THEORIES OF DYEING. The question as to whether dyeing true chemical combina tion of the fibres with the dyestuffs or not has not yet been definitely answered. Sonic in vestigators believe that dyed fibres are merely mechanical others hold that dyed fibres are true chemical compounds: finally, still others hold that they constitute what are now termed 'physical mixtures,' or 'solid solutions.' Those who accept the chemical theory argue that if the dyestuff did not combine chemically with the fibre, then there would be no reason why chemically neutral colored substances should not act as yet true dyes are generally either acid or basic, while the fibres themselves may he either basic or acid, or else may include both basic and acid constituents. Another argu ment in support of the chemical theory is found in the fact that some substances, them selves colorless, are capable of imparting color in fabrics when directly applied to them: the view being that colored substanees must he transformed chemically in order to produce color. The principal arguments in favor of the 'physical mixture' theory. advanced by O.
Witt in 1890, are as follows: (I ) Solid fuchsin is greet). with a metallic lustre: solutions of fuchsin are colored red: and fabrics dyed with fuelisin are likewise colored red. (2) Solid rhodamine shows no trace of fluorescence; its solutions are fluorescent; and silk dyed with rhodamine likewise shows distinct Iltioreseence. The fact that the same dyestuff may impart dif ferent colors to different fabrics is compared with the tact that iodine gives different colors in different solvents, and yet there is no reason for assuming that iodine forms chemical com pounds with its ordinary solvents. Finally, in accordance with Witt's theory, the function of mordants is explained by the assumptifm that solutions of these substances in fibres are much better solvents for the dyestuffs than are the pure fibres. However, while interesting because capable of development and quantitative verifica tion, this physico-chemical theory is yet far front being definitely established or generally accepted by chemists.