The great secret of tanning consists in so preparing the cuticle or porous surface of the hide as that the gluey or gelatinous tissue shall unite with the tannin. This is produced by immersing the skins in an infusion of oak bark, or other substances containing tannin. Therefore any process which will open the pores of the unhaired skins so that they become light and spongy and thus facilitate the union of the tannin with the tissue of the skin, is desirable. The name of _tan_ takes its origin from a coarsely powdered bark, which contains the active principle in the tanning process. The component is called _tannin_. To Seguin, a celebrated French chemist, the discovery of a property in nut-galls of combining with the albumen and gelatine of skins, thus forming an insoluble and unalterable compound, is due. This is the key to the theory of tanning. Morfit’s definition of the principle is, “to saturate a skin with tannin in such a manner as to promote the slow combination of this principle with the gelatine, albumen and fibrine contained in the former, so as to form with them a new compound. This reaction, in the operation of tanning, does not proceed spontaneously, but is the result of a slow process, requiring great care and skilful manipulation.” Tannin exists in the bark of the trunks of nearly all perennial trees, and especially that portion of the bark next to the fibre and sap wood. It is found in the leaves of trees in small quantities. The oak-tree bark, leaves and fruit, contain a larger amount of tannin than any other tree species in North America. The bark is usually gathered from May to July, as during those and the intervening months they contain more tannin than during the other portions of the season. There are various kinds of oak in the United States, all of which yield a large amount of tannin. There is a species of oak which grows in Delaware, Maryland and Virginia, known by the name of Spanish Oak, which is superior for its tannin properties. The rock-chestnut oak, the black and white oaks, and Spanish oak, are considered the most valuable, and are used almost exclusively by the manufacturers of leather in the United States. The wood of almost every tree, the bark of which contains tannin, is also useful for the same purpose. There are several varieties of tannin. The pure tannin and the impure tannin is a generic term for the different varieties. It was a long time before tannin was separated in a pure state. It is so intimately combined with other extractive matter that it was with difficulty separated. Chemists give us four varieties. _Natural tannin_, comprising several species, originating from the different proportions of extractive and gallic acids. Also three kinds of artificial tannin, obtained by the action of nitric acid upon charcoal; also upon indigo and the various resins; also that obtained by the action of sulphuric acid upon the resins and camphor.