228 SOILS AND MANUEES 



phosphate thus added is converted into the basic form, 

 which is the special characteristic of the slag phosphates, 

 its manurial value will be no greater than that of the native 

 tricalcic phosphates. This, perhaps, may partly account 

 for the differences in the solubility of the phosphates of 

 slags obtained from different sources. The extraordinary 

 differences sometimes observed in the fertilising power of 

 slags containing approximately the same proportion of 

 phosphates may be due to the same cause. 



The average quantity of lime employed is about one 

 part to five or six of pig iron, but by using a smaller 

 quantity, a product much richer in phosphates can be 

 obtained. This necessitates subjecting the iron to a 

 second treatment in order to complete the dephosphorisa- 

 tion, but as only a small amount of impurity is left in 

 the iron, the lime does not suffer much change in the second 

 treatment and can be used over again. 



Under ordinary circumstances, the proportions of the 

 principal constituents may vary within the following 

 limits : 



Per cent. 



Lime 4060 



Oxides of iron ........ 10 20 



Silica 515 



Oxide of manganese 3 6 



Magnesia ......... 2 6 



Alumina ......... 1 3 



Phosphoric acid (P 2 O 5 ) . . . 1020 



The average proportion of phosphoric acid is about 

 16 per cent., which is equal to 35 per cent, of tricalcic 

 phosphate, but commercial samples containing as little 

 as 25 per cent, and as much as 45 per cent, of phos- 

 phates are quite common. The average of English makes 

 is about 30 per cent. 



More than two-thirds of the total lime (calcium oxide) 

 are always present in combination with phosphoric acid 



