902 INDIGO 



to the atmosphere, which is not always the case. The necessity for a long process of 

 fermentation in order to obtain the colouring matter would also not be very apparent, 

 the mere contact with oxygen being, it might be supposed, all that was necessary for 

 the purpose. The facility with which the indigo-blue is destroyed if the process of 

 fermentation is carried too far, is also inconsistent with the supposition that it is con- 

 tained in plants either as such, or in a de-oxidised state, since indigo-blue is a body 

 not easily decomposed, except by very powerful agents. 



In order to throw some light on this subject, an investigation was undertaken by 

 Schunck into the state in which indigo-blue exists in the Isatis tinctoria, or common 

 woad, which is the only plant indigenous to Europe that yields any considerable 

 quantity of the colouring matter. Schunck succeeded in obtaining from that plant a 

 substance of very peculiar properties, to which he gave the name of Indican. This 

 substance has the appearance of a yellow or light brown transparent syrup. It has 

 a bitter taste. It is very easily soluble in water, alcohol, and ether ; its solutions are 

 yellow, and have an acid reaction. Its compounds with bases are yellow. When its 

 watery solution is mixed with a strong acid, such as muriatic or sulphuric acid, no 

 change takes place at first, but on leaving the solution to stand, or on heating it, it 

 becomes blue and opalescent, then acquires a purple colour, and at length deposits 

 a quantity of purplish-blue flocks, which are quite insoluble in water. These flocks 

 consist for the most part of indigo-blue, but they contain also a red colouring matter 

 and several brown substances of a resinous nature. The supernatant liquid contains 

 a peculiar kind of sugar, and on being distilled, yields carbonic, formic, and acetic 

 acids. Hence it follows that the plant does not contain indigo-blue ready formed 

 either in the blue or colourless state, that the latter exists in the vegetable juice in a 

 state of combination with sugar, forming a compound of that peculiar class known to 

 chemists as glucosidcs. This compound is readily dissolved by water, and the indigo- 

 blue may then be liberated and precipitated from the solution by means of acids, and 

 probably also by other agents, but the simultaneous action of oxygen is not necessary 

 during the process of decomposition, which the compound undergoes in yielding 

 indigo-blue. Now if, as seems probable, the various species of indigofera contain 

 indican or some similar substance, the phenomena which took place during the process 

 of manufacturing indigo may easily be explained. During the steeping process the 

 indican is dissolved, and, in consequence of the fermentation which then takes place 

 in the liquor, it is decomposed into indigo-blue and sugar. The former would then 

 be precipitated ; but since ammonia is, according to most authors, evolved at the same 

 time, the indigo-blue is, by the simultaneous action of the alkali and the sugar, or 

 other organic matters contained in the liquid, reduced and dissolved, forming a true 

 indigo vat, from which the colouring matter is afterwards precipitated by the 

 combined action of the atmospheric oxygen and the lime, during the beating process. 

 According to Schunck, two distinct periods may be observed in the decomposition of 

 indican. During the first period, indigo-blue is the chief product of decomposition ; 

 during the second, the red and brown resinous matters make their appearance with 

 very little indigo-blue. The formation of carbonic, acetic, and formic acids is, 

 according to Schunck, dependent on that of the brown resinous matters. It would 

 appear, therefore, that the copious disengagement of carbonic acid, as well as the arid 

 taste, attributed to acetic acid, sometimes observed during the manufacture of indigo, 

 are phenomena which indicate the formation, not of indigo-blue, but of other sub- 

 stances, which may prove very injurious to the quality of the indigo. These substances 

 being soluble in alkalis, but insoluble in water, are precipitated, as soon as the liquid 

 loses the alkaline reaction which it possesses at the commencement, and becomes acid. 

 Though indigo-blue is a body of very stable character, not easily decomposed when 

 once formed, except by potent agencies, still the assertion of Porottet and others, that 

 ' nothing is more fugitive, and more liable to be acted on by destructive agencies, than 

 the colouring principle of the indigoferae,' will be easily understood when the follow- 

 ing facts, mentioned by Schunck, are taken into consideration. If a watery solution 

 of indican, this indigo-producing body, be boiled for some time, it then yields by 

 decomposition, not a trace of indigo-blue, but only indigo-red ; and if it be boiled with 

 the addition of alkalis, it then gives neither indigo-blue nor indigo-red, but only the 

 brown resinous matters before mentioned. The mere action of alkalis is, therefore, 

 sufficient to cause the molecules, which would otherwise have gone to form imli^o- 

 blue, to arrange themselves in a totally different manner, and yield products which 

 bear very little resemblance to it. It is evident, therefore, that one of the chief 

 objects to be kept in view by the manufacturer of indigo, is the proper regulation of 

 the process of fermentation, so as to prevent the formation of the other products, 

 which take the place of indigo-blue, and are formed at its expense. 



The indigo of commerce occurs in pieces, which are sometimes cubical, sonit times 

 of an irregular form. These pieces are firm and dry, and are easily broken, the 



