( 346 ) 
or very loosely combined carbon dioxide!) in very large quantity, 
which may be driven out not only by warming but also by a current 
of gas free from oxygen *). 
The substance which yields indigoblue on oxidation may be extracted 
from the solution by means of chloroform either with or without 
addition of acetic acid. When the chloroform solution, which has a 
light greenish yellow colour, is evaporated by blowing a current of 
air into it, a greenish coloured residue is obtained which is partially 
soluble in water. The aqueous solution, which possesses a splendid 
fluorescence, gives indigoblue at onee when shaken with air and 
ammonia ; indigoblue is also formed by exposure to the air, and very 
rapidly when warmed. 
An extract of the leaves of Marsdenia tinctoria, which is also very 
distinctly acid to litmus, behaves in a similar way. It has not yet 
been possibie to obtain the substance which yields the indigoblue in 
a state fit for analysis, nor to prepare crystallized derivatives of it.” 
The fact that the indigo-yielding substance is formed from the 
leaves by dilute organic acids was confirmed by Messrs. vAN LOOKEREN 
CAMPAGNE and VAN DER VEEN in 1895 3); notwithstanding the inso- 
lubility in acids it is still taken for indigowhite. The solubility of 
the indigowhite in the extract is explained, by these investigators, 
by the formation of an unstable compound with substances which 
yield indigored and indigorubin on oxidation. The ready oxydation 
of the substance which is extracted by chloroform, in presence of 
mineral acids or of alkaline carbonates, shows, according to the same 
authors, that we are not dealing with indoxyl. 
In 1897 I again took up my researches, and in the “Verslag” 
for that year the following is to be found : 
“Tf Indigoleaves (Guatemala or Natal) or leaves of Marsdenia 
tinctoria are placed in an atmosphere of chloroform or carbondioxide 
they retain their green colour. If, after some time, they are brought 
in contact with the air, they quickly become bluish, proving that the 
indican in the dead leaves, which had probably escaped from the 
cells in the form of a solution, had been decomposed. 
1) If carbon dioxide is passed into a solution of indigowhite in lime water until 
the lime is converted into bicarbonate, the whole of the indigo is precipitated and 
no indigoblue can be obtained from the filtrate by means of air and ammonia. 
(Note of 1893.). 
*) If the current of gas is passed through the liquid for a long time, it is well to 
mix it with chlorcform vapour in order to render living organisms, which might give 
rise to carbon dioxide during the experiment, inactive. 
3) Landwirtsch. Versuchsstationen. XLVI, 249. 
