338 
COLORING MATTER OP DUTCH LITMUS. 
to the presence of indigo in cake litmus; yet the facts 1 am 
about to lay before the Society leaves no doubt, in my mind, 
of its existence in all the specimens of Dutch litmus which 
I have hitherto examined. Ferber, of course, makes no 
mention of it ; and I have no doubt that one reason why 
he was not permitted to inspect the mill was, that he might 
not discover the fact of the introduction, not merely of the 
earthy matter to give body to the cake, but also of indigo 
to give color. 
The existence of indigo in cake litmus is proved by the 
following facts: 
1st. The color of the cake is indigo-blue. 
2dly. Their odor, when fresh, is essentially that of indigo. 
There is also a slight violet smell, which is developed by 
the lichens when undergoing fermentation, and which has 
led some writers into the error of supposing that Florentine 
orris is used in the manufacture of litmus. 
3dly. If the cakes be rubbed with the nail they assume 
the well-known coppery lustre which characterizes indigo ; 
but which of course is much less intense in the litmus cake 
than in a mass of pure indigo. 
4thly. If the cakes be heated by a spirit lamp, in a pla- 
tinum capsule or watch glass, a vapor is evolved, which is 
known to be that of indigo by the following characters — 
a. Its peculiar and very characteristic odor. 
p. Its reddish-violet color. 
y. Its condensation in the form of beautiful coppery-pur- 
ple crystals, which, when rubbed with oil of vitriol, form the 
blue liquid known as sulphate of indigo, and which is 
decolorized by hypochlorite of lime. The crystals, when 
examined by the microscope, present, when viewed as 
opaque objects, a beautiful coppery brilliancy ; and when 
viewed as transparent objects, by transmitted light ; some 
of them are of a deep blue color. 
5thly. Litmus cakes, when deprived of their lichen-blue 
by digestion in distilled water or spirit, still retain a blue 
