IV.] COMPOSITION OF BASIC SLAG 131 



the whole of the phosphoric acid in basic slag also goes 

 into solution when it is shaken with an alkaline solution 

 of ammonium citrate, in which tri-calcium phosphate 

 is not very soluble. The analysis of certain flat square 

 plate crystals, occasionally found in cavities in the balls 

 of slag, proved them to consist of a tetra-basic phos- 

 phate of calcium of the formula {0,2,0)^^^, the mole- 

 cule of phosphorus pentoxide being combined with four 

 molecules of lime instead of with three as in ordinary 

 calcium phosphate. To this tetra-basic phosphate of 

 lime the properties of basic slag have usually been 

 ascribed, it is supposed to be readily acted upon by 

 carbon dioxide with the formation of calcium carbonate 

 and di-calcium phosphate, and as this latter phosphate 

 is readily soluble in water containing carbonic acid, the 

 availability of the basic slag is accounted for. 



But it is by no means certain that this association 

 of tetra-calcium phosphate with basic slag is correct. 

 In the first place, the detailed analysis of the basic slag 

 hardly bears out this view ; there is more lime than is 

 necessary to make up tetra-calcium phosphate even 

 when every allowance is made for silica and sulphur, 

 and the amount of free lime that can be determined 

 is not sufficient to make up the balance. Moreover, 

 the crystals of tetra-calcic phosphate are only to be 

 found in basic slags made from irons poor in silicon; 

 the usual crystals found in the basic slag cavities are 

 long hexagonal needles, pale green or blue in colour, 

 of which considerable quantities can be picked out from 

 the cindery portions of the slag. The appearance also 

 of a fractured surface of the ordinary molten parts of the 

 slag would agree much better with a structure built up 

 of such prismatic crystals than of the flat crystals of tetra- 

 calcium phosphate. The prismatic crystals, according to 

 Stead, consist of a double silicate and phosphate of lime 



