The glue-worker first removes the fat, which is supplied to the soap and candle trades; the bones are now treated for glue (q.v.); and the residue is worked up for manures, &c. These residues are ground to a fine or coarse meal, and supplied either directly as a fertilizer or treated with sulphuric acid to form the more soluble superphosphates, which are more readily assimilated by growing plants. In some places, especially South America, the residues are burned in a retort to a white ash, the "bone-ash" of commerce, which contains some 70-80% of tricalcium phosphate, and is much used as a manure, and in the manufacture of high-grade superphosphates. In the gelatin industry (see GELATIN) the mineral matter has to be recovered from its solution in hydrochloric acid. To effect this, the liquors are freed from suspended matter by filtration, and then run into vats where they are mixed with milk of lime, or some similar neutralizer. The slightly soluble bicalcium phosphate, CaHPO4, is first precipitated, which, with more lime, gives ordinary tricalcium phosphate, Ca3(PO4)2. The contents of the vats are filter-pressed, and the cakes dried on plates supported on racks in heated chambers. This product is a very valuable manure, and is also used in the manufacture of phosphorus. Instead of extracting all the gelatinous matter from degreased bones, the practice of extracting about one half and carbonizing the residue is frequently adopted. The bones are heated in horizontal cast-iron retorts, holding about 5 cwt., and the operation occupies about twelve to thirteen hours. The residue in the retorts is removed while still red-hot to air-tight vessels in which it is allowed to cool. It is then passed through grinding mills, and is subsequently riddled by revolving cylindrical sieves. The yield is from 55 to 60% of the bones carbonized, and the product contains about 10% of carbon and about 75% of calcium phosphate, the remainder being various inorganic salts and moisture (6-7%). Animal charcoal has a deep black colour, and is much used as a filtering and clarifying material. The vapours evolved during carbonization are condensed in vertical air condensers. The liquid separates into two layers: the upper tarry layer is floated off and redistilled; the distillate is termed "bone oil,"[1] and mainly consists of many fatty amines and pyridine derivatives, characterized by a most disgusting odour; the residue is "bone pitch," and finds application in the manufacture of black varnishes and like compositions. The lower layer is ammoniacal liquor; it is transferred to stills, distilled with steam, and the ammonia received in sulphuric acid; the ammonium sulphate, which separates, is removed, drained and dried, and is principally used as a manure. Both during the carbonization of the bones and the distillation of the tar inflammable