1902.] on Recent Developments in Colouring Matters. 105 



costly. The whole indigo problem stood thus reduced to the problem 

 of transforming naphthalene cheaply and economically into phtalic 

 acid. This has been accomplished by the Badische Anilin- und Soda- 

 Fabrik by heating naphthalene with fuming sulphuric acid in the 

 presence of mercury salts. Torrents of sulphur dioxide escape, and 

 the whole process can only be carried out properly if the means be 

 given to convert this gas again into fuming sulphuric acid, which 

 may be used again for treating fresh quantities of the hydrocarbon. 

 The new sulphuric acid process of the Badische Anilin- und Soda- 

 Fabrik has thus been of paramount importance for the working out 

 of the indigo problem. 



Some of the older synthetical methods of producing indigo are so 

 easy and rapid that they can easily be shown as a lecture experiment. 

 If, for instance, we add a caustic potash solution to a solution of 

 orthonitrobenzoic aldehyde in acetone, indigo is formed at once and 

 settles out in dark-blue crystalline flakes (Exp. IX.). The synthesis 

 now in practical use is a little more delicate in its execution, but 

 there are certain modifications of it which are rapid enough to be 

 shown in a lecture experiment (Exp. X.). 



The action of the alkali on the phenylglycine-carbonic acid 

 does not at once produce indigo, a colourless derivative of the dye, 

 indoxylcarbonic acid, or rather its potash salt, is formed at first, but 

 if we dissolve this in hot water and introduce a current of air, it is 

 at once and with a quantitative yield transformed into indigo which 

 settles out in the shape of a crystalline deposit of infinitely fine 

 division. This is collected in filter-presses and delivered into com- 

 merce in the shape of a paste or a powder. 



The industrial synthesis of indigo is extremely interesting, be- 

 cause it is a triumph not wholly due to chemical science. Science 

 has shown the way to success, bnt it was quite unable to clear away 

 the difficulties arising from practical and economic considerations. 

 Here the representatives of our great industry had to advance inde- 

 pendently and on paths for which theoretical knowledge could not 

 serve them as a guide. Unlimited praise and admiration is certainly 

 due to them for the masterly way in which they grappled with colossal 

 difficulties and for the courage with which they staked millions on 

 the realisation of one great idea. 



At the same time we cannot help feeling some regret for the 

 indigo planters in the far East, who, after enjoying more than a 

 century of easy prosperity, see now that more serious times are in 

 store for them. They see the day coming when the indigo planta- 

 tions will disappear, in the same way in which the madder fields of 

 Avignon have vanished. But we are consoled by the knowledge that, 

 especially for India, the time has already come, which has been so 

 vividly described by Sir William Crookes, in one of his addresses to 

 the British Association, as the future in store, sooner or later, for all 

 humanity, the time when bread begins to be scarce. It seems to me 

 that any one who, by bringing about some great commercial revolu- 



