=Bone-black.=--When bones are heated in a retort or crucible, the organic constituents are decomposed and carbonised. A mixture of combustible gases is given off; some of these do not condense on cooling, others condense in the form of a heavy oil, called bone-oil. Also much water containing tarry water and ammoniacal salts in solution passes over. The residue in the retort or crucible consists of finely divided carbon, in intimate mixture with the inorganic constituents of the bones: this mixture constitutes ordinary bone-black, or animal charcoal, as it is sometimes called. The inorganic portion may, if required, be removed by washing the residue in dilute hydrochloric acid. The process, as worked on the large scale, is carried on in different ways, according as it is desired to collect the volatile condensable portion of the distillate, or to allow it to escape. In the latter case, when it is required to obtain only bone-black, the apparatus employed is of a very simple nature, and the amount of fuel needed is comparatively small. The carbonisation is effected in fire-clay crucibles, 16 in. high and 12 in. diameter. These are to be preferred to crucibles made of iron, which were much used at one time, since they do not lose their round form when subjected to a high temperature; in consequence of this, they fit more closely together in the furnace, less air can penetrate, and therefore less of the charcoal is consumed by oxidation. The furnace is an ordinary flat hearth, having a superficial area of about 40 square yards, and is covered in with a flat arch, all of brickwork. The fireplace is situate in the middle of the hearth; the crucibles are introduced through doors in the front, which are bricked up when the furnace is filled; each furnace holds eighteen crucibles. The crucibles, filled with the coarsely broken bones, are covered with a lid luted on with clay. To economise fuel, the furnaces should be in a row, and placed back to back. The arrangement of the furnace and pots is shown in Figs. 1 and 2. A is the fireplace; B, the crucibles, eighteen in number, spread over the floor of the furnace in a single layer; _c_, _d_, _e_, and _f_ are the flues for conducting away the heated gases arising from the calcination of the bones, as well as the waste heat itself; the last portion of the flue is fitted with a damper _g_. The furnaces are intended to be built in fours, back to back, the waste heat serving in a great measure to conduct the operation of the revivifying apparatus placed in the centre of the group, and marked C. [Illustration: Figs. 1 and 2.--BONE-BLACK FURNACE.]