1881.] on Indigo, and its Artificial Production. 585 



Step No. 3. — The next advanco of importance in tliis some- 

 wliat complicated matter is the discovery by Erdmann and Laurent 

 independently, that indigo on oxidation yields a crystalline body, 

 which, however, possesses no colouring power, to which they gave the 

 name of isatin. 



Indigo. Isatin. 



Step No. 4. — The reverse of this action, viz. the reduction 

 of isatin to indigo, was accomplished by Baeyer and Emmerling in 

 1870 and 1878, by acting with jihosphorus pentachloride on isatin, 

 and by the reducing action of ammonium sulphide on the chloride 

 thus formed. 



Understanding now something of the structure and of the re- 

 lationships of the body which we wish to build up, let us see how 

 this edifice has, in fact, been reared. Three processes have been suc- 

 cessfully employed for carrying out this object. But of these three 

 only one is of practical importance. A synthetic process may yield 

 the wished-for result, but the labour incurred may be too great and 

 the losses during the campaign may be too severe to render it pos- 

 sible to repeat the operation with advantage on a large scale; just 

 as it costs, at the usual rate of wages, more than twenty shillings to 

 wash a sovereign's worth of gold out of the Rhine sands, so that this 

 employment is only carried on when all other trades fail. 



For the sake of completeness, let us, however, consider all three 

 processes, although Nos. 1 and 2 are at present beyond the pale of 

 practical schemes. 



These three processes have certain points in common. (1) They 

 all proceed from some compound containing the benzene nucleus. 

 (2) They all start from compounds containing a nitrogen atom. (3) 

 They all commence with an ortho-compound. 



They differ from one another ; inasmuch as process No. 1 starts 

 from a compound containing seven atoms of carbon (instead of eight), 

 and to this, therefore, one more atom must be added ; process No. 2, on 

 the other hand, starts from a body which contains exactly the right 

 number (eight) of carbon atoms; whilst No. 3 commences with a 

 compound in which nine atoms of carbon are contained, and from 

 which, therefore, one atom has to be abstracted before indigo can be 

 reached. 



Process No. 1 (Kekule — Claissen and Shadwell). — So long ago as 

 1869 Kekule predicted the constitution of isatin, and gave to it the 

 formula which we now know that it possesses, viz. 



CeH4 ^ 



