=1. Oil Tanning.=—This, while of less importance than the other two methods, is still used in considerable quantities for lighter and cheaper qualities of leather. The process most commonly used is often called chamoising, or “shamoying,” because it is used principally for the production of “chamois leather” or wash leather. The hides used for this form are usually thin and light, the flesh sides of split sheepskins being the commonest, and the resultant leather is not only soft and flexible and strong, but is also unaffected by water. For this reason it is more difficult to dye than other varieties. =2. Mineral Tanning or Tawing.= _Alum._—For thousands of years it has been known that if a solution of alum is rubbed or soaked into a raw hide the fibres of the leather become changed to an insoluble and permanent condition, and by afterwards rubbing and rolling, and working in some greasy material, like the yolk of eggs, a useful variety of leather can be produced. The alum in this case does not form a permanent compound with the animal fibres, but can be washed out by working in warm water. Chemists have agreed, therefore, to call this temporary reaction by the name “tawing” as opposed to “tanning” where the chemical action is a permanent one. The “kid” leathers used for gloves are commonly made by this process. _Chrome._—During the last few years a new process has been introduced, based upon the use of chromium salts, which are absorbed by the hide in the form of the yellow or orange-colored salts, chromate and bichromate of sodium, and then are reduced in the fibres to a green compound by the use of hydrosulphite of sodium, or some other strong but harmless reducing agent. This chrome leather is extremely valuable, and is freely used, especially for the “uppers” of good quality in the boot and shoe trade. This leather is very strong, and is water-proof, but possesses a serious disadvantage for the dyer, in that when it is once dry it can never be again wetted, and therefore it must be dyed fresh from the tannery wash tanks, or not at all. =3. Vegetable, or Bark Tanning.=—At some very early period in the world’s history it was discovered that certain vegetable extracts, possessing in general a peculiar “puckery” taste, also possessed valuable properties in the treatment of raw hide. This process was certainly well known to the Romans, for Pliny mentions, as tanning materials, the three great sources of tannin to-day, namely, gall nuts, the bark of trees, and sumach. These and many other vegetable materials, used for tanning, all contain a peculiar substance, known as “tannin” or tannic acid, which gives them their useful properties.