MANUFACTUEE OF BASIC SLAG. 211 



rarely exceed 2 per cent. But the more these calcareous agglo- 

 merates are present, the higher will be the free lime content. It 

 is true that that lime not being combined chemically with the 

 phosphate, but in a state of simple mixture, and up to a certain 

 point as impurity, cannot exercise any influence on the solubility of 

 the phosphoric acid in the soil, for those bodies can only act as 

 solvents which are of a nature to modify the chemical nature of 

 another body. The neglect of this principle has led some authorities 

 to ascribe to the uncombined lime in basic slag an importance 

 which it absolutely does not possess. What determines the solubility 

 of the phosphoric acid in basic slag is the strongly basic calcareous 

 silicates with which it is combined. 



The magnesia likewise forms with the phosphoric acid a tetra- 

 phosphate, but the solubility of this body considerabl}' exceeds the 

 solubility of the corresponding calcareous compounds. Basic slags 

 rich in magnesia dissolve much more rapidly in organic acids than 

 those in which magnesia is absent. The action of citric acid on the 

 slags shows that the magnesia is chiefly combined with the phos- 

 phoric acid and not with the silica. It is perfectly possible, and in 

 all cases very likely, that the action of phosphoric acid of basic slag 

 in the soil depends, in the first instance, on the presence of these 

 silicates. We would thus be confronted with double compounds^ 

 phosphates, and silicates of calcium which are dissolved with the 

 greatest facility by water containing carbonic acid, hence, therefore, 

 the solution of the phosphoric acid and its absorption by plants 

 follows. It has been remarked, in fact, that the most active basic 

 slags are always those with a high percentage of silica. Steel works, 

 therefore, possess a very simple and in no way costly method of in- 

 creasing the solubility of basic slag in citrate ; it suffices to add hot 

 sand to it in the convertor, the sand melts like butter, is converted 

 into silicic acid and combines with the phosphate of lime to increase 

 its solubility. It is clear, in that case, that lime should not be de- 

 ficient in the slag. Sihca, therefore, plays a very important role 

 in the basic slag. CaSiO^ converts the shghtly soluble phosphate 

 of the slag into Ca^PoOg according to the following equations : — 



CagfPO,)^ + CaoSiO^ = Ca.P.O, + CaSiOg 

 Fe3(POJ2 + 2Ca"_,SiO^ = Ca.P'oOg + Fe3Si,0, 



The introduction of an excess of silica helps to decompose Ca^PoO^ 

 afresh. There exists in Germany a factory which makes basic slag 

 with 24 per cent of phosphoric acid according to Scheibler's patent, 

 German patents 31:,416 and 41,303. A little less hme than that re- 

 quired by the complete dephosphorization of the cast-uon is added 

 to the convertor, and the slag so obtained run out. The remainder 

 of the lime is then added, and the low strength slag which results 

 is used as a reducing substance in the convertor. .To increase the 

 strength of the slag in phosphoric acid, phosphated chalk may also 



