a 
blackish hue. The planter studies to seize the period at which the maximum 
portion of colouring matter is formed, that he may then transfer the leaves to the 
steeping vat. 
Three different processes are employed for extracting the indigo, each of which 
must modify more or less the nature of the product. In the first and second, the 
dried leaves are operated on ; in the third the recent plant. For the perfect 
success of the two former processes, the plant should be very speedily deprived of 
its water of vegetation ; hence the Indigofera is reaped only in fair weather. An 
hour and a half before sunset, the plants are cut down, carried off the field in 
bundles, and immediately spread on a dry floor. Next morning, at six o’clock, 
the reaping is resumed for an hour and a half before the sun acts too powerfully 
on vegetation, and the plants are treated in the same way. Both cuttings become 
sufficiently dry by 3 o’clock, p. m. to permit the leaves to be separated from 
the stem by threshing. The leaves are now thoroughly dried by exposure to the 
sun, then coarsely bruised, or rather ground to powder in a mill, and packed up 
for the use of the manufacturer of indigo. 
From these powdered leaves, the dye stuff is extracted, either by simply digest- 
ing them in water, heated to InOo or 180° F. in as small a proportion as may be 
practicable, and subsequently beating the infusion with paddles till the blue indigo 
granulates, as Roxburgh recommended ; or by mashing the ground leaves with 
twice their bulk of water, at the atmospheric temperature, drawing off the liquor 
into a vat, where it speedily undergoes fermentation, and is beat as above with 
paddles or oars, till the blue indigo forms. Some persons prescribe the addition 
of lime water at this stage of the process ; others reject its use. 
In operating on the recent plant, it is laid in bundles in the steeping trough, 
( trempoir ) which contains sufficient water to stand about two inches above plants 
slightly pressed down by crossing bars of wood. A brisk fermentation soon begins, 
with copious extrication of air bubbled. This pi*ocess is suffered to proceed till 
the liquor has become green, and casts up a pellicle of a copper-red hue. A 
sour smell is now perceived, and the blue colouring particles seem ready to 
separate. This happens commonly at the end of from ten to twenty hours, 
according to the temperature of the weather. The liquor is then run off 
into the beating vat, and lime water is added, or not, according to the fancy 
of the operator. In all cases of fermentation, whether the dried leaves or 
the receut plant be employed, it is proper to watch the progress of that change 
with solicitude ; because, when too violent, it not only decomposes entirely some 
of the indigo blue, but introduces much foreign vegetable matter into the pre- 
cipitate; when too feeble, it is said to leave some indigo unextracted. 
From the differences which exist in the nature and culture of the Indigoferee, and 
of their treatment by the manufacturer, the product, indigo, as found in commerce, 
differs remarkably in quality and chemical composition. In this respect, it forms 
a complete contrast to the simple crystalline product sugar. Besides impurities, 
accidentally present, from a bad season, want of skill or care, the purest coin - 
meicial indigo consists of no less than five constituents : — 1. Indigo-blue, a very 
singular vegetable compound of carbon, hydrogen, and oxygen, with fully 10 pf r 
C y&' °f azote : 2. Indigo- gluten, a yellow or brownish yellow varnish, which 
differs trom wheat-gluten by its solubility in water. It has the taste of osmazome, 
or of beef-soup, melts when heated, burns with flame, and affords an erapyreu- 
matic oil along with ammonia by distillation. — 3. Indigo-brown ; this constituent 
is more abundant than the preceding ; it is extracted by a concentrated water of 
p° ash, made to act on powdered indigo, previously digested in dilute sulphuric 
'■'^vreuil s indigo-green seems to have consisted of this substance, mixed 
a . lme mattcr > ancl indigo-blue.— 4. Indigo-red; this is readily dis- 
ii ¥ or ling alcohol, out of indigo previpusly subjected to the action of an acid 
or alkaline menstruum ; the alcohol acquires a beautiful red-tinge, and leaves, by 
