588 Professor H. E. Boscoe [May 27, 



acid, giving rise to ortho-nitro-phenyl-propiolic acid, and (6), and 

 lastly, the conversion of this latter into indigo by heating its alkaline 

 solution with grape sugar, xanthate of soda, or other reducing agent. 

 These reactions are thus represented : — 



3. CeHsCHZlCHCOOH CgH^CNOJCHZlCH.COOH. 



Cinnamic acid yields Ortho-nitro-ciunamic acid. 



In this process the para-acid is also obtained, and as this is 

 useless for the manufacture of indigo, it has to be removed. This 

 is effected by converting the acids into their ethyl ethers, which, 

 possessing different degrees of solubility, can be readily separated 

 from one another. 



4. This is next converted into the dibromide 



C6H4(N02)CHBr . CHBrCOOH. 



5. And by careful treatment with caustic soda this yields ortho- 

 nitro-phenyl-propiolic acid, thus : — 



C.H/NOolCHBrCHBrCOOH + 2NaOH = 



C6H,(N02)C2.COOH + 2NaBr + 2H2O. 



6. n[CeH4(N02)C2 . COOH + H^ = C3H5NO +00^ + H^O]. 



Ortho-nitro-phenyl-propiolic acid. Indigotin. 



(Q. E. D.) 



The last of these reactions is in reality not so simple as the equa- 

 tion indicates. For only about 40 per cent, of indigo is obtained, 

 whereas according to theory 68 per cent, should result. Indeed, 

 although, as we have seen, indigo can be prepared by these three 

 methods, chemists are as yet in doubt as to its molecular weight, the 

 probability being that the molecule of indigo contains twice 16 

 atoms of carbon, or has the formula ^(CgHgNO) or C32H20N4O4. 

 Still it must be remembered that according to Sommaruga the vapour 

 density of indigo is 9 • 45, a number corresponding to the simpler 

 formula C^gHjoNaOa. 



The artificial production of indigo may even now be said to bo 

 within measurable distance of commercial success, for the ortho-nitro- 

 phenyl-propiolic acid, the colourless substance which on treatment 

 with a reducing agent yields indigo-blue, is already in the hands of 

 the Manchester calico printers, and is furnished by the Baden 

 Company for alkali and aniline colours at the price of 6s. per lb. for 

 a paste containing 25 per cent, of the dry acid. 



With regard to the nature of the competition between the artificial 

 and the natural colouring matters it is necessary to say a few words. 

 In the first place, the present price at which the manufacturers are 

 able to sell their propiolic acid is 50s. per kilo. But 100 parts of 

 this can only yield, according to theory, 68 * 58 parts of indigo-blue, 



