35 
I have already stated that isatin can be reduced to 
indigo-blue ; Baeyer endeavoured now to find a more simple 
method for effecting this. By acting with phosphorus penta- 
chloride on isatin he obtained a compound which he called 
isatin chloride, which nascent hydrogen converts into 
indigotin. 
2C 8 H 4 C1N0 + 2H 2 = Ci 6 H 10 N 2 O 2 + 2HC1. 
As far back as 1869, Kekuld predicted isatin to possess 
the constitution which it has been proved to have by 
Baeyer’s researches, and two of Kekule’s pupils, Claisen 
and Shadwell, discovered in 1879 a very simple synthetical 
method for preparing it. By acting with phosphorus 
chloride on orthonitro- benzoic acid C 6 H 4 (N0 2 )C0 2 H the 
chloride, C 6 H 4 (N0 2 )C0C1 is formed, which when heated with 
silver cyanide yields the nitril, C 6 H 4 (N0 2 )C0CN, on heating 
the latter with a solution of caustic potash, it is converted 
into orthonitro-phenylglyoxylic acid, C 6 H 4 (N0 2 )C0.C0 2 H and 
this is converted by nascent hydrogen into the amido- 
compound, which, like other ortho-compounds, loses water 
and yields isatin. 
I have now given you a sketch of the history of artificial 
indigo up to 1879, when I wrote : € ‘ The artificial production 
of indigo has so far merely a theoretical interest; whether 
the time will come when simplified methods will admit of its 
manufacture on a large scale remains to be seen. But even 
if not, the indigo-purpurin, which is always formed together 
with the blue, may become of importance as a colouring 
matter. This body, as Dr. Schunck has shown, is identical 
with his indigorubin, which always occurs, but in small 
quantity only, in indigo. Dr. Schunck has traced the 
formation of this beautiful purple colour in Polygonum 
tinctoriurm a plant used in China and Japan for the 
preparation of indigo. He has cultivated it for several 
years, and found that the young plants do not contain a 
trace of it. It can be only obtained from plants having 
