74 CHEMICAL FERTILIZERS 



between the upper layer of benzine and the lower layer of 

 water. By means of appropriately -placed taps, the benzine 

 is run off into store, and the condensed water run to waste. 

 When the bones have become sufficiently dried, the distilla- 

 tion of benzine is stopped by checking the circulation of 

 steam through the steam coil. The benzine, which now 

 contains the fat from the bones is run off from the bottom 

 of the vessel, whence it flows to a still in which the benzine is 

 distilled, condensed and returned to store. The grease 

 left behind is then completely freed from benzine by blowing 

 in steam; the resulting vapours are passed to the condenser 

 and separator tank. Four or five washes with benzine in 

 the extractor are generally needed. Other extractives than 

 benzine, some of which are non-inflammable, are occasionally 

 used ; they are not very popular, owing to their cost. The 

 grease can be used for soap making and other purposes. 



In some works " degreasing " is conducted by older 

 systems, in which the bones are merely boiled in open pans ; 

 in other factories the action of steam in closed vessels is 

 employed. Under both of those systems, the manufacture of 

 gelatine as well as the production of fat is generally carried 

 out. For this double purpose, some factories slightly acidulate 

 the water, in which the bones are boiled, by the addition of 

 a very small quantity of sulphuric acid, say | % of the 

 weight of bones taken. The grease is set free from its cal- 

 careous compounds and 4-5 % of fat can thus be ob- 

 tained from the bones. The steam process is carried out in 

 large cast-iron cylinders, in which steam enters at the top 

 and the fat runs off at the bottom; steam of from 

 2 to 4 atmospheres is generally used. As soon as the liquid 

 contains a large proportion of gelatine, the solution becomes 

 colloidal, the fat refuses to rise to the surface, so that the 

 liquor appears milky. Care must be taken that the operation 

 of boiling does not reach the stage of producing a colloidal 

 solution. The operation must be stopped in time to allow 

 the fat to separate by standing, after which the gelatinous 

 solution is concentrated for the production of glue. The 

 bones, having been freed from grease by any of these methods, 



