117. We shall conclude this division of adjective co louring matters, with a short account of those properties of Prussian blue which ire have not considered under the head of Prussic acid : (See CHEMISTRY, p. 60.) 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 iu excess, the solution is yellowish, but when it is in a pro per proportion, it is blue. The excess of the oxide of iron may be taken up by an acid : the niuriatic is found to answer the purpose best, and when it is added, the compound becomes of a beautiful blue.
118. An alkali digested on Prussian blue or prussiatc of iron, combines with the Prussic acid, and separates the iron from it. The addition of an acid which has a more powerful attraction for the alkali than the Prussic, detaches the latter from the alkali, and a double play of affinities is brought into action. The acid which is add ed unites with the alkali, while the Prussic acid forms with the oxide of iron the Prussian blue. At thc mini mum of oxidation, iron is incapable, in whatever propor tion It may be applied, of producing a blue with the Prus sic acid ; and hence, if the prussiate 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 sulphurous, or phosphorous acids to the white prus siate, it becomes blue, though none of these acids can af ford oxygen.
119. Whatever theoretical opinion may be entertain ed with respect to the cause of the formation of Prussian blue, the uncommon beauty ,and lustre of the colour it self have recommended it as a dye, and occasioned va rious attempts to fix it equally and permanently upon cloth. No method which has hitherto been proposed 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 Bancroft lias in deed described a method, (which we shall afterwards no tice,) by which, he says, he was able to obviate the diffi culties hitherto attending the use of this colouring mat ter. Sonic pieces of cloth so dyed possessed an intense
ly full colour, and a lustre greatly surpassing every thing before seen in wool, emulating even the transparency and brilliancy of the finest sapphire to such a that the cye which has once seen the Prussian blue so communi cated, disdains afterwards 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 lrom the different solutions of copper in the sulphuric, the nitric, the muriatic, and the acetous acids, and particularly well by that in the vola tile alkali. The prussiate of copper is distinguished from the prussiate of iron by its extraordinary perma nency ; for though all the alkalies readily decompose the Prussian blue, they have no effect upon the combination of the Prussic acid with copper. Such, indeed, is its durability, that neither the acids nor washings with soap, however numerous, nor exposure 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 application upon cottons, and perhaps in dyeing cotton yarn for stripes of muslins, borders of handkerchiefs, Stc.
II. Of Vegetable ?djective Colours.
121. One of the most important colouring matters belonging to this class is madder. This substance is the root of a plant, which Linnaeus divides into two species, the rubia tinctorum foils senis, and the rubla peregrina foliis quaternis. The madder, in a state of preparation for dyeing, is distinguished into different sorts: that ob tained from the principal roots is called grape madder, and that produced from the stalks, which, by being bu ried in the earth, are transformed into roots, non grape. When the madder roots are gathered. the latter are se parated to form the non grape, together with the fibres of the roots, which do not exceed a certain degree of thickness, and also such as are too thick, and contain much woody matter. The best roots are about the thick ness of a goose quill : they are semitransparent, of a reddish colour, with a strong smell, and a smooth bark.