PHOSPHATIC MANUEES 229 



and silica, and the remainder usually only a small amount 

 occurs in the free or uncombined state, i.e., as lime. 

 The larger the proportion of phosphoric acid, silica, and 

 other ingredients which unite with the lime, the smaller 

 will be the proportion left in the free or uncombined state. 



When the proportion of iron, silica, etc., is large, that 

 of the phosphoric acid, lime, and other ingredients is 

 correspondingly diminished and rice versa. A large 

 proportion of iron adds considerably to the difficulty of 

 grinding, but has not been found otherwise objectionable. 



Basic Character of Slag Phosphates. An extremely 

 important point in connection with the composition of 

 basic slag is the character of the phosphate it contains. 

 It may be amorphous or crystalline. When slowly cooled 

 it naturally assumes the latter form, and it was by 

 examination of the crystals that Hilgenstock discovered 

 that it belongs to the class of substances known as basic 

 salts, i.e., salts which contain a larger proportion of 

 basic oxide than is normally required to combine with 

 the acid oxide. 



In ordinary tricalcic phosphate the normal salt the 

 ratio of lime (CaO) to phosphoric acid (P 2 5 ) is 1/18 

 to 1, but in slag phosphates it is 1*57 to 1. It is not 

 simply a mixture of free lime with ordinary tricalcic 

 phosphate, but appears to be a genuine chemical com- 

 pound in which the additional lime enters into the com- 

 position of the molecule and cannot be distinguished 

 from the other parts of lime present. The proportion of 

 lime corresponds, in ordinary tricalcic phosphate, to the 

 formula P./) 5 ,3CaO and in slag phosphates to the formula 1 

 P 2 5 ,4CaO. The latter is therefore called tetracalcic, or 

 basic phosphate of lime. 



The question is one not merely of scientific interest, 



1 The exact constitution of the basic phosphate is not known ; it 

 is probably very complex, but the difference between the molecular 



