DYEING. 



227 



Adjective 



Colouring 



Matters. 



form, lac might be used with considerable advantage 

 as a dye, and even as a substitute for cochineal. 



117. We shall conclude this division of adjective co- 

 louring matters, with a short account of those proper- 

 ties of Prussian blue which we have not considered un- 

 der the head of Prussic acid: (See CHEMISTRY, p. iiO.) 

 Prussian blue is a triple compound of Prussic acid, an 

 oxide of iron, and potash. The oxide of iron may com- 

 bine in different proportions with the acid ; when it is 

 in excess, the solution is yellowish, but when it is in 

 a proper proportion, it is blue. The excess of the oxide 

 of iron may be taken up by an acid : the muriatic is 

 found to answer the purpose best, and when it is add- 

 ed, the compound 1 ecomes of a beautiful blue. 



118. An alkali digested on Prussian blue or prussi- 

 ate of iron, combines with the Prussic acid, and sepa- 

 rates the iron from it. The addition of an acid which 

 lias a more powerful attraction for the alkali than the 

 Prussic, detaches the ktter from the alkali, and a double 

 play of affinities is brought into action. The acid 

 which is added unites with the alkali, while the Prus- 

 sic acid forms with the oxide of iron the Priir-i.ui 

 blue. At the minimum of oxidation, iron is incapable, 

 in whatever proportion it may be applied, of prod'i- 

 cing a blue with the Prussic acid ; and hence, if the 



::ite of potash be added to a recently prepared 

 sulphate of iron, the precipitate is white, and remains 

 colourless until it absorbs oxygen from the atmosphere. 

 Berthollet supposes that the white prussiate of iron 

 differs from the blue, not because it is less oxygenated, 

 but because the sulphuric acid in the green sulphate 

 of iron adheres more strongly to its basis than when 

 the iron is more highly oxidized ; and as a proof of 

 this, he states, that, by adding the muriatic, the sul- 

 phurous, or phosphorous acids to the white prussiate, 

 it becomes blue, though none of these acids can afford 

 oxygen. 



1 19. Whatever theoretical opinion may be entertain- 

 cd with respect to the cause of the formation of Prus- 

 sian blue, the uncommon beauty and lustre of the co- 

 lour itaelf have recommended it as a dye, and occasion- 

 ed various attempts to fix it equally and permanently 

 upon cloth. No method which has hitherto been pro- 

 posed can be said to have completely answered the 

 purpose ; for whatever care has been taken, the colour 

 is frequently weak and dull, and always uneven. Dr Ban- 

 croft lias indeed descr'.L*d a method, ( which we shall 

 afterwards notice,) by which, he says, he was able to 

 obviate the difficulties hitherto attending the use of 

 this colouring matter. Some pieces of cloth so dyed 

 possessed an intensely full colour, and a lustre great- 

 ly surpassing every tiling before seen in wool, emula- 

 ting even the transparency and brilliancy of the finest 

 sapphire to such a degree, that the eye which has once 

 seen the Prussian blue so communicated, disdains af- 

 terwards to fix Itself upon the common indigo blue. 



120. The Prussic acid produces coloured compounds 

 with other metallic oxides besides those of iron. The 

 most remarkable, and perhaps the most important of 

 these, is the red prussiate of copper. This colour was 

 obtained by Dr Bancroft from the different solutions of 

 copper in the sulphuric, the nitric, the muriatic, and 

 the acetous acids, and particularly well by that in the 

 volatile alkali. The prussiate of copper is distinguish- 

 ed from the prussiate of iron by its extraordinary per- 

 manency ; for though all the alkalies readily decom- 

 pose the Prussian blue, they have no effect upon the 



Adjectire 



Colouring 



Matters. 



combination of the Prussic acid with copper. Such, 

 in;leed, is its durability, that neither the acids * 

 nor washings with soap, however numerous, nor ex- 

 posure to the weather for the longest space of time, 

 seem capable, in the least degree, of diminishing either 

 its body or its lustre. Dr Bancroft says, that this colour 

 might prove highly useful, by way of topical applica- 

 tion upon cottons, and perhaps in dyeing cotton yarn 

 for stripes of muslins, borders of handkerchiefs, &c. 



II. Of Vegetable Adjective Colours. 



121. One of the most important colouring matters Madder, 

 belonging to this class is madder. This substance is 



the root of a plant, which Linnaeus divides into two 

 species, the rnbia ti/ic.'ornm foliis senis, and the rubia 

 pen grind fuliia quaternis. The madder, in a state of 

 preparation for dyeing, is distinguished into different 

 sorts : that obtained from the principal roots is called 

 gran? madder, and that produced from the stalks, 

 which, by being buried in the earth, are transformed 

 into roots, iwn irmpc. When the madder roots are 

 gathered, the latter are separated to form the non grape, 

 or with the fibres of the roots, which do not ex- 

 ci'fil a certain degree of thickness, and also such as are 

 too thick, and contain much woody matter. The best 

 roots are about the thickness of a goose quill : they are 

 semitransparent, of a reddish colour, with a strong 

 smell, and a smooth bark. 



122. If the roots of madder be examined with a Appearance 

 microscope, the interior part is observed to contain of madder 

 a considerable proportion of specks of a bright red co- wuh a '" 

 lour ; but the ligneous part which surrounds these, as " 



well as the bark, abounds with a brownish yellow co- 

 louring matter, which tends greatly to degrade the red 

 that madder would otherwise afford. The pernicious 

 effects of this yellow matter, when madder is used as 

 a dye, may in some degree be avoided, by extracting 

 the colouring matter in water which is but moderately 

 warm, the yellow matter being but imperfectly dissol- 

 ved so long as the heat is below the boiling point. The 

 outer bark and ligneous parts are also more easily pound- 

 ed than the parachymatous parts, to which the red co- 

 lour is more immediately attached, and, on this account, 

 a separation may, to a certain extent, be effected by 

 mechanical mear.s. This separation established among 

 the French dyers the distinction of madder into robee, 

 mis-robie, and courle. After the first operation of the 

 mill, the madder is passed through a sieve with a cover 

 fitted to it, and, by this means, what is called the short 

 madder, which is used for inferior colours, is obtain- 

 ed ; the remainder being again ground and sifted, 

 produces the mis-robee ; and a third operation affords 

 the robee or finest madder. 



12:;. Water of the ordinary temperature of the at- Solvents of 

 mosphere may be made to dissolve almost all the red madder. 

 colouring matter from madder ; but for this purpose it 

 must be employed in large quantity ; and the colour is 

 more beautiful when the solution is obtained by cold 

 than by hot water. Alkalies increase tlie solving power 

 of water, particularly with respect to the yellow mat- 

 ter. The residuum is very inconsiderable, so tliat mad- 

 der appears to consist almost wholly of colouring mat- 

 ter. The latter portions of the extract, which seems to 

 be chiefly yielded by the ligneous and cortical 'parts, 

 is fawn-coloured. 



124. The red colouring matter of madder is soluble 

 in alcohol, and, on evaporation, a deep red residuum is 



Except the oxymurUtic, which decomposes it. 



