Isotherms for adsorption of organic solutes are divided into four main classes, according to the nature of slope of the initial portion of the curve, and thereafter into sub-groups.The main classes are: (i) S Curves, indicative of vertical orientation of adsorbed molecules a t the surface. (ii) L Curves, the normal or " Langmuir " isotherms, usually indicative of molecules adsorbed flat on the surface, or, sometimes, of vertically oriented adsorbed ions with particularly strong intermolecular attraction. (iii) H Curves ( ' I high affinity ") (commencing a t a positive value on the " concentration in solid " axis) , often given by solutes adsorbed as ionic micelles, and by high-affinity ions exchanging with lowaffinity ions. (iv) C Curves (" constant partition "), linear curves, given by solutes which penetrate into the solid more readily than does the solvent.The sub-groups of these classes are arranged according to the shape of the parts of the curves farther from the origin, and the significance of plateaux and changes of slope are described. Thus, if the adsorbed solute molecules in the monolayer are so oriented that the new surface they present to the solution has low attraction for more solute molecules, the curve has a long plateau ; if they are oriented so that the new surface has high attraction for more solute, the curve rises steadily and has no plateau.The choice of solutes for reliable measurement of specific surface areas is very restricted. It is suggested that p-nitrophenol may be one of the best compounds for this purpose.The significance of their features is discussed.* Part I, J., 1954, 4360; previous papers in this series were divided into two series, for inorganic and organic substrates respectively. Parts I-V, organic series, are considered as Parts VI-X in the present series. Thus, Part X is J . SOC. Dyers Colourists, 1958, 74, 846 (ref. 12a).

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An outline of the causes and nature of fading of organic colouring matters is given. Atmospheric fading, in absence of light, is produced by nitrogen oxides in urban air, especially with blue and violet disperse dyes on some hydrophobic fibres, but sulphur dioxide and ozone can also cause fading. Light fading occurs with all dyes ; some of the factors which affect it are discussed, including the physical state of the dye, the chemical and the physical nature of the fibre, the temperature and humidity of the atmosphere, and the quality and intensity of the illumination. The hue of many soluble dyes becomes redder on fading. The types of fading rate are described ; the fading of many dyes is rapid at first, but later it slows to a constant rate, so that a dye, which by normal testing methods has low fastness, may yet withstand prolonged exposure.