120 CHEMICAL FERTILIZERS 



where some of the phosphate occurs as a sclero-apatite, 

 the gelatinous tissue of the bones taking the place of the 

 fluorine. Some authorities consider bone phosphates to be 

 a carbono-apatite [Ca^PO^^CO-j. There is also consider- 

 able evidence for supposing that the nitrogenous matter, 

 insoluble in water, which occurs in milk, is a casein-apatite. 



There are also special cases where calcium occurs as a 

 basic phosphate, with more calcium in it than tri-basic phos- 

 phate, and where the acid concerned is less dehydrated than 

 ortho-phosphoric acid. In addition, there are many complex 

 silico phosphates, which are met with in some fertilizers. 



In the presence of hydrochloric acid, mono-calcium phos- 

 phate is the stable form. If any of the calcium phosphates 

 are dissolved in hydrochloric acid, and evaporated to dryness, 

 it is mono-calcium phosphate that is left behind. Similarly, 

 if a mixture of phosphoric acid and calcium chloride be 

 evaporated, mono-calcium phosphate is left behind. Di- 

 calcium phosphate will absorb hydrochloric acid gas passed 

 over it. The weak acid carbonic acid can attack tri-calcium 

 phosphate to a slight extent, and produce solutions which 

 probably contain some di-calcium phosphate. The phos- 

 phates of the alkalies are all soluble in water. The phos- 

 phates of the alkaline earths are soluble in acetic acid, but 

 insoluble in water; while ferric and aluminium phosphates 

 are insoluble in acetic acid. 



Basic Slag. The occurrence of basic slag as a by-product 

 in the manufacture of high-grade steel has been mentioned 

 on p. 63. The crude slag is first obtained either as 

 large blocks which have cooled slowly in the waggon into 

 which the molten slag was originally run, or in large slabs 

 obtained by pouring the molten slag on to a flat surface. 

 Basic slag may be quenched in water for rapid cooling, but 

 this process is somewhat dangerous, as the water may 

 suddenly burst through a crack and boil explosively. Basic 

 slag is a particularly difficult substance to grind in a satis- 

 factory manner, as it is nearly always mixed with pieces of 

 steel of varying sizes. These lumps of metal give much 

 trouble in many forms of mill, but in mills which grind by 



