INVISIBLE ultra‐violet rays possess the property of making many substances, whether solids or liquids, fluoresce in darkness or half light. Substances so irradiated re‐emit light in distinct colours which are quite unlike their normal appearance. Physical variations, or chemical differences, too minute for easy detection by other means, determine the different fluorescent colours. For example, raw olive oil will fluoresce yellow, refined olive oil bluishgreen. Fatty oils which have been solvent‐extracted generally have a violet fluorescence, whereas the normal pressed oils give a characteristic yellow reaction. By comparison with known standards, admixtures can readily be graded, and a preliminary treatment with an adsorption agent such as fuller's earth or activated carbon facilitates detection by removing from the oil, impurities which interfere with the fluorescent reactions.
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