292 _ INDUSTRIAL PLANTS 
stance called imdican (C.gH3,NO,;) 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 (Cy,H,N.O,). A sub- 
stance which thus decomposes into a sugar and some other 
compound 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 (Cyg.H,2N.2O.2) 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 
(Cy,H,,0,) which in turn readily oxidizes to form the deep 
violet-purple compound known as hematein (C,,Hy20,). 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, 
