MANUFACTURE OF BONE SUPERPHOSPHATE. 181 



serve for several successive operations, and finally gives a boiling of 

 gelatine concentrated enough for making glue. If gelatine be not 

 made, the boiUngs are collected and concentrated by evaporation 

 to add them to the bone dust as shown in the sequel. But the 

 same water cannot serve indefinitely for fat extraction. After a 

 certiin time, it is remarked that the fat which it dissolves no 

 longer rises to the surface. The solution takes a milky appearance ; 

 it is a sign that it is saturated with gelatine and that its concentra- 

 tion obstructs the ascent of the globules of fat. It must then be 

 drawn off and replaced by fresh water. Unless fresh bones are 

 operated on, the fat obtained on extraction by water is generally 

 of inferior quality, it gives off a bad smell and is more or less dark 

 in colour. It is purified as indicated further on. 



Fat Extraction from Bones by Steam. — Fat extraction from 

 bones by steam is done in large cast-iron cylinders, capable of con- 

 taining 4 to 5 tons of crushed bones, the upper opening serving for 

 the introduction of the bones, the lower opening for their discharge. 

 These openings are closed by hinged lids as in an autoclave. Steam 

 enters at the top and the fat runs off from the bottom by means of 

 pipes situated near the aperture. Steam of from two to four 

 atmospheres is used ; for one to two hours the steam entrains the 

 fat with it. The condensed water, charged with fat and gelatine, 

 collects in the space reserved below the false bottom and is after- 

 wards added in the manufacture of nitrogenized superphosphates, 

 although the fat exercises an unfavourable influence on the dis- 

 solving of the phosphate. This water generally contains 1 to 2 per 

 cent of nitrogen and 0-3 per cent of P2O5, in the concentrated state. 

 It contains as much as 7 per cent of nitrogen, 3 per cent of ash, 50 

 per cent of organic matter, and 45 per cent of water. The solution 

 of fat and gelatine is withdrawn from time to time, and the treat- 

 ment by steam continued until a sample of the liquid contains no 

 more fat. To separate the two, the different draw-offs are united in 

 a wrought-iron pan with a conical bottom, fitted with a steam 

 jacket into which steam is injected, and a tap for drawing off the 

 fat. The object of heating is to keep the gelatine fluid enough for 

 the fat to separate on standing. When separation is complete, 

 the fat runs off by the above-mentioned tap, and the gelatine is run 

 into an evaporation pan through a valve in the conical bottom of 

 the pan. Whatever may be the quality of this gelatine, it may be of 

 advantage to reduce it to a marketable form by a series of mani- 

 pulations which the author has described elsewhere. By submitting 

 bones to systematic storing, all the gelatine may be extracted, and 

 the resulting bone dust, almost destitute of nitrogen, contains 35 per 

 cent of phosphoric acid. Bone dust is often mixed with moist 

 superphosphate to dry it. 



Some manufacturers only make degelatinized bone dust which 



