1881.] on Indigo^ and its Artificial Production. 583 



that by careful treatment tlio leaves of all these indigo-yielding 

 plants can bo shown to contain a colourless principle termed indican, 

 and that this easily decomposes, yielding a suf,'ar-likc body and 

 indigo-blue. That white indigo is not present in the loaves is 

 proved by the fact that this compound requires an alkali to 

 bo present in order to bring it into solution, whereas the sap of 

 plants is always acid. The decomposition is represented by Schunck 

 as follows : — 



C26H31NO,, + 2H,0 = C,H,NO + SC.H.oO^. 



Indican. Indigotin. Indiglucin. 



So readily does this change from indican to indigo take place, 

 that bruising the leaf or exposing it to great cold is sufficient to 

 produce a blue stain. Even after mere immersion in cold alcohol or 

 ether, when the chlorophyll has been removed, the leaves appear 

 blue, and this has been taken to show the pre-existence of indigo in 

 the plant. But these appearances are deceptive, for Schunck has 

 proved that if boiling alcohol or ether be used, the whole of tho 

 colour-producing body as well as the chlorophyll is removed, tho 

 leaves retaining only a faint yellow tinge, whilst the alcoholic 

 extract contains no indigo-blue, but on adding an acid to this liquid 

 the indican is decomposed and indigo-blue is formed. 



Passing now to the more immediate subject of his discourse, the 

 speaker again reminded his hearers that indigo w\as the second 

 natural colouring matter which has been artificialy prepared ; 

 alizarin the colouring matter of the madder-root being the first. As 

 a rule, the simpler problems of synthetic chemistry are those to 

 which solutions are the soonest found, and these instances form no 

 exception to the rule. The synthetic production of indigo is a more 

 difficult matter than the artificial formation of alizarin, and hcnco 

 the speaker did not apologise for leading up to the complex through 

 the more simple phenomenon. 



When the ingenious Japanese workman who had never seen a 

 watch had one given to him with an order to make a duplicate, he 

 took the only sensible course open to him, and carefully pulled the 

 watch to pieces, to see how the various parts were connected to- 

 gether. Having once ascertained this, his task was a comparatively 

 easy one, for he then had only to make the separate parts, and fit 

 them together, and he thus succeeded so well in imitating the real 

 article that no one could tell the dificrence. So it is witli the 

 chemist, until he knows how the compound is built up, that is, 

 until he has ascertained its constitution, any attempt at synthesis is 

 more like groping in the dark than like shaping the course by 

 well-known landmarks into harbour. 



In the case of alizarin it was comparatively easy to reduce it to 

 its simplest terms, and to show that the backbone of this colouring 

 matter is anthracene C14H10, a hydro-carbon found in coal-tar. This 



