452

DYEING AND CALICO PRINTING.

the process of the fermentation of the plant, and under what influence andfrom what substance present in the plant is indigo derived ? Since noresearches on the fresh indigo plants have yet been made, and since, more-over, the different phases of the operations whereby indigo is produced on thelarge scale have never been scientifically examined, we can therefore onlyreason by analogy or from observation of what happens with other similarplants, and give an explanation which is probably, but not necessarily, correct.It has been at one time supposed that the leaves of the Indigoferce contained in-digotin in the hydrogenised state, and that this substance became soluble underthe influence of a peculiar fermentation, while the stirring and beating of thefermented liquid afterwards by promoting contact of the liquid with air causedthe oxidation of the white indigo. Although this explanation agrees with thewell-known chemical properties of indigo, it is not tenable on account of thefollowing faCts :—The yellow liquid which runs off from the fermentationtanks has a decidedly acid reaction (according to Dr. Croockewit, who, bya lengthy residence in Java, has been frequently enabled to test this liquid),and contains both acetic and laCtic acids; and we know by experience thatwhite indigo is not soluble in an acid liquid, and only dissolved by the aid ofalkalies or alkaline earths ; and unless we therefore admit the existence of anunknown principle, capable of keeping white indigo dissolved in such a liquidas that running off from the fermentation tanks until it is oxidised by the oxygenof the air, the existence of the hydrogenised indigotin in the plant is not veryprobable. Another opinion is, that indigo does not pre-exist in the leaves orplant at all, but is formed during the process of fermentation jvhich is usuallyemployed for the extraction of the drug (dye material) from the indigoferce aswell as from woad.

Dr. Schunck has discovered in the Isatis tinctoria* a principle very solublein water and alcohol, viz., a kind of glucoside, capable of being split up bythe action of dilute acids and alkalies, and even simply by evaporation of itssolutions into a peculiar sugar (indiglucin) and into indigotin; to this produCtthe chemist just named gives the name of indican, its formula beingC 2 6H 3 i NO i7 : the splitting-up of indican into indigotin and indiglucin maybe elucidated by—

C 26 H 3I NO I7 + 2 H 2 0 = C 8 H 5 NO+ 3 (C 6 H io 0 6 )

Indican. Indigotin. Indiglucin.

The preparation of the indican will be referred to when we.treat of woad;but it is probable indican may exist also in the Indigoferce ; and, if so, theindican will become soluble in the fermentation tanks, and this body insolution will be entirely split up into indigotin and indiglucin in the beatingtanks, a result which, under the prevailing high temperature (often 30° to 35° C.),will cause the further decomposition of the indiglucin, and also cause theproduction of other brown, red, and yellow matters, which remain mixed upwith the indigo. During the process of beating up the liquor a large quantityof carbonic acid is disengaged, and consequently a good ventilation has to bekept up in the buildings wherein the tanks are placed. If the temperature ofthe liquid is too low (the workpeople know by experience the most suitabletemperature) hot water is added, but is rarely needed.

Indigo is only, or)at least chiefly, prepared from the real Indigoferce, although

* This dyeing material, known as woad, will be more fully mentioned below.