The chief principle of extraction is so to arrange the process that both the material and the extracted liquor are maintained at high temperatures for the shortest possible time. As we have observed, gelatine is readily hydrolyzed by hot water, and as hot water is needed for its extraction or solution, care must be taken to remove the solution as soon as possible from the source of heat. In practice this can only be done somewhat imperfectly, as it is necessary to obtain a gelatine sol of several per cent. strength before removing it from the extraction vessel. The stronger this sol is made before removal, the less the time, trouble and expense is incurred in evaporation subsequently, but the more is the exposure to heat with consequent weakening of the gelatine. Hence in practice it is necessary to compromise. The matter is complicated further by the necessity of obtaining a clear sol, for which it is desirable that the sol obtained in extraction should not be too concentrated, as impurities settle and filter much more readily from weaker and less viscous sols. It will be understood, therefore, that whatever material is being extracted, the most favoured procedure is to extract in fractions. The first fraction, which is least exposed to hydrolytic decomposition, produces the highest quality products, and the subsequent fractions (nearly always two more, and sometimes several) yield products which gradually become of inferior quality owing to the number of times the raw material has been re-heated. Within limits, the precise temperature of extraction does not have the importance one would expect. Lambert suggests the temperature of 185° F. as suitable for both skin and bone gelatine, and most manufacturers would, on the whole, endorse this. If, however, a higher temperature be preferred, the hydrolytic action is increased in intensity but decreased in its time of operation, whilst if a lower temperature be adopted the decomposition is retarded in speed, but is increased in totality because of the longer time needed to obtain a suitable strength of liquor. Thus, with care, much the same result is obtained by extraction at near boiling-point for a short time as by extraction at 160° F. for a long time. The higher temperatures have the definite advantage of speed, whilst the lower temperatures have the advantage that one may choose to be satisfied with a weaker extract, and so gain a little in the strength of the gel, by throwing more work on the evaporator. One other point should, however, be borne in mind in this connection, viz. that a gelatine sol kept at temperatures above 185° F. begins to deteriorate in colour. Whilst, therefore, much depends upon the precise class of material, it is broadly true to say that the higher temperatures are advantageous for glue, whilst the lower temperatures are preferable for the highest quality gelatine.