21)2 INDUSTRIAL PLANTS 



stance called indican (CooHsiNOn) which is extracted by 

 water from the leafy shoots, and, under the influence of an 

 enzym which accompanies it, gives rise to a compound re- 

 sembling glucose and to indigo blue (CinHmNiOj. A sub- 

 stance which thus decomposes into a sugar and some other 

 compound is known as a yluco.sidc. Indigo l:)lue is insoluble 

 in water and can therefore be separated along ^vith certain 

 impurities by filtration. The pasty mass retained is dried 

 iu cakes to form the indigo of commerce. The inscjiubilit}' 

 of indigo blue in water presents a peculiar difficulty to its 

 use as a dye, yet at the same time gives it a great advantage 

 when once it is incorporated with a fiber. Tlie difficulty is 

 overcome l)y taking advantage of the fact that indigo blue 

 may be readily changed (in various ways which increase the 

 proportion of hydrogen) into a colorless substance called 

 indigo white (Cn-iHi-iSJ^^) which is soluble in tlilute alkaline 

 solutions and has the fortunate jjroperty of c|uickly changing 

 back to indigo-blue on exposure to the air. The means com- 

 monly employed by d\'ers to change the indigo-blue is to add 

 indigo to ^'ats containing lime-water in ^'\dlich l>rau or mo- 

 lasses or some other sul>stance is undergoing fermentation. 

 When the indigo is all transfoi-nu>d and dissolved, a, piece 

 of white woolen or cotton soaked in the solution and then 

 exposed to the air soon takes on a permanent lilue color. 



A considerable number of jilants have l)een found to con- 

 tain indican, and s(>veral different sp(^eies are eultivated in 

 India and other warm coimtries for the manufacture of 

 indigo. Of these ])lants the most important one is the d>'er's 

 indigo shrub (Fig. 275). 



Logwood is obtained from a small ('entral .-Vnicrican tree 

 (Fig. 27(i). It is exported in the form of logs l'r<mi ^^•hich the 

 sap-wood has been removed. The coloring matter which 

 it yields, is, like indigo, not present in the li\'ing j^lant 

 but is derived fi'oni a colorless glucoside called iKnnaioxylin 

 (i\iMuO^) which in turn readily oxitlizes to form the deep 

 ^■iolet-purl)le compound known as hauudein (( ', JIi./),,). It 

 is interesting to observe thai this li-ansfornuttion iuA'olves 

 the loss of two atoms of hydrogen just as does the change 

 of the white indigo into the blue. Inlike indigo, however, 



