For some descriptions of hides, and notably for India kips, putrid soaks were formerly much employed, the putrefactive action softening and rendering soluble the hardened tissue. In India the native tanners soften their hides in very few hours by plunging them in putrid pools, into which every description of tannery refuse is allowed to run. Putrefactive processes, however, are always dangerous, as the action, through changes of temperature, or variation in the previous state of the liquor, is apt to be irregular, and either to attack one portion of the hide before another, or to proceed faster than was expected. Hides are also frequently more or less damaged by putrefaction and heating during the process of cure, and these damages are accentuated in a putrid soak. Hence hides in the soaks require constant and careful watching, and the goods must be withdrawn as soon as they are thoroughly softened, for the putrefaction is constantly destroying as well as softening the hides. It is possible that putrefactive softening is less injurious to kips, and such goods as are intended for upper-leather, than to those for sole purposes, as it is generally considered necessary in the former case that a good deal of the albumen and interfibrillary matter be removed, and that the fibre be well divided into its constituent fibrils for the sake of softness and pliability; and thus the putrid soak, if acting rightly, accomplishes part of the work which would afterwards have to be done by the lime and the bate, as the actual fibre of the hide seems less readily putrescible than the softer cementing substance. Putrefaction is caused, as we have seen, by a great variety of living organisms, each of which has its own special products and modes of action. It is quite possible that, if we knew what precise form of putrefaction was most advantageous, we might by appropriate conditions be able to encourage it, to the exclusion of others, and obtain better results than at present. Putrid soaks (in the old sense) are, however, disused in the present day by all enlightened tanners, as it is recognised that the risks outbalance the advantages, and when drysalted hides are worked, the soluble salts of the cure accumulate to an injurious extent. The modern method, where no chemicals are used, is to give one fresh water at least to each pack of hides or skins. Even in this case considerable putrefaction takes place where the soaking occupies 7 to 14 days, as is the case with kips and hides, and it is probable that the use of chemical and antiseptic methods of soaking will ultimately be generally adopted, both on technical and sanitary grounds.