tan-burning furnaces on a different principle from the American, the essential point being the use of a "ladder-grate" (_Treppenrost_), on which the burning tan is exposed to a draught of air playing over its surface. Fig. 39 shows a portable stove of this construction. Gläser also makes furnaces of larger size for heating air for drying-rooms, and for boiler purposes. The essentials of successful tan-burning are good draught, a large grate-surface, and a high temperature of the combustion-chamber, and hence the ordinary Cornish or Lancashire boiler, with its limited grate-area, surrounded by the comparatively cool boiler-tube, is peculiarly ill-adapted for the purpose. The writer has profitably burnt a mixture of wet tan and very small coal in such a boiler by the aid of a steam-jet under-grate blower, but such a method can only be regarded as a makeshift in default of better appliances. [Footnote U: Such chains with attachments for elevators and conveyers, are manufactured in this country by Ley's Malleable Casting Company, in Derby.] [Illustration: Fig. 36.] [Illustration: Fig. 37] In England, the tanning material is usually exhausted in pits called "leaches," "latches," or "taps." These, in large yards, are made capable of holding about 50 cwt. of material. The new material is first flooded with a pretty strong liquor. When this has gained as much strength as possible, it is pumped off, and is followed by a weaker one, and so on till the material is exhausted. Much of the economy of a tan-yard depends on the way, systematic or otherwise, in which this is done. It is customary to complete the exhaustion with hot liquors, or water, but opinions differ on the expediency of the practice. By the use of heat, however, stronger liquors and more rapid spending are attained; and with some materials, such as mimosa, complete exhaustion is impossible in the cold. [Illustration: Fig. 38.] [Illustration: Fig. 39.] The worst tap is frequently boiled by inserting a steam-pipe; but if heat is used at all, it would probably be better to heat a strong liquor by a steam-coil, and run it on the new material, which would be softened and swollen, and yield a much larger proportion of its strength to the first liquor; while it is stated by Eitner that the colouring matters of tanning materials are much less soluble in strong than in weak infusions. Boiling weak old liquors containing lime is specially prejudicial, causing great darkening and discoloration.