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THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [December 16,1871. 
mulgate several enactments against its use. So severe 
•were some of them, that Henry IV. of France issued an 
"edict condemning to death any one who used that per¬ 
nicious drug called the “devil’s food.” It is only since 
the year 1737 that the French dyers have had the right 
of using indigo without restriction. 
Indigo exists in several varieties of plants, among 
which may he mentioned the Polygonum tinctorium and 
the Isatis tinctoria , or woad; hut, as in the case of 
nearly all the dye-stuffs already spoken of, it is found 
most abundantly in plants of the leguminous order. It 
is extracted commercially from the genus Indigo fey a, 
•varieties of which grow abundantly in India, Java, 
♦China and in some of the South American States. 
To extract the indigo which the plant contains, it is 
mowed when in full flower, made into bundles, and 
-carried into large vats containing water. Fermentation 
ensues, which is .allowed to proceed for eight or nine 
hours, when the liquor, which was yellow at the begin- 
ming of the operation, assumes a dark green colour, 
forming a blue scum on the surface. This liquor is then 
run off into shallow vats, where it is violently agitated 
with sticks, stirred by men or women, or with a dasher, 
which produces the same result, the conversion of the 
white soluble indigo into blue insoluble indigo. A little 
lime-water is now added, and the whole allowed to rest. 
The blue indigo thus deposited is boiled with water, 
when a scum, composed of vegetable and animal matters, 
comes to the surface and is removed. The blue paste is 
thrown on a filter, pressed and placed in wooden frames, 
■which are divided into small squares, where the indigo 
is allowed to dry first in the sun and afterwards in the 
shade. The quality of the indigo produced depends 
mainly on the care bestowed on its manufacture. 
Until 1855 we had no correct ideas as to the state in 
which the indigo existed in the Indigofera plants, nor of 
the changes which it underwent during the process of 
extraction. It had been stated by M. Chevreul, many 
years before, that white indigo was oxidized into blue. 
But in the above year, and subsequently, in 1857 and 
1865, Dr. Schunck published a series of papers, in which 
he described the true nature of the chemical changes 
which take place in the manufacture of indigo. He 
-operated on the Isatis tinctoria , or woad, which contains 
the same colour-giving principle as the Indigofera , and 
ns the only plant yielding indigo that grows freely in 
this country. 
By these researches he demonstrated that indigo 
-existed in the plants, combined with a sugar, forming 
a glucoside, to which he gave the name indican; this 
compound, under the influence of fermentation in the 
manufacturing process, was unfolded into indigo and 
sugar. By treating the dried woad with ether or alco¬ 
hol, Dr. Schunck obtained indican, and, among other 
processes for its extraction, gives the following simple 
•one. Ho treats the pounded dried woad leaves with 
■•other in a displacement apparatus, and distils off the 
greater part of the ether. The remaining green liquor 
is then evaporated at a moderate temperature, a little 
cold water added to the syrupy residue, and the insoluble 
chlorophyl and other matters separated by filtration. 
The yellow liquid thus obtained is evaporated, either 
spontaneously or in vacuo. 
Indican is a yellow, transparent, glutinous solid. As 
■may be seen from the above, it is soluble in alcohol, 
ether and water. When boiled with caustic alkalies it 
-evolves ammonia, but its most remarkable and inter- 
-esting property is that of yielding indigo blue when 
treated with strong acids. If sulphuric or muriatic acid 
be added to its water solution, no change whatever is 
perceptible for some time ; but on heating to near the 
Boiling-point, the solution immediately becomes sky- 
blue. On boiling for a short time, the solution becomes 
opalescent. On continuing to boil it acquires a purple 
tcolour, and then, provided the solution is tolerably con¬ 
centrated, a copious deposit of dark purple flocks is 
formed. These are collected on a filter and washed with 
water. By treating the washed flocks with alcohol, Dr. 
Schunck obtained pure indigotine, and a reddish-blue 
substance was dissolved, called by Berzelius indigo-red, 
and by Schunck indirubine, which may be considered 
as a product of the decomposition of indican. 
The colour-giving principle of the plant may be con¬ 
sidered, like those of the dye-woads, as a glucoside, 
yielding by its decomposition a colourless principle, 
which is afterwards converted into a colour. Dr. Schunck 
explains by the following formula the decomposition of 
indican into indigo :— 
C 26 H 33 N0 1S + H 2 0 = C 8 H 5 NO + 3 (C 6 H 10 O 6 ). 
Indican. "Water. Indigo. Indiglucine. 
In 1864 he published a most valuable paper, throwing 
much light on the manufacture of indigo, and the 
management of what is called a woad vat. In it he 
gives the chemical actions which take place, and ex¬ 
plains why, if the indigo manufacturer does not take 
the greatest care in conducting the process of fermenta¬ 
tion, to extract the indigo from the plant, he will either 
get an inferior quality of indigo, or a great decrease in 
the yield of the product, or even in some cases entirely 
lose the colouring matter. He shows that these results 
take place when the fermentation in the vats is allowed 
to be either too rapid or too prolonged, in which cases 
not only is indican decomposed, but the indiglucine, or 
sugar, is converted into alcohol and acetic acid; and he 
shows that the indigo combines with this alcohol and 
acetic acid, yielding a compound which docs not permit 
the indigo it contains to be oxidized on exposure to the 
air. This explains the loss of indigo in its manufacture. 
Dr. Schunck represents the change by the following 
formulae :— 
C 8 H 5 NO + 8 (C 2 H c O) + 3(C 2 H 4 0 2 ) + C0 2 
Indigo. Alcohol. Acetic Acid. 
= C 31 H 3j N0 4 + 13H.O. 
He produced this compound more easily by treating 
pure indigo blue with alcohol, to which was added an 
alkaline solution of protoxide of tin, until the indigo 
was dissolved, acetate of soda was then added, and the 
whole digested at a moderate temperature. The indigo 
blue after some time ceased to be deposited on exposure 
to air, having entirely disappeared, and been converted 
into the new compound. 
Bure indigo, or indigotine, can be prepared from in¬ 
dican, by the process already described, or by reducing 
indigo to powder, placing it in a small dish on which is 
a cover, and applying a heat of about 300° or 400° F., 
when beautiful prismatic needles of indigotine are sub¬ 
limed, which arc removed mechanically. It is insoluble 
in water, alcohol or ether, or in weak acids or alkalis, 
but is slightly soluble in creasote, carbolic acid, and an¬ 
hydrous acetic acid, to which a very small quantity of 
sulphuric acid has been added. From this latter solu¬ 
tion, the indigotine may be precipitated by the addition 
of water. It is the only solution of blue indigo that can 
be applied directly on a fabric. 
Indigotine, when heated, gives off beautiful violet va¬ 
pours, having a characteristic odour. Under the action 
of oxidizing agents, such as nitric acid, it yields three dis¬ 
tinct products, isatin , C 8 H 5 N 0 2 ; indig otic or nitrosalycilic 
acid, C-H 3 (N0 2 )0 3 ; and picric acid, C 6 H 3 3(N0 2 )0. 
The first is formed by simple oxidation, whilst in the 
second and third, part of the carbon is oxidized, and 
hyponitric acid is substituted for part of the hydrogen. 
These chemical reactions lead me to call your attention 
to some very interesting scientific deductions, showing 
indigo to bear a very close relation to aniline and car¬ 
bolic acid, both products derived from coal-tar. If in¬ 
digotine is heated with potash in a small retort, aniline 
is produced, and distils over. When indigo is heated 
for some time with caustic potash solution of sp. gr. 
1*35, it yields anthranilic acid, which may be considered 
