902 COLT'S-FOOT 



assnmption was made in regard to other colouring matters, all of which were supposed 

 to exist originally in a deoxidised and colourless state. In regard to indigo, however, 

 the hypothesis is disproved at once by the fact that reduced indigo is only soluble in 

 alkaline liquids, and that the juice of the indigo-bearing plants is always acid. In 

 regard to the other colouring matters it seems also to be quite untenable. The first person 

 to throw some light on this obscure department of organic chemistry was Robiquet. This 

 distinguished chemist succeeded in obtaining from lichens in their colourless state a 

 beautifully crystallised, colourless substance, soluble in water, having a sweet taste, and 

 consisting of carbon, hydrogen, and oxygen. This substance he denominated orcine. 

 By the combined action of ammonia and oxygen, he found it to be converted into a red 

 colouring matter, containing nitrogen, and insoluble in water, which was in fact 

 identical with the colouring matter of archil. This beautiful discovery furnished 

 chemists with a simple explanation for the curious phenomena observed in the forma- 

 tion of this and other colouring matters, and it was soon followed by others. Heeren 

 and Kane obtained from various lichens other colourless substances of similar pro- 

 perties, and it was shown by Schunck that orcine is not even the first link in the 

 chain, but is itself formed from another body, lecanorine, which, by the action of 

 alkalis, yields orcine and carbonic acid, just as sugar by fermentation gives alcohol 

 and carbonic acid. In like manner it was discovered by Erdmann that the colouring 

 matter of logwood is formed by the simultaneous action of oxygen and alkalis from 

 a crystallised colourless substance, hamatoxyline, which is the original substance 

 existing in the wood of the tree, and is like the others, not itself strictly speaking, a 

 colouring matter, but a substance which gives rise to the formation of one. 



There is, however, another class of phenomena connected with the formation of 

 colouring matters, entirely different from that just referred to. It was discovered by 

 Robiquet that if madder be treated for some time with sulphuric acid, and the acid be 

 afterwards completely removed, the madder is found to have acquired a much greater 

 tinctorial power than before. This fact was explained by some chemists by supposing 

 that the sulphuric acid had combined with or destroyed some substance or substances, 

 contained in the madder which had previously hindered the colouring matter from 

 exerting its full power in dyeing, such as lime, sugar, woody fibre, &c. By others it 

 was suspected that an actual formation of colouring matter took place during the 

 process, and this suspicion has been verified by recent researches. Schunck suc- 

 ceeded in preparing from madder a substance resembling gum, very soluble in water, 

 amorphous, and having a very bitter taste, like madder itself, and to which he gave 

 the name of rubian. This substance, though not colourless, is incapable of combining 

 with mordants, like most colouring matters. When, however, it is acted on by strong 

 acids, such as sulphuric or muriatic acid, it is completely decomposed, and gives rise 

 to a number of products, the most important of which is alizarine, one of the colouring 

 matters of madder itself. Among the other products are a yellow crystallised colouring 

 matter, rubianine, two amorphous red colouring matters resembling resins, rubiretine and 

 verantine, and lastly, grape-sugar. When subjected to fermentation, the same products 

 are formed, with the exception of rubianine, which is replaced by a yellow colour- 

 ing matter of similar properties. This process of decomposition evidently belongs to 

 that numerous class called by some chemists ' catalytic,' in which the decomposing 

 agent does not act, as far as we know, in virtue of its chemical affinities. It is evident, 

 then, that when madder is acted on by sulphuric acid an actual formation of colouring 

 matter takes place, and it is even probable that the whole of the colouring matter 

 found in madder in its usual state was originally formed from rubian, by a process of 

 slow fermentation, the portion of the latter still remaining undecomposed being that 

 which is acted on -when acids are applied to madder. From the Isatis tinctoria or 

 common woad plant, Schunck, in like manner, extracted an amorphous substance, 

 easily soluble in water, called by him indican, and which, by the action of strong 

 acids, is decomposed into indigo-blue, indigo-red, sugar, and other products, among 

 which are also several resinous colouring matters. Looking at them from this point 

 of view, colouring matters may be divided into two classes, viz., first, such as' are 

 formed from other substances, not themselves colouring matters, by the action of 

 oxygen and alkalis; and, secondly, such as are formed from other substances by 

 means either of ferments or strong acids, without the intervention of oxygen. To the 

 first class belong the colouring matters of archil, litmus, and logwood ; they yield 

 comparatively fugitive dyes, and are usually decomposed Vy allowing the very process 

 to which they owe their formation to continue. To the second class belong indigo- 

 blue, indigo-red, and alizarine, bodies which show a remarkable degree of stability 

 for organic compounds. More extended research will probably show that many other 

 colouring matters are formed either in one manner or the other, which will probably 

 afford us the means of arriving at a rational mode of classifying these bodies, and of 

 distinguishing them as a class from others. E. S. 



COI.T S-FOOT. See TUSSJLAQO. 



