Erodin is being used most successfully in several large works both in England and abroad, and on calf-skins and sheep-skins has proved quite as effective and much safer than dog-dung; the skins coming out clean and free from stains. It has been a good deal used in the experimental tannery of the Yorkshire College, and has proved a satisfactory substitute for puer, but with the present bacterial cultures can only be employed warm, and does not answer used cold like the ordinary pigeon-dung bate. No doubt a suitable bacterial medium and culture can be found for cold bating, which for thicker leathers is often preferable to puering, and experiments in this direction are being undertaken. From the multiplicity of germs present, and the adaptability of the dung infusion as a nutrient medium for any putrefactive organisms which may gain access to it, the bating and puering process is necessarily a dangerous one for the goods, always leading to loss of weight, and, if the process is carried on too long, to the more or less complete destruction of the skins. Loss of weight, however, in greater or lesser degree is inevitable, and indeed necessary where a soft leather is to be produced. If the skins are allowed to lie in the bate or puer liquor, mud, containing organisms, and zooglœa-forms of bacteria settle in the folds, and produce marbled markings, streaks and lines by the destruction of the grain surface (hyaline layer). Black or bluish stains are also often produced, known as bate-stains, and either due to bacterial pigments, or in some cases, to the action of evolved hydrogen sulphide on iron present from salting or other sources. Frequent change of position is therefore necessary, especially when the liquor is active from being used at a high temperature, but it does not seem to be desirable to keep the skins in constant motion, and if puering is done in a paddle, it should only be run at intervals. T. Palmer[105] determined in experiments on pigeon-dung bates that there is considerable loss of nitrogen during the process, and recommended bating in pits from which the air was excluded as much as possible, both as effecting a considerable economy in the dung, and in excluding false ferments, which, he concludes, are mostly aerobic. It is not improbable that the method is advantageous, since it has been shown by Roscoe and Scudder that liquefaction of gelatin only takes place in presence of oxygen, and its partial exclusion would therefore lessen the risk of overbating, and consequent damage and loss of weight. [105] Leather Trade Circular, 22nd Sept., 1891; 1887, p. 667; and Sanford, Journ. Soc. Chem. Ind., 1893, p. 530.