132 PHOSPHATIC MANURES [chap. 



of the composition (CaO)5Po05Si02, and contain about 

 29 per cent, of phosphoric acid, 1 1 per cent, of silica, 

 and 56 per cent, of lime. Moreover, when separated 

 from the mass of the cinder, finely ground, and attacked 

 with water charged with carbon dioxide or with very 

 dilute citric acid, the phosphoric acid they contain shows 

 approximately the same solubility as that of the phos- 

 phoric acid in an ordinary sample of basic slag, whereas 

 the crystals of tetra-basic phosphate of lime are markedly 

 less soluble. On the whole, it seems more probable that 

 the typical phosphoric acid compound of basic slag is 

 this (CaO)5P206Si02 — and not the tetra-calcium phos- 

 phate, (CaO)^?^©^, especially as there is plenty of other 

 evidence to show how large a part silica will play in 

 bringing phosphoric acid into a soluble state. 



Whatever may be the form of combination of the 

 phosphoric acid in basic slag, it is undoubtedly easily 

 attackable by the soil water, so that it is more available 

 to the plant than any of the forms of tri-calcium 

 phosphate, though as a rule it falls below superphos- 

 phate. In this availability the fineness of grinding is 

 a very important factor, and all samples should be 

 carefully tested ; at least 90 per cent, of the material 

 should pass through the standard wire sieve of 100 

 meshes to the inch. The content in phosphoric acid 

 varies with the amount of phosphorus present in the 

 iron employed in the steel-making process ; of late years 

 basic slags have been on the whole richer than they 

 were in the earlier years of its manufacture, and it is 

 possible to obtain material containing 23 per cent, of 

 phosphoric acid (equivalent to 50 per cent, of tri-calcium 

 phosphate). Lower grade material is more common, 

 but has been shown to be equally valuable when 

 quantities containing equal amounts of phosphoric acid 

 are compared ; consequently basic slag should always 



