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CHAPTER III.

COLOURING MATTERS OF ORGANIC ORIGIN ARTIFICIALLY OBTAINED,

HE discoveries of scientific chemists made during the last thirty yearshave brought about the use, on a large scale, of substances suitable fordyes obtained by purely chemical reactions ; in practice on the large scalethere have been, however, a few instances of a somewhat similar nature,in so far as regards the obtaining of dyes and pigments from perfectly colourlessmaterials. We may mention, as an example, the conversion of lichens under thejoint a&ion of air and putrid urine into a beautiful reddish violet matter longbefore scientific chemists proved that the colourable acids of these plants arechanged first into orcine and then into coloured orceine under the influenceof oxygen and ammonia. Among the colouring matters of organic originartificially obtained, we distinguish the following groups:—(i). Colours derivedfrom uric acid ; (2). Colours derived from aniline and the oily non-oxygenatedalkalies homologous, isologous, and analogous to aniline; (3). Colours obtainedfrom phenol and substances analogous therewith; (4). Naphthaline colours;(5). Colours derived from vegetable alkaloids; (6). Some colouring matters ofvarious origin. All these substances agree in this point, that carbon is anessential element of their constitution. Some of them contain beside,hydrogen and oxygen; others, again, nitrogen, in addition to the hydrogen,while the oxygen may or may not be present. As a consequence of theirconstitution these substances are unstable under the influence of high tem-peratures, and combustible, i.e. t converted into water, carbonic acid, andnitrogen at a red heat with access of air; some of these substances are weakacids, others, again, strong alkalies comparable with ammonia, and not a feware quite neutral.

Colours Derived from Uric Acid—Purpurates, Murexide.

Purpuric acid, the ammoniacal salt of which is termed murexide, is one ofthe most interesting groups of a series of produ&s of decomposition.*Murexide was discovered by Scheele in the year 1776, as a produft of thesplitting up of uric acid, C IO H 4 N 4 06, a substance which has not beenhitherto artificially prepared. This material, when adled upon by nitric acidor other oxidising agents, is converted into urea, C 2 H 4 N 2 0 2 , a neutral sub-stance, and into alloxan—

(C IO H 4 N 4 06) + (N0 5 .HO) = C 2 H 4 N 2 0 2 +C8H 4 N 2 0 IO +N03.

'-.-' v -»-' '-,-'

Uric acid. Urea. Alloxan.

* It is better to apply here the term splitting up (Spaltung), as the Germans call it.