_Bating and puering_, though differing practically in many ways, are identical in theory, and most of what follows applies to both of them. The action is much more complex than that of the drench, involving both chemical reactions and those of organised and unorganised ferments, and it is a matter of no little difficulty to say what proportion of the observed effect should be ascribed to each of these agencies. Formerly, the principal effect was attributed to organic salts of ammonia and its homologues, and to amido-acids which combine with lime. Phosphoric acid is also present, and if any exists in the form of soluble salts, it will combine with lime, and render it insoluble and inactive. It is probable, however, that most if not all the phosphoric acid is already in the form of tricalcium phosphate, and therefore without effect. It is now, however, recognised that the effects of these chemicals are of no importance as compared with the products of bacterial action, and the researches of J. T. Wood have cleared up much that was until recently quite inexplicable.[101] [101] Journ. Soc. Chem. Ind., 1894, p. 218; 1895, p. 449; 1898, pp. 856, 1010; 1899, pp. 117, 990. Much effect has been ascribed to the digestive ferments, such as pepsin and trypsin, which are present in fresh dung. It is known that the animal organism secretes these in considerable excess of its requirements, but it is doubtful whether any exist undecomposed, even in fresh dung; though they are apparently more resistant to putrefaction and decomposition than would _a priori_ have been expected of such complex organic compounds, and there is therefore a possibility of their existence in the dung, even as it comes to be used in the tannery. Both pepsin and trypsin are enzymes (see p. 16), and belong to the great class of albuminoids. They are soluble in water, but insoluble in alcohol, and hence are precipitated by the addition of the latter to their solution, but are not altered by it, and regain their activity on solution in water. By heat they are coagulated and decomposed, and their activity permanently destroyed.