COLOURING MATTERS 001 



weak acid, such as acetic acid. It is remarkable that of all bases none show so much 

 affinity for colouring matters as alumina, peroxide of iron, and peroxide of tin, bodies 

 which occupy an intermediate position between acids and bases. If a solution of any 

 colouring matter be agitated with a sufficient quantity of the hydrates of any of these 

 bases, the solution becomes decolourised, the whole of the colouring matter combining 

 with the base and forming a coloured compound. It is accordingly these bases that 

 are chiefly employed in dyeing, for the purpose of fixing colouring matters on par- 

 ticular portions of the fabric to be dyed. When used for this purpose they are called 

 mordants. Their compounds with colouring matters are denominated lakes, and are 

 employed as pigments by painters. The colours of the compounds usually differ, 

 either in kind or degree, from those of the colouring matters themselves. Red 

 colouring matters often form blue compounds, yellow ones sometimes give red or 

 purple compounds. The compounds with peroxide of iron are usually distinguished 

 by the intensity of their colour. When a colouring matter gives with alumina and 

 oxide of tin red compounds, its compound with peroxide of iron is usually purple 

 or black ; and when the former are yellow, the latter is commonly olive or brown. 

 Almost all the compounds of colouring matters with bases are decomposed by strong 

 acids, such as sulphuric, muriatic, nitric, oxalic, and tartaric acids, and even acetic 

 acid is not without effect on some of these compounds. The compounds with earths 

 and metallic oxides are also decomposed, sometimes by alkalis. A solution of soap 

 is sufficient to produce this effect in many cases, and dyes are therefore often tested 

 by means of a solution of soap in order to ascertain the degree of permanence which 

 they possess. 



No property is so characteristic of colouring matters, as a class, as their behaviour 

 towards bodies of a porous nature, such as charcoal. If a watery solution of a colouring 

 matter be agitated with charcoal, animal charcoal being best adapted for the purpose, 

 the colouring matter is generally entirely removed from the solution and absorbed by 

 the charcoal. The combination which takes place under these circumstances is pro- 

 bably not due to any chemical affinity, but is rather an effect, of the so-called attraction 

 of surface, which we often see exerted by bodies of a porous nature, such as charcoal 

 and spongy platinum, and which enable the latter to absorb such large quantities of 

 gases of various kinds. That the compound is indeed more of a physical than a 

 chemical nature seems to be proved by the circumstances that sometimes the colouring 

 matter is separated from its combination with the charcoal by means of boiling alcohol, 

 an agent which can hardly be supposed to exert a stronger chemical affinity than water. 

 It is this property of colouring matters which is made use of by chemists to decolourise 

 solutions, and by sugar manufacturers to purify their sugar. The attraction manifested 

 by colouring matters for animal or vegetable fibre is probably also a phenomenon of 

 the same nature, caused by the porous condition of the latter, and the powerful affinity 

 of the so-called mordants for colouring matters, may, perhaps, be in part accounted 

 for by their state of mechanical division being different from that of other bases. 

 Colouring matters, however, vary much from one another in their behaviour towards 

 animal or vegetable fibre. Some, such as indigo-blue, and the colouring matters of 

 safflower and turmeric, are capable of combining directly with the latter, and imparting 

 to them colours of great intensity. Others are only slightly attracted by them and 

 consequently impart only feeble tints ; they therefore require, when they are employed 

 in the arts for the purpose of dyeing, the interposition of an earthy or metallic base. 

 To the first class Bancroft applied the term substantive colouring matters, to the second 

 that of adjective colouring matters. 



One of the most interesting questions connected with the history of colouring matters 

 is that in regard to the original state in which these substances exist in the animal and 

 vegetable organisms from which they are derived. It has been known for a long time 

 that many dye-stuffs, such as indigo and archil, do not exist ready formed in the plants 

 from which they are obtained, and that a long and often difficult process of preparation 

 is required 'in order to eliminate them. The plants which yield indigo exhibit, while 

 they are growing, no trace of blue colour. The colouring matter only makes its 

 appearance after the juice of the plant has undergone a process of fermentation. The 

 lichens employed in the preparation of archil and litmus are colourless, or at most 

 light brown, but by steeping them in liquids containing ammonia and lime a colouring 

 matter of an intense red is gradually generated, which remains dissolved in the alkaline 

 liquid. Other phenomena of a similar nature might be mentioned, as for instance 

 the formation of the so-called Tyrian purple from the juice of a shell -fish, and new 

 ones are from time to time being discovered. In order to explain these phenomena 

 various hypotheses have been resorted to. It was supposed, for instance, that the 

 indigofera contained white or reduced indigo, and hence were devoid of colour, and 

 that the process of preparing indigo-blue consisted simply in oxidising the white indigo, 

 which was for this reason denominated indigogciw, by some chemists. The same 



