The adsorption law indicates that however much colouring matter is removed from the volume concentration (continuous phase) there must always be some left. After all that the decolorization processes can do, there still remains much colour that can only be removed by a chemical action of the ordinary sense. The amount of colouring matter of this kind is not large, but it is a deep red-brown, and when the gelatine sol has been evaporated and dried out the final product, if untreated, possesses this typical colour, and is known as glue. If, before gelation, a chemical bleaching action is applied to destroy this pigment, the product may be then dried out in a nearly colourless condition and is known as gelatine. Gelatine, therefore, is simply bleached glue. Many other definitions have been given, and many elaborate distinctions drawn, but the fact of bleaching is the essential difference. In these days when gelatine is so valuable, the higher-grade products are nearly always bleached, and the term "glue" is consequently more often applied to a lower-grade product, and is sometimes used in a sense implying this fact. If it be desired to manufacture gelatine, it is fairly obvious that the task is lightened by observing the axiom that prevention is better than cure. If steps are taken to prevent the presence or development of such colouring matter, a great advantage is attained, for not only is the problem of bleaching easier, but also quicker and less expensive in chemicals. The nature of the colouring matters is but imperfectly investigated, but in the case of skin gelatine the pigment of the hair roots and epidermis is doubtless one factor. A long liming is said to assist in its destruction, possibly because this completes the loosening of epithelial structures and possibly because the alkali causes some hydrolysis of the pigment. In both skin and bone gelatine sols, however, there is a considerable tendency to develop the brown colouring matter typical of glue. This tendency is enhanced by an increase in temperature and also by the presence of acid or alkali. These facts seem to indicate that its development is associated with a partial hydrolysis of the gelatine in some direction. Rideal says this colouring matter is allied to caramel. In harmony with this is the experience that its development is greatest in products which have been "burnt," _i.e._ subjected to unusually high temperature. The practical maxims which arise from these considerations are fairly obvious and widely known, viz. to conduct the extraction and evaporation at as low a temperature as possible and in as neutral a condition as practicable. The temperature is particularly important during evaporation (see Section VI.).