In many cases the trough is V-shaped with the chain running in the angle; in others flat-bottomed as in the illustration, or rectangular. The scrapers may consist either of metal or of wood; and where materials have to be carried up a steep incline buckets instead of scrapers should be employed. The arrangement of such a conveyor is illustrated by Fig. 76. A useful form of conveyor for dry materials consists in a woven cotton belt running in a smooth trough and with laths riveted across it at intervals. These laths should project slightly beyond the edges of the belt so as to prevent wear. Care must be taken with belts of this sort that the material does not get between the belt and the pulley. Chain-carriers are often used for conveying the spent tan to the furnaces from the leaches, and occasionally for carrying skins. Several other kinds of conveyor are in use in corn-mills, spiral or worm conveyors which work on the screw principle being very largely used for carrying corn. They are not very suitable for tanning materials on account of the coarseness of the latter, by which the friction is greatly increased; they are however occasionally used. Those built up of separate blades are specially to be avoided. An ingenious form of conveyor has been recently introduced from Germany, and consists of a light trough supported on steel springs and vibrated longitudinally by means of an eccentric in such a way as to shake the material from one end of the carrier to the other; the velocity of motion of the trough being less in the outward than the return stroke, so that the material is carried with it as it moves forward and slides over it in its return. It is obvious that the principle may also be applied to screening or sifting. CHAPTER XXII. _THE EXTRACTION OF TANNING MATERIALS, AND THE MAKING OF EXTRACTS._ _Leaching._--The material, having been reduced to a suitable state of fineness, is ready for extraction. This requires a considerable amount of time, as the tannin is contained in cells whose walls are of a wood-like substance (cellulose and lignine), through which the water diffuses but slowly. Hence, unless the material be very finely ground, a long soaking will be necessary before it becomes “spent.” It should be the aim of the tanner to have his barks, etc. ground so finely that they may be extracted as rapidly as possible, and yet not be so fine that they settle to a compact mass in the leaches and so prevent circulation. Using the present methods of extraction on the large scale it is necessary to have the material only somewhat coarsely ground or crushed, so as to render its percolation practicable; but it is quite possible that in the near future some better mechanical means will be found of treating the dust and other excessively finely ground matter so as to bring about a very rapid extraction.