The effect of various factors (oxygen, temperature and humidity, atmospheric contaminants and concentration of dye) on the fading of dyes is discussed. The phototropic behaviour of azo and thioindigoid dyes is described. The fading products resulting from the prolonged irradiation of dyes are discussed, as is the relation between the light‐fastness properties of a dye and its chemical constitution.

The effect of sensitising dyes on the photochemical degradation of textile fibres is discussed. The degradation is dependent upon the presence of oxygen, and is accelerated by water vapour and rise in temperature. The study of simple model photochemical systems intended to simulate the dyed fibre system indicates that there are significant differences between the two systems. The sensitising effect of dyes in the textile systems seems to be due to the formation of excited singlet oxygen and hydrogen peroxide. The latter is not formed in the absence of water vapour.

The degradation of undyed yarns of viscose rayon, cellulose (secondary) acetate rayon, and nylon 6.6, both free from and containing titanium dioxide (anatase), has been studied at different temperatures. The degradation in moist air is particularly marked at high temperatures. A possible reaction mechanism for the sensitizing effect of titanium dioxide is discussed.

The photochemical degradation of textile materials is discussed under two main heads— I. PHOTOLYTIC DEGRADATION WITH Short‐WAVE ULTRA‐VIOLET RADIATION‐ The degradation of cotton and other textile fibres with radiation of wavelength 2537 A. is not dependent upon the presence of oxygen in the surrounding atmosphere. In some cases (e. g. cotton and visoose rayon) the loss in strength in carbon dioxide or nitrogen is nearly as great as in oxygen; in other cases (e. g. nylon) there is greater degradation in oxygen, but still substantial degradation in carbon dioxide. The effect of vat dyes on the rate of degradation of cotton is discussed. All the vat dyes examined were found to desensitise the photolytio degradation of cellulose. Dyes such as Caledon Yellow 5G and Cibanone Yellow R decreased the degradation of cotton in atmospheric air, dry oxygen, and inert gases. II. PHOTOCHEMICAL OXIDATION WITH NEAR ULTRA‐VIOLET AND VISIBLE LIGHT‐ The photochemical degradation of cotton and other textile fibres by light transmitted through glass is increased by various dyes and pigments. The sensitising dyes may belong to the following classes‐ vat, acetate, direct cotton, basic, acid, and sulphur; the pigments include zinc oxide, zinc sulphide, and titanium dioxide. The degradation of the various textile fibres is dependent upon the presence of oxygen in the surrounding atmosphere and is negligible in its absence. The mechanism of the degradation is discussed, and the evidence for the suggestion that the degradation is due to oxidation of the fibre by activated oxygen and hydrogen peroxide is examined. Evidence is described which favours the belief that activated oxygen is a metastable form of molecular oxygen.

The use of rigid solvents and flash photolysis in the study of photochemistry is described, including applications of the techniques and the problems involved. Fundamental photochemical processes are discussed in some detail, with particular reference to quinone compounds. The significance of the lowest excited state in determining the sensitivity to light of a molecule is emphasised and the relation between colour and sensitising ability is discussed on this basis.