The charge of wood, cut into suitable lengths, is thrown in until the cylinder is as full as possible. 'With ordinarily dry wood a eharge will weigh about 24 cwt. ; if the pieces are erooked they should be sawn up, that no roem may be wasted. The doer is now lowered down, secured by the crossbar, 9.nd carefully plastered round with a loam er Clay luting, se as to prevent either ingress of air or egress of the products of distillation. At first the fire should be kept under, to dry the charge, but after about two or three hours driven hard to effect perfect decomposition of the wood and the separation of all volatile constituents. The rate at which the fire is driven must of eeurse depend upon the time allowed to work off the batch. If a large turn-out is necessary, carbonization may be effected in twelve or thirteen hours, but a slower process, say sixteen hours, gives better results. Not only is a slowly eharred wood better in quality, but the amount of uneondensable gases is reduced. During earbonization the following changes are effected. First, all extraneous moisture is driven off ; then, as the temperature is raised, and decomposition of the wood takes place, acetic acid and water ; then the tar and volatile oils ; and, finally, carbonic oxide, earbonie 9.cid, and marsh gas. In the eylinder is left ehareoal, retaining, pieee by piece, the shape of the original wood. When the run of liquid from the condenser ceases, and the exit pipe from the cylinder becomes cool, it is known that the distillation is complete. The fire is allowed to die down, the deer opened, and the charcoal raked out, by means of the diaphragm above described, into deep, iron waggons, shown at C, Fig. 7, and run away to cool. The waggons are fitted with a sheet-iron cover, which is luted down with clay to prevent absolute eotubustion of tho ehareoal by the air. To effect this purpose, a little water is occasionally sprinkled over the eharge when raked out into the waggon, but, as a rule, this is unnecessary. In the ease of seme old fashioned plants, the charcoal is raked into a luted box, sunk in the ground underneath the eylinder door, but it need hardly be pointed out that this plan creates an unnecessary amount of " smalls " and dust.
To economize fuel in the distilling operation, and prevent nuisance, some of our best manufac turers are now beginning to adept the very excellent plan of bringing the waste gases back from the end of the eondensers, returning them into or under the fires, and burning them. The advantages of this plan are apparent—it is only surprising that it is not more generally adopted.
In the Forest of Dean, and some other parts ef the country, instead of cast-iron cylinders, wrought-iron ovens, set in somewhat simil:ir fashion, are used. This arrangement, without the drying house—which is the same as in Figs. 5, 6. and 7—is shown in Figs. 8, 9, and 10. Figs. 8 and 9 give the elevation of back and front, partly in section, and Fig. 10 the longitudinal section of an oven. It will be noticed that the charge is introduced and withdrawn in small sheet-iron waggons. By this arrangement labour is saved, and the removal of the charge effected with a iniuinnitu of breakage. The waggons are piled up about 18 in. above the sides. These ovens nro usually about 8 ft. long by 5 ft. square, and the charge is contained in two waggons. It would, however, be an improvement on the usual plan to have only ono, longer, waggon, as the charge would thereby be more speedily with drawn and covered up. The fire is
sometimes allowed to play direct upon the oven after about 2 ft. from the arch, up to which point a 41-in. casing gives protection; sometimes tho oven is set " naked," and occasionally a 41-in. casing is carried all round. The heat and products of combustion pass up the sides, along an arched flue formed on the top of the oven, from thence into a series of flues forming the bed of the drying house, and finally escape up the chimney.
Each of these two systems of distilling—in east-iron cylinders and wrought-iron boxes—has its stanch upholders, and it would be difficult to assign to either the pre-eminence. As a general rule, it may be laid down that the Welsh ovens are the hest for the small timber which is generally distilled there, and the cast-iron cylinders more suitable for the heavier cuts preferred in other parts of the country. The disadvantages of the cylinders are, a liability to crack, and a somewhat larger consumption of fuel through the thickness of the plate. On the other hand, the wrought iron oven is apt to leak at the joints and doors, to warp with the heat, and is more appreciably acted upon by the acid products of distillation—especially when cooling down. With judicious and careful patching, a cylinder, even cracked, may be worked for eight or nine years with only the ne.-,ssary renewal of the door and lighter parts.
Oeeasionally, yet another style of oven is used, made of sheet iron, and divided into compart ments, into each of which a charge of wood is lowered through a hole in the top. The compart ments rest upon a movable framework, and are brought in turn under the door to receive the charge. By this plan only one opening is necessary, but the warping and general wear and tear present formidable objections.
From the retort, of whatever description it may be, the gases pass along the exit pipe shown at D in Figs. 6 and 7 into the condensers. These are of various forms. A most effectual one is shown in plan in the general arrangement of works set forth in Plate I., and iu perspective in Fig. 11. At the rear of the ovens a large tank is built, the same length as the range of furnaces, about 14 ft. wide and 3 ft. 6 in. deep. The exit pipe from each oven or cylinder is carried straight through the side of the tank and continued to the opposite side, at its full size of 9 or 10 in. as the case may be (see A, Fig. 11). It then turns sharply back, and is branched into two copper pipes B and C, one above the other, These copper pipes should not be less than 5 in. in diameter. They run back to the oven side of the tank, and are once more returned sharply, passing finally through the side of the tank farthest from the ovens, and dipping about 2 in. into a spout, which carries over the condensed products to a suitable receiver. The uncondensed gases are conducted away by a small pipe, after the manner shown in Fig. 13. The whole tank is kept full of water, a. continual flow of which, on and off, must be secured by any convenient arrangement. The copper pipes must_also have a considerable fall, to prevent their being choked by the deposit of tar, &c. This tarry deposit, from its clinging and heavy nature, is difficult to deal with, and the one great objection to this form of condenser is that it cannot be readily and speedily cleaned out.