192 SYSTEMS OF PERMANENT AGRICULTURE 



By this means the impurities of the low-grade phosphate and 

 the calcium sulfate formed in the first reaction are left behind, and 

 the monocalcium phosphate is then formed as a condensation prod- 

 uct, with only the impurities of the high-grade phosphate and a 

 small amount of calcium sulfate made from the excess of sulfuric 

 acid which is carried with the phosphoric acid. In practice, double 

 superphosphate is made to contain about 20 per cent of the ele- 

 ment phosphorus, corresponding to about 75 per cent of mono- 

 calcium phosphate, and to 400 pounds of phosphorus per ton of 

 product. This material is not made in the United States, but is 

 produced to a considerable extent in Germany. It has advantage 

 over ordinary acid phosphate in long-distance shipping, and it also 

 permits the use of phosphate rock containing more iron and alumi- 

 num than can be used for the manufacture of common acid phos- 

 phate on account of the deliquescent properties of the products. 



Slag phosphate. Basic slag phosphate results as a by-product 

 when pig iron, made from phosphatic iron ores and thus contain- 

 ing considerable phosphorus, is converted into steel by the basic 

 process in which an excess of lime is used. By proper methods a 

 slag is produced which may contain about 8 per cent of phosphorus, 

 or 1 60 pounds per ton. It is commonly held that the phosphorus 

 is in the form of tetracalcium phosphate, Ca 4 O(PO 4 ) 2 , whose struc- 

 tural composition might be represented thus: 



Tetracalcium Tricalcium Phosphoric 



phosphate phosphate acid 



/\ /\ H -\ 



Ca-O-7P=O Ca cAp=O H O^P=O 



Ca/ / H - 



cl> < 



X O X O H (X 



Ca-cAP=O Ca*-(Ap=O H O~P=O 



X CK ^0' H CT 



Whether these formulas express the relationship of tetracalcium 

 phosphate to tricalcium phosphate and to phosphoric acid, is not 

 fully known, and it is even questioned if the phosphorus in basic 



