The tannins from different plants are not identical, although closely related to each other. They all have a strong astringent taste, and dissolve readily in water, forming weak acid solutions. They make dark-colored compounds with iron salts, and convert the hide tissue of animals into a tough, insoluble, and comparatively indestructible material which, when loosened and softened by some mechanical action, is known as leather. _Tannin._—Pure tannin can best be obtained from gall nuts—small excrescences on the leaves and twigs of certain plants caused by the puncture of some insect preparing to deposit its eggs there. The best varieties, called Aleppo galls, come from Turkey and Austria, where they are found on oak trees, and contain from 60 to 70 per cent. of tannic acid. From these it can be extracted in a very pure form, and it comes to market as an extremely light, fine, grey or light tan-colored powder, which dissolves in very little water to an almost colorless solution. Tannin in this form is largely used for dyeing, especially in the dyeing of cotton or linen goods with the Basic colors. For tanning purposes it is customary to use the bark of various trees, oak bark being the most esteemed in Europe and, in this country, hemlock bark being the most used. These contain from 12 to 15 per cent. of tannin, as a rule, with a moderate amount of brown coloring matter. Pine bark is also frequently used, and the bark of fir, spruce, and larch, while, in Russia especially, much willow bark and birch bark is used for light grades, the so-called Russia leather. The next most valuable source of tannin is known as sumach, consisting of the finely-ground twigs and leaves of several species of that plant. The American sumach contains more tannin—18 to 25 per cent—than other varieties, but it is less valuable than the Sicilian sumach, which contains less coloring matter, and therefore can be used for tanning light shades of leather. All the materials can be used in the tannery either directly, or in the form of previously prepared extracts. From the Far East come some very important sources of tannin, used for dyeing as well as for leathermaking, in the form of dried extracts of various plants. One of these is Catechu or Cutch, now of value only for its tannin contents, but in former years used as a brown dyestuff as well. A similar product, known as Gambier, is still imported on a large scale from Singapore and other Eastern ports. It contains less tannin than Cutch, but less coloring matter as well. It is used not only for leather but for black silk dyeing with logwood. =The Tanning Process.=—Without going too much into detail, the conversion of raw hide into leather by means of tannin is a very lengthy and mechanical process. The hides are first softened by soaking in water, and then are dehaired, usually by steeping in a bath of slaked lime until the hair is loosened and can be scraped off with a blunt knife.