Basic Slag, Slag, Thomas's Slag 



taken up in the dross as silicates and other forms 

 of phosphate. This floating dross is drawn off and 

 the purified iron is left. The dross is black, very 

 hard, and heavy. It is difficult to grind, but it 

 has to be ground very finely, because its fineness is 

 an important feature of the product. Two qualities 

 of fineness are made — one of 50 per cent, and one of 

 80 per cent., that is to say, qualities of which 

 respectively 50 per cent, and 80 per cent, will pass 

 through a sieve having about 10,000 regular holes 

 to the square inch. The phosphate, which forms in 

 the slag at a temperature of from 1800° to 2000° 

 Centigrade, is generally understood to be in the 

 tetrabasic form [Ca04P205], soluble in citric — 

 which is a weak acid ; whilst tribasic phosphates 

 are only soluble in strong acids, which obviously 

 makes a great difference. The tetrabasic form or 

 any form other than tribasic in which phosphoric 

 acid occurs is very useful, since it represents a 

 reasonable degree of assimilability ; but all the 

 phosphate is not tetrabasic, and another portion is 

 present in the tribasic form. 



To determine the solubility of the phosphate 

 the solubility in citric acid is fixed at 2 per cent., 

 which corresponds approximately to the phosphoric 

 acid absorbed by plants. With regard both to 

 solubility and assimilability, tetracalcic phosphate 

 is very similar to the bicalcic phosphate of preci- 

 pitated phosphate, and to basic and reverted super- 

 phosphate. Even if it acts less rapidly than 

 superphosphate it does nevertheless act in the same 

 year. Turnips, which do not easily extract phosphoric 

 acid from its combinations, but which at the same 



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