For some descriptions of hides, however, and notably for India kips, putrid soaks seem actually to be an advantage, the putrefactive action softening and rendering soluble the hardened tissue. Putrefactive processes 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. Hence hides in the soaks require constant and careful watching, and the goods must be withdrawn as soon as they are thoroughly softened, for the putrefaction is constantly destroying as 'well as softening the hides.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 probable 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 necessary in the former case that 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 ; the putrid soak, if acting rightly, only accomplishes a part of the work, which would afterwards have to be done by the lime and the bate. The actual fibre of the hide seems less readily putrescible than the albuminoid parts ; hence the putrefaction may soften the latter better, and even at less expense of valuable hide-substance, because more rapidly, than fresh water. On this point, there is room for investigation. Putrefaction is a general name for a class of decompositions which are caused 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. It will be neces sary to revert to this subject when speaking of the bates used in preparing dressing-leather, which also owe their activity to putrid fermentation.
Beside merely soaking the hides, it is necessary to work them mechanically, to promote their softening, which was formerly accomplished by "breaking over" the hides on the beam with a blunt knife. This process is now usually superseded or supplemented by the use of the "stocks"; these consist of a wooden or metallic box, of peculiar shape, wherein work two very heavy hammers, raised alternately by pins in a wheel, and let fall upon the hides, which they force op against the side of the box with a sort of knead ing action. The ordi nary form of this ma chine is shown in Fig. 900. A more modern form, which seems to possess some advan tages, is the American double-shover, seen in Fig. 901.
The number of hides which can be stocked at once naturally varies with the size of both hides and stocks, but should be such that the bides work regularly and steadily over and over. The whole number should not be put in at once, but should be added one after another, as they get into regular work. The duration of stock ing is 10-30 min., according to the condition and character of the hides. Hides should not be stocked till they are so far softened that they can be doubled sharply, witliiin• breaking or straining the fibre. After stocking, they must be soaked again for a short time, and then be brought into an old lime. A small quantity of sulphide of sodium added to the soaks or in the
stocks bas been recommended as of great value in softening obstinate hides, and probably with justice, from its well-known softening action upon cellular and horny tissues.
Unhairing.—In England, lime is the agent universally used for this purpose, though every tanner admits its deficiencies and disadvantages. It is hard, however, to recommend a substitute which is free from the same or greater evils, and lime has one or two valuable qualities, which will make it very difficult to supersede. One of these is that, though it inevitably causes loss of sub stance and weight, it is also impossible, with any reasonable care, totally to destroy a pack of hides by its use ; this is by no means the case with some of its rivals. Another advantage is that, owing to its very limited solubility in water, it is a matter of comparatively small consequence whether much or little is used ; and even if the hides are left in a few days longer than usual, the mischief, though certain, is only to be detected by careful and accurate observation. With all other methods, exact time and quantity are of primary importance, and it is not easy to get ordinary workmen to pay the necessary atten tion to snob details. Again, the qualities of lime, its virtues and failings, have been matter of experience for hundreds of years, and so far as such experience can teach, we know exactly how to deal with it. A new method, on the other hand, brings new and unlooked-for difficulties, and often requires changes in other parts of the process, as well as in the mere unliairing, to make it successful. As our knowledge of the chemical and physical changes involved be comes greater, we may look to overcoming these obstacles more readily ; this constitutes one of the main advantages of a really scientific knowledge over an empirical one.
Slacked lime is soluble in water at 15° (60° F.) to the extent of 1 part in 778. Unlike most substances, it decreases in solubility at higher temperatures, requiring 972 parts of water at 54° (130° F.), and 1270 parts at 100° (212°F.). Its action upon animal tissues increases rapidly, how ever, with temperature, though no doubt it is moderated to some extent by the lessened solubility. Calculating from Dalton's numbers, pure lime-water at 15° (60° F.) contains 1.285 grm. of Ca0 per litre, and should require 459 cc. of decinormal acid to neutralize it. This estimate in some cases appears to be slightly too high ; e. g. a saturated lime-water from carboniferous limestone at 13° (56.5° F.) required only 433 cc. of decinormal acid, which equals 1.211 grm. of Ca0 per litre, and this lime-water gave nearly constant results for many mouths together : on the other hand, any traces of other soluble bases would raise the strength of the lime-water above its normal amount. Thus a magnesian limestone lime-water tested at the same time required 472 cc. of normal acid, confirming the old observation of tanners, that such lime is stronger than that made either from chalk or carboniferous limestone. This increased strength must arise from the presence of some soluble base other than lime, and may be due to the magnesia, which, however, is very slightly soluble.