292 INDUSTRIAL PLANTS 



stance called indican (CogHsiNOiy) 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 (CicHioNoO:). A sub- 

 stance which thus decomposes into a sugar and some other 

 compountl is known as a glucoside. Indigo blue is insoluble 

 in water and can therefore be separated along with certain 

 impurities by filtration. The pasty mass retained is dried 

 in cakes to form the indigo of commerce. The insolubility 

 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. The difficulty is 

 overcome by 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 (CieHijNoOo) which is soluble in dilute alkaline 

 solutions and has the fortunate property of quickly changing 

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

 monly employed by dyers to change the indigo-blue is to add 

 indigo to vats containing lime-water in which bran or mo- 

 lasses or some other substance is undergoing fermentation. 

 When the indigo is all transformed and dissolved, a piece 

 of white woolen or cotton soaked in the solution and then 

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



A considerable number of plants have been found to con- 

 tain indican, and several different species are cultivated in 

 India and other warm countries for the manufacture of 

 indigo. Of these plants the most important one is the dyer's 

 indigo shrub (Fig. 275). 



Logwood is obtained from a small Central American tree 

 (Fig. 276). It is exported in the form of logs from which the 

 sap-wood has been removed. The coloring matter which 

 it yields, is, like indigo, not present in the living plant 

 but is derived from a colorless glucoside called hematoxylin 

 (CieHiiOe) which in turn readily oxidizes to form the deep 

 violet-purple compound known as htrmatein (CicHjaOe). It 

 is interesting to observe that this transformation involves 

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

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



