126 AGRICULTURE 



of purely phosphatic manures, of which by 

 far the most important are Superphosphate 

 of Lime and Basic Slag. The former is 

 obtained by treating ground phosphatic rock 

 with sulphuric acid, during which process 

 most of the raw insoluble phosphate is 

 converted into a form soluble in water. In 

 former times superphosphate was frequently 

 put on the market in somewhat poor mechani- 

 cal condition, a state of things due partly to 

 the character of the rock phosphate employed, 

 and partly to defective manufacture. Of 

 late years, however, manufacturing details 

 have been more carefully attended to, and 

 the mechanical condition of superphosphate 

 now seldom leaves anything to be desired. 



Basic slag, which has come into such 

 general use during the last twenty years, is 

 a bye-product in the manufacture of steel 

 from pig-iron, which contains considerable 

 quantities of phosphorus. In the manu- 

 facture of steel it is desirable to get rid of the 

 phosphorus, and this is effected in a con- 

 verter by blowing air through the molten 

 iron mixed with lime until all the phosphorus 

 is oxidized to phosphoric acid, which is then 

 withdrawn as a phosphate of lime in the slag 

 that forms on the surface of the molten mass, 

 and in the lime bricks which line the walls 



