( 125 ) 
the indigo-blue. On warming an indican solution with isatine and 
dilute hydrochloric acid, all the indoxyl which is set free precipi- 
tates as indigo-red, and I presume that a good quantitative indican 
determination may be based upon this reaction. 
All the here mentioned characteristics of the indoxyl-containing 
plant-saps are also announced in the literature of the chemically 
prepared indoxyl, except the conduct towards isatine and hydrochloric 
acid which has perhaps not been examined. 
Natural indigo prepared from woad, contains a small quantity of indigo- 
red; but whether this originates from the same indoxyl as the blue, or 
from an isomeric indoxyl, I cannot decide. Indigo-red I could also 
find in the indigo made from indican, whether chemically by boiling 
with acids, or by bacteria, or by enzymes. Consequently, if two 
indoxyls should exist, there should also exist two indicans. 
2. Demonstration of Indigo in the Indigo-plants themselves. 
For the demonstration of indigo in the plants themselves, 
Mr. Motiscu described in 1893 his ,aleohol-experiment” to which 
he afterwards repeatedly recurred '). In this experiment the parts of 
the plants to be examined are exposed, in a confined atmosphere, 
to alcohol- or chloroform-vapour, for instance by putting them into 
a glass-box, in which a small vessel with these substances is placed. 
Thus slowly dying all the indigo-plants become more or less 
blue, which is perceptible after the chlorophyll has been removed 
by extraction with alcohol. I found, however, that never all the 
present indoxyl or indican changes into indigo. The „alcohol-experi- 
ment” succeeds the best with Polygonum tinctorium, where at least 
most of the indoxyl changes into indigo. For woad the result is 
greatly dependent on the length of time which the experiment 
requires, even on the season, but invariably only a part, though it 
may be a great part, of the indoxyl passes into indigo. With Indigo- 
fera only a little indigo precipitates in the youngest leaflets and 
buds, while the older leaves become quite colourless by the alcohol- 
extraction though they are extremely rich in indican, so that, for 
this plant, the ,alcohol-experiment”’ is without any value 2). 
") Sitz.ber. der k. Akad. d. Wiss. za Wien Bd. 102, Abt. 1, pag. 269, 1893; Bd. 
107, pag. 758, 1898, and Berichte d. deutschen Botan. Gesellsch. Bd. 17, pag. 230, 1899. 
*) Quite wrongly Mr. Mo.iscu declares: „Die präcisesten Resultate erhält man 
bei Indigofera mit der vAlkoholprobe,” and as wrong is his assurance /Durchwegs 
war zu bemerken, dass die in Europa gezogenen Pflanzen (von Indigofera) auffallend 
viel weniger Indigo lieferen wie die tropischen” (Berichte d. deutsch. Bot. Ges. Bd. 
17, pag. 231, 1899). 
