_Pepsin_ is the active principle of the secretion of the glands of the stomach, and large quantities are prepared for medical use as an aid to digestion from the stomachs of pigs. Pepsin only acts in slightly acid solution, and, though fresh bate liquor is slightly acid to litmus, it speedily becomes alkaline from the lime of the skins and the ammonia present, so that the action of pepsin in a bate can only be a very limited one. Wood[102] compared the action of a 1 per cent. solution of pepsin, acidified with 0·2 per cent. of hydrochloric acid, with that of a dogs’ dung puer liquor, both at the temperature of 40° C. At the end of one hour the skin in the pepsin-solution was considerably fallen, but that in the puer-solution was almost dissolved. Since the solution here employed was much stronger than is likely to occur in practice, and the conditions much more favourable to its action, it may be assumed that the practical effect of traces of pepsin in the bate may be neglected. [102] Journ. Soc. Chem. Ind., 1894, p. 220. _Trypsin_ or _pancreatin_[103] if present, is more likely to have an effect, since it is active in neutral and in alkaline solutions. It is the product of the pancreas, and is largely concerned in intestinal digestion. Chemically it much resembles pepsin, but is more resistant to heat, retaining its power of digestion after heating to a temperature of 160° C. in a dry condition. Its warmed solution dissolves fibrin almost instantly, and in large quantity, and peptonises gelatin and hide-fibre, so as to render them soluble in water. Wood found that a 1 per cent. solution of pancreatin acted far more rapidly than a solution of pepsin of equal strength. At 40° C. in neutral solution, the skin fell rapidly, and the action continued even in the cold. In 15 hours the liquid was swarming with minute bacteria. At the suggestion of the Author, the experiment was therefore repeated, with the addition of 15 per cent. of chloroform, which prevented the development of bacteria, while it did not stop the action of the pancreatin. The skin fell as before, but in neither case had it the peculiar touch of puered skin, nor were the characteristics of the leather produced from it the same. We may therefore conclude that, though trypsin may contribute to the action of the bate or puer, it can only do so in a minor degree, and that the principal effect of the bate or puer is due to other causes. It is certain, however, that fresh bird-dung, and probably that of all animals, contains ferments capable of liquefying gelatin. An instance of this is found in the observation, common in glue manufacture, that if the dropping of a sparrow falls on a cooler full of solidified gelatine size, it will liquefy a track quite down to the bottom of the cooler. Trypsin, or at least the secretion of the pancreas, as well as the gall from the liver, have great power of wetting and emulsifying fats, and