CHEMISTRY APPLIED TO THE ARTS. 



when the chemical nature of indigo, and the form 

 in which it occurs in the plant, have been ex- 

 plained. Indigo does not exist as such in the 

 plant. The indigo plants contain a substance 

 called indican, which undergoes fermentation, 

 yielding indigo blue, and a kind of sugar called 

 indiglucine. This decomposition is therefore 

 analogous to that by which alizarine is obtained 

 from ruberythric acid (p. 342) ; indeed, we may 

 remark in passing, that there are in many plants 

 substances called glucosides, which, when fer- 

 mented and many of them when boiled with 

 acids yield a kind of sugar and another sub- 

 stance. Thus, amygdaline, contained in the bitter 

 almond, yields sugar, and prussic acid, and benzoic 

 aldehyd ; myronic acid, contained in the black 

 mustard-seed, yields sugar and essential oil of 

 mustard. Many other instances might be given. 



Indigo blue has the composition C 8 H 5 NO. It 

 has a deep blue colour, and when rubbed with a 

 smooth surface, a copper-coloured metallic lustre. 

 It can be volatilised unchanged, when very 

 cautiously heated in small quantities. It is quite 

 insoluble in water, alcohol, ether, oils, and in 

 dilute acids and alkalies. 



When treated with reducing agents in alkaline 

 solutions, it takes up hydrogen, and is converted 

 into a colourless substance, which dissolves in the 

 alkali. This colourless substance can be pre- 

 cipitated from such solutions by neutralising them 

 with an acid. It is called indigo white, and has 

 the composition C 16 H 12 N 2 O 2 = 2C 8 H 5 NO + H 2 . 

 When indigo white, or its alkaline solution, is 

 exposed to the air, the oxygen rapidly takes 

 hydrogen from it, and indigo blue is produced. 

 Now, in the fermenting vat, ammonia and formic 

 acid are produced that is, an alkali and a re- 

 ducing agent ; the indigo blue, therefore, formed 

 by the fermentation of the indican, is converted 

 into indigo white, which dissolves in the alkaline 

 liquid. This is the yellow solution which flows 

 from the fermenting to the beating vat In the 

 beating vat it is brought into thorough contact 

 w th the oxygen of the air, and oxidised to indigo 

 blue : 



C 16 H 1S N,0 S 



Indigo White. 



O = 2C 8 H 5 NO 



Indigo Blue. 



H S O. 



We shall see that this conversion of indigo blue 

 into soluble indigo white, and the reoxidation of 

 the latter into insoluble indigo blue, is of the 

 greatest importance in dyeing. 



Action of Sulphuric Acid on Indigo Blue. 

 By the action of strong, or, most conveniently, 

 fuming, sulphuric acid (p. 337) on indigo blue 

 two acids are formed : i. Sulphophenicic acid 

 (C, 6 H 10 N,SO $ = 2C 8 H 5 NO + SO :1 ), and 2. Sulph- 

 indigotic acid (C 8 H 6 NSO 4 = C 8 H 6 NO + SO^ ; 

 the latter is always formed if the sulphuric acid 

 is strong and the action prolonged. The solu- 

 tions of these acids are deep blue. Their salts 

 with alkalies, such as sulphophenicate of soda 

 (Ci 6 H 9 NaN 1 SO 6 ) and sulphindigotate of soda 

 (C 8 H 4 NaNSO4), are soluble in water. A variable 

 mixture of the two salts just named is sold under 

 the names indigo carmine and ceruleine. It is 

 prepared by taking advantage of the fact, that 

 sulphophenicates and "sulphindigotates of the 

 alkalies, although soluble in water, are insoluble 

 in solutions of most salts. The solution of indigo 

 in sulphuric acid, which always contains a large 



excess of sulphuric acid, is neutralised with car- 

 bonate of soda. Sulphophenicate and sulphin- 

 digotate of soda are formed, and also sulphate of 

 soda : the presence of this in the solution renders 

 the two former salts insoluble ; they are accordingly 

 precipitated, and can be collected on a filter. 

 Indigo carmine usually consists chiefly or entirely 

 of sulphindigotate of soda. 



We have now considered in some detail the two 

 most important of the natural vegetable dye-stuffs, 

 madder and indigo ; the others need not occupy us 

 long. 



Logwood is the wood of the tree Hamatoxylon 

 campechianum, belonging to the natural order 

 Leguminosae. The tree grows in Central America 

 and the West Indies. The colouring power of the 

 wood depends on a substance called hcematoxyline, 

 having the composition C 16 H 14 O 6 . This substance 

 occurs in the wood, partly free, partly as a ' gluco- 

 side ' (see above). The glucoside is decomposed 

 by fermentation of the wet powder. Haematoxy- 

 line is a colourless or faintly yellow crystalline 

 substance. With acids it forms a yellow solution, 

 which, on the addition of an alkali, changes to red. 

 It acts as a weak acid, forming salts with strong 

 bases. These salts are very readily oxidised, and 

 although themselves colourless, become blue or 

 black by absorbing oxygen. Thus, with alumina, 

 we obtain a violet blue ; with oxide of iron, a bluish 

 black ; with oxide of zinc, a purple ; with stannous 

 oxide, a violet ; bichromate of potash is at once 

 reduced, oxidising the hasmatoxyline, and produc- 

 ing a deep violet black compound. It is used for 

 dyeing blue and black, advantage being taken of 

 the above-mentioned reactions. 



Brazil wood is obtained from various trees be- 

 longing to the natural order Leguminosae. Thus, 

 we have Pernambuco wood from. Ccesalpinia crista; 

 Brazil wood proper from C<zsalpinia braziliensis ; 

 Nicaragua wood from Ccesalpinia echinatos ; 

 Sapan wood from Ccesalpinia Sapan; Bresillet 

 wood from Ccesalpinia vesicaria. We do not as 

 yet know certainly what is the colouring principle 

 of these woods, or whether there is only one or 

 more such principles ; at all events, all of these 

 woods act in very nearly the same way. The 

 colouring principle exists in the wood as a gluco- 

 side. It is itself colourless, but absorbs oxygen, 

 and becomes red. 'It combines with metallic 

 oxides, forming ' lakes.' 



Sandal-wood is the wood of the Pterocarpus 

 santalinus, a beautiful tree, growing in Ceylon and 

 the Coromandel Coast. Its colouring principle, 

 called santalin, is of a red colour. 



Fustic. What is called ' old fustic ' is the wood 

 of the Morus tinctoria, a tree which grows in the 

 East Indies, in South America, and West Indies. 

 It contains two colouring-matters, Morin and 

 Maclurin, having the composition respectively 

 C 12 H 8 O 6 and C 13 H 10 O 8 . The wood is used for 

 dyeing yellow. 'Young fustic,' 'bois de fustet,' 

 is the wood of the Rhus cotinus, growing in the 

 West Indies, the Levant, Spain, Italy, Hungary, 

 the Tyrol, and the south of France. Its colouring 

 principle has been named ' fustine,' and appears 

 to be identical with ' quercetine.' It is used for 

 dyeing yellow. 



Quercitron is the bark of the Quercus tinctoria, 

 a kind of oak growing in Pennsylvania, North 

 and South Carolina, and Georgia. It contains a 

 crystalline substance, ' quercitrine,' which, when 



