In all probability these various preferences are determined by the nature of the varnish, which differs widely in various parts of the globe. In this country the varnishes are made largely from linseed oil by boiling it with "driers." This oil contains much triglyceride of an unsaturated relative of stearic acid. The double bonds are very susceptible to oxidation with the production of resinous bodies of unknown constitution. This phenomenon is known as "drying the oil," and has been extensively used in the manufacture of linoleums. The driers are either oxidizing agents or oxygen carriers, such as litharge, Prussian blue, raw umber, manganese dioxide, manganese borate, and "resinate." Prussian blue is most preferred for British japans, as it always materially assists the attainment of the desired black colour. The exact details of the boiling, and the manufacture of the varnishes is still largely the trade secret of the master japanners, and differs indeed for the various stages of japanning. The varnish for the earlier coats is boiled longer, and the drying carried further, than in the case of the later coats. This is partly to obtain a product of such stiffness that it will not penetrate the leather. The driers and the pigments should be finely powdered and thoroughly mixed in. The boiling takes several days when at a low temperature, but if done in 24 hours the temperature may be up to 570° F. In the later coats driers are often not used, and the product is often mixed with copal varnish, pyroxylin varnish, etc., which greatly help in obtaining smoothness and gloss. Turpentine, petroleum spirit and other solvents are also used to thin the varnishes. Before boiling, the oil is often purified by a preliminary heating with nitric acid, rose spirit and other oxidizing agents, which precipitate impurities and thereby assist in obtaining a bright gloss. Before the application of the varnishes, the leather is first dried thoroughly in a stretched condition. This is accomplished by nailing down on boards which fit like movable shelves into a "stove," a closed chamber heated by steam pipes. The temperature of the stove varies widely in different factories, from 140°-200° F., according to the nature of the varnishes. The first coat of warm and rather stiff japan is laid over the hot leather in a warm room, being spread over first by hand, then by a serrated slicker, and then again smoothed by hand. The goods are then put into the stove for several hours to dry. When dry the surface is pumiced and brushed and a second coat applied in a similar manner, but with increased care. This is repeated with finer japans until the desired result is obtained. Brushes are used to apply the later coats. Up to seven coats may be applied for the production of a smooth japan--three coats of ground japan, two coats of thinner japan, and two coats of finishing varnish.