pendently studied the action of oxidising agents on indigo 
and obtained isatin, C 8 H 6 N0 2 which is not a colouring matter. 
The further examination of this body led to most interesting 
results, but as those are not directly connected with the 
subject of this paper, I cannot discuss them here. 
We must therefore proceed at once to 1865, when Baeyer 
and Knop found that by acting on isatin in an alkaline 
solution with hydrogen in the nascent state it is converted 
into a j^ellow crystalline body, which they called dioxindol 
C 8 H 7 N0 2 . This is easily further reduced in an acid solution 
to oxindol , C 8 H 7 NO, which forms colourless needles, and on 
its vapour being passed over red-hot zinc-dust it loses its 
oxygen, indol , C 8 H 7 N being formed, which is also a colourless 
crystalline compound and a most interesting body, inasmuch 
as it is also formed, as Nencki and Kiihne have shown, 
in pancreatic digestion, and is contained in the faeces. 
In 1869, Baeyer and Emmerling obtained indol from 
cinnamic acid, which occurs in several plants, and can be 
obtained artificially from coal-tar, as I shall show further 
on. By the action of nitric acid it yields two isomeric nitro- 
compounds; one of them, called orthonitro- cinnamic acid, 
loses, when heated with caustic potash and iron filings, 
carbon dioxide, and oxygen and indol is formed : 
C 8 H 6 (N0 2 )C0 2 H = c 8 h 7 n + o 2 + co 2 
The same chemists discovered, 1870, a method by which 
isatin can again be reduced to indigo-blue. By heating it 
with a mixture of phosphorus trichloride, acetyl chloride, 
and phosphorus to 70 — 80°, they obtained a green liquid 
which, when poured into water, deposited, on standing ex- 
posed to the air, a blue powder containing indigotin. At the 
same time a purple colouring matter was formed, which 
they called indigo-purpurin. 
It has been known for some time that urine, on standing, 
sometimes deposits indigo-blue. Jafie, in 1870, found that 
he could produce it by the subcutaneous injection of indol ; 
