84 FIRST PRINCIPLES OF AGRICULTURE. 



ods of extracting the fat have the advantage of increas- 

 ing the relative proportion of the nitrogen, this element 

 not being attacked by the solvents. 



The more complete extraction of the fat and moisture 

 by these methods also aids in the final preparation of the 

 bone by grinding. Bone prepared in this way frequently 

 contains as high as six per cent of nitrogen, and twenty 

 per cent of phosphoric acid. 



The nature and composition of animal bone is such as 

 to make it a valuable source of phosphoric acid; and, 

 while it is largely used with nitrogenous and potassic 

 materials in the manufacture of artificial manures, its 

 best use is, perhaps, in the fine ground form, particularly 

 for soil improvement and for slow-growing crops. 



Phosphoric acid applied in this form gradually gives 

 up nitrogen and phosphoric acid to the plant ; and its 

 physical and chemical conditions are such that it forms 

 in the soil, during the growing season, no compounds 

 more insoluble than the bone itself. Of all the phos- 

 phatic materials available as manure, bone is the only 

 one that is now used to any extent without further treat- 

 ment than simple grinding. 



Bone-black or Animal Charcoal. This material 

 becomes an important source of phosphoric acid for arti- 

 ficial manures after it has served its chief and first pur- 

 pose in clarifying sugar. In making bone-black only the 

 best bones are used; they are cleaned and dried, and 

 placed in air-tight vessels, and heated until all volatile 

 matter is driven off; the resultant product, which retains 

 in part the original form of the bone, is then ground to 

 a coarse powder ; it then becomes a bone charcoal, con- 



