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 DYE 1 N G In dyeing wool it is the fibre itself which acts as the mordant. In the case of the Acid Colours the explanation is similar. In many of these the free colour-acid has quite a different colour (purple) from that of the alkalisalt (yellow), and yet on dyeing wool or silk with the free colour-acid, the fibre exhibits the colour of the alkali-salt and not of the colour-acid. In this case the fibre evidently plays the part of a base. Another fact in favour of the view that the union between fibre and colouring matter is of a chemical nature, is that by altering the chemical constitution of the fibre its dyeing properties are also altered; oxycellulose and nitrocellulose, for example, have a greater attraction for Basic Colours than cellulose. Such facts and considerations as these have helped to establish the view that in the case of dyeing animal fibres with many colouring matters the operation is a chemical process, and not merely a mechanical absorption of the dyestuff. A similar explanation does not suffice, however, in the case of dyeing cotton with the Direct Colours; also in some cases of dyeing wool and silk—e.g., with Orchil. These are attracted by cotton from their, solutions as alkali salts, apparently without decomposition. The affinity existing between the fibre and colouring matter is somewhat feeble, for the latter can readily be removed from the dyed fibre by merely boiling with water. The depth of colour obtained in dyeing varies with the concentration of the colour solution, or with the amount of some neutral salt—e.g., sodium chloride, added as an assistant to the dyebath; moreover, the dyebath is never completely exhausted. The colouring matter is submitted to the action of two forces, the solvent power of the water and the affinity of the fibre, and divides itself between the fibre and the water. After dyeing for some time, a state of equilibrium is attained in which the colouring matter is divided between the fibre and the water in a given ratio, and prolonged dye^g does not intensify the dyed colour. An interesting fact bearing on this question is that some basic colouring matters are capable of dyeing—amorphous sulphur, gelatin, silicic acid, infusorial earth, &c. These and other similar facts have led to the adoption by various investigators of the view that in some cases the fibres exert a purely physical attraction towards colouring matters, and that the latter are held in an unchanged state by the fibre. The phenomenon is regarded as one of purely mechanical surface-attraction, very similar to that exercised by animal charcoal when employed in decolorizing a solution of some colouring matter. Some consider such direct dyeing as mere diffusion of the colouring matter into the fibre, and others that the colouring matter is in a state of “ solid solution ” in the fibre, similar to the solution of. a metallic oxide in coloured glass. According to this latter view, the cause of the dyeing of textile fibres is similar to the attraction or solvent action exerted by ether when it withdraws colouring matter from an aqueous solution by agitation. In the case of colours which are dyed on mordants, the question is merely transferred to the nature of the attraction which exists between the fibre and the mordant, for it has been conclusively established that the union between the colouring matter and the mordant is essentially chemical in character. From our present knowledge it will be seen that we are unable to give a final answer to the question of whether the dyeing process is to be regarded as a chemical or a mechanical process. There are arguments and facts which favour both views ; but in the case of wool and silk dyeing, the prevailing opinion in most cases is in favour of the chemical theory, whereas in cotton-dyeing the mechanical theory is widely accepted. Probably no single theory can explain satisfactorily the fundamental cause of attraction

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in all cases of dyeing, and further investigation is needed to answer fully this very difficult and abstruse question. The poisonous nature or otherwise of the coal-tar dyes has been frequently discussed, and the popular opinion, no doubt dating from the time when magenta and its derivatives were contaminated with elusion. arsenic, seems to be that they are for the most part really poisonous, and ought to be avoided for colouring materials worn next the skin, for articles of food, &c. It is satisfactory to know that most of the colours are not poisonous, but some few are—namely, Picric acid, Victoria Orange, Aurantia, Coralline, Metanil Yellow, Orange II., and Safranine. Many coal-tar colours have, indeed, been recommended as antiseptics or as medicinal remedies, e.g., Methyl Violet, Auramine, and Methylene Blue, because of their special physiological action. In histology and bacteriology many coal-tar colours have rendered excellent service in staining microscopic preparations, and have enabled the investigator to detect differences of structure, Ac., previously unsuspected. In photography many of the more fugitive colouring matters, e.g., Cyanine, Eosine, Quinoline Bed, Ac., are employed in the manufacture of ortho-chromatic plates, by means of which the colours of natural objects can be photographed in the same degrees, of light and shade as they appear to the eye—blue, for example, appearing a darker grey, yellow a lighter grey, in the printed photograph. Since the year 1856, in which the first coal-tar colour, mauve, was discovered, the art of dyeing has made enormous advances, mainly in consequence of the continued introduction of coal-tar colours having the most varied properties -and suitable for nearly every requirement. The old idea that the vegetable dyestuffs are superior in fastness to light is gradually being given up, and, if one may judge from the past, it seems evident that in the future there will come a time when all our dyestuffs will be prepared by artificial means. Those who desire further information on the subject of dyeing may consult the following works of reference :— Authorities.—Bancroft. Philosophy of Permanent Colours. 2 vols. London, 1813.—Berthollet-Ure. Elements of the Art of Dyeing. 2 vols. London, 1824.—Chevreul. Eecherches chimiques sur la teinture. Paris, 1835-1861.—J. Persoz. TraiU theorique et pratique de Vimpression des tissus. Paris, 1846.— O’Neill. The Chemistry of Calico-Printing, Dyeing, and Bleaching. Manchester, 1860. Dictionary of Calico-Printing and Dyeing. London, 1862.—Schutzenberger. Traite des matteres colorantes. 2 vols. Paris, 1867.—Bolley. Die Spinnfasern und die im Pflanzen- und Thier-kdrper rorkornmenden Farbstoffe. 1867. —Crookes. A Practical Handbook of Dyeing and Calico-Printing. London, 1874.—Napier. A Manual of Dyeing and Dyeing Pleceipts. London, 1874.—Bolley, Kopp. Traite des matteres colorantes artificielles. 1874.—Jarmain. Cantor Lectures: Course of Six Lectures on Wool-Dyeing. 1876.—O’Neill. Textile Colourist. 4 vols. Manchester, 1876.—Calvert. Dyeing and Calico-Printing. Manchester, 1876.—Moyret. Traite de la teinture des soles. Lyon, 1877.—O’Neill. The Practice and Principles of Calico-Printing, Bleaching, and Dyeing. Manchester, 1878.—Girardin. Matteres textiles et matteres tinctoriales. Paris, 1880.—Smith. The English Dyer. Manchester, 1882.—Crookes. Dyeing and Tissue-Printing. London, 1882.—Hummel. The Dyeing of Textile Fabrics. London, 1885.—Wolff. Die Beizen. Wien, 1885.—Schultz. Die Chemie des Steinkohlentheers. 1886. —Parnell. Life and Labours of John Mercer. London, 1886. —Sansone. The Printing of Cotton Fabrics. 1887.—Julius. Die kunstlichen organischen Farbstoffe. 1887.—Kertesz. Die Anilinfarbstoffe. Braunschweig, 1888.—Schultz and Julius. Tabellarische Uebersicht der kunstlichen organischen Farbstoffe. Berlin, 1888.—Sansone. Dyeing. Manchester, 1888.—Witt. Chemische Technologic der Gespinnstfasern. Braunschweig, 1888. —Paul. The Practical Ostrich-Feather Dyer. 1888.—Benedikt and Knecht. The Chemistry of the Coal-Tar Colours. London, 1889.—DepArre. The Finishing of White, Dyed, and Printed Cotton Goods. Manchester,. 1889.—Ganswindt. Handbuch der Fdrberei. Weimar, 1889.—Mohlau. Organische Farbstoffe welche in der Textilindustrie Verwendung finden. Dresden, 1890. —DepArre. Traiti de la Teinture et de VImpression. 4 vols.