IV] DONE MANURES 109 



collagen, chondro-mucoid, etc. — whicli arc insoluble in 

 acids and are left behind in a soft tough condition if the 

 bone be put to soak for some time in weak acid. Mixed 

 with the cartilage is a certain amount of fat, and the 

 first treatment the bones receive is to steam them 

 under a pressure of 15 to 20 lb. in order to melt and 

 remove the fat, which is sold as tallow or used for soap- 

 making forthwith. In some cases the fat is extracted 

 even more thoroughly by the action of benzene. The 

 boiled or steamed bones thus obtained contain 4 to 5 

 per cent of nitrogen and 43 to 50 per cent of calcium 

 phosphate, and are read)' for conversion into manure. 

 They arc sometimes merely crushed into i-inch or |-inch 

 bones, though there is no longer much demand for 

 material so coarse ; more generally they are ground 

 down into "bone meal." A really fine powder is, 

 however, rarely obtained, because the cartilage inter- 

 feres materially with the disintegration unless special 

 methods are employed. It is this crushed material 

 which is also treated with sulphuric acid for the 

 manufacture of "dissolved" or " vitriolised " bones. 

 In factories making glue the cleaner bones are picked 

 out, and, after the fat extraction, they are broken up 

 and steamed afresh at a much higher pressure and 

 temperature, 50 to 60 lb. to the square inch, by which 

 process the collagen takes up water and becomes con- 

 verted into gelatine, which dissolves. The solution 

 is concentrated and allowed to set, when it becomes 

 glue : the bone residue, which now contains only i 

 to 1-5 per cent, of nitrogen but 55 to 60 per cent, of 

 calcium phosphate, is ground and sold as " steamed 

 bone flour." Owing to the removal of the cartilage, 

 this material can be ground finely, and forms a dry 

 friable powder very convenient for use as a manure. 

 The coarser kinds of bone meal are converted into 



