The usual method of liming is carried out in brick pits of square or rectangular shape with a sloping front on which the hides or skins can be piled to drain (Fig. 16). There is a great variation in the quality of lime, and in all cases it should be tested for the available percentage of caustic lime. A good sample should contain between 70 and 80 per cent. of pure lime. Buxton lime, which can be obtained in powdered form, is particularly suitable for the tannery. The ordinary lime is preferable to the chalk lime, as it is usually stronger, though it sometimes contains more impurities. Gas lime is the poorest of all. Lime should be stored in a dark place, otherwise the outside of the heap carbonates quickly, forming chalk (CaO + CO_{2} = CaCO_{3}) which is of no use for the liming process. To prepare the lime for the pits it is slaked (_i.e._, formed into a paste with water. All the lumps should be reduced to paste in order to avoid lime burns, which are caused by direct contact of the hides with pieces of unslaked lime. A certain quantity of the paste is then added to the water or old lime liquor in the pit, and the liquor is well plunged up to hasten solution and diffusion of the lime. A long wooden pole, with a flat block of wood attached, is used for this purpose and also for pressing down the hides under the surface of the liquor. Two methods of liming are in vogue: (1) the single-pit system, and (2) the three-pit system; but the latter is the better method, as it is more easily controlled, and causes less loss of gelatine than the former. In this system the liquor is strengthened with fresh lime for each pack of skins. Its great fault is that the pit is only cleared out at long intervals in order to take advantage of the mellowness of used lime liquors; hence, there is frequently large accumulation of insoluble limestone and other sediment from the lime, in addition to a quantity of dissolved gelatine, which rapidly accumulates putrefactive bacteria. When the pit is cleared out the process of liming is disturbed for a time, as fresh lime liquors are not beneficial to hides and skins, and the loosening of the hair is delayed. On the other hand, the three-pit system permits liquors of uniform strength, and the process is continued without interruption. Each new pack is first placed in the oldest of the three liquors, which is then cleared out, and a new liquor prepared. From the weakest pit the hides pass to a stronger liquor, and the process is finished in the third pit, which should contain a new lime liquor. The mellow liquors, being charged with bacteria, facilitate the loosening of the hair, while the third liquor, consisting of fresh lime, serves to swell the fibres of the hide, by means of which the flesh is more effectually removed. The lime also forms a soap with the natural grease of the hide; this grease can therefore be removed. In some cases, however, especially in the cattle fed up for