PHOSPHORUS 55 



is likewise obtained from the structure of the rock 

 itself. This condition is observed in the hard-rock 

 section of Florida. 



Basic Slag. It has been found within recent 

 years that the phosphatic slag from the basic proc- 

 ess of steel-making possesses considerable value as 

 a fertilizer. The content of phosphoric acid varies 

 from 10 to 25 per cent, and, according to some, 

 the phosphoric acid exists as calcium phosphate. 

 The slag is, however, of little value as a manure 

 unless it be very finely ground e.g., 80 per cent 

 of it should pass a sieve having 100 meshes per 

 linear inch. The attainment of this condition con- 

 stitutes the preparation of the slag for the market, 

 and is expensive on account of hardness of the ma- 

 terial. Grinding is commonly performed by a ball 

 mill which consists essentially of a drum, the inner 

 surface of which is polygonal; the drum contains 

 a number of cast-steel balls of various sizes. Rota- 

 tion of the drum breaks up the slag by the rolling 

 and percussive action of the balls, and the commi- 

 nuted material is systematically separated by 

 sieves in the sides of the drum. The sieves which 

 effect the final separation are protected from 

 direct contact with the balls by perforated iron 

 plates. 



Bone Meal. This is of manurial value on 

 account of its phosphates, but it also contains ni- 

 trogenous matter. Fresh bones contain about f 

 per cent of nitrogen; they decompose very slowly 

 when used as a manure. When the bones are 



