BASIC SLAG, THOMAS SLAG 227 



in the soil, these bases are liberated again when the phosphat< 

 redissolves in water and carbon dioxide. Some authorities 

 claim that the acidity is due to the presence of calcium 

 sulphate, a necessary by-product in the manufacturing 

 process. But it is doubtful if this has any more residual 

 acid effect than any ordinary fertilizer or than anv normal 

 soil treatment. 



175. Basic Slag, Thomas Slag. — Basic slag is a very 

 popular fertilizer in Germany. 



(a) How Made. — In the basic Bessemer process of making 

 steel from phosphatic iron, devised by Thomas and Gil- 

 christ, of England, the molten cast iron is placed in a con- 

 verter lined with calcium oxide or calcium and magnesium 

 oxides. By blowing a stream of air through the molten 

 mass, phosphorus and silicon are oxidized and unite with 

 the calcium to form a double, basic phosphate and silicate of 

 calcium. The molten slag is poured off and when cool is 

 broken up and finely ground. 



(b) Composition. — The exact compound of the phos- 

 phorus in basic slag is not known, but there have been 

 found crystals of so-called tetracalcium phosphate. Most 

 of the phosphorus, however, is probably in the form of a 

 pentacalcium silico-phosphate. These compounds may best 

 be compared with the other compounds of calcium and 

 phosphorus so far studied: 



1. Monocalciwn phosphate, CaO.(H 2 0)2.P20 5 , found in 

 dissolved phosphates such as acid phosphate and dissolved 

 bone-black. 



2. Dicalcium phosphate, (CaO^.H^O.PoC^, found to some 

 extent with acid phosphate as the so-called reverted phos- 

 phate. 



3. Tricalcium phosphate, (CaO) 3 .P20 5 , found in rock 

 phosphate and bones. 



4. Tetracalcium phosphate, (CaO^^Os, and penta- 

 calcium silico-phosphate, (CaO)s.Si02.P205. 



The formulas for the first three have here been modified 

 from their usual form to bring out the differences between 

 them and the last two. 



