168 Composition oj Basic Slag 



believed and taught that the body was a calcium phosphate of the 

 composition (CaO)4P20„. In 1887, Stead and Ridsdale' described some 

 large and apparently pure crystals of this composition that they obtained 

 from basic slag. 



This and a statement of Hilgenstock's seem to be the ground on 

 which this belief has been based in spite of the fact that in Jan. 1895 

 Stead published another paper^ in which the former paper is practically 

 contradicted. As this latter work seems very generally to have been over- 

 looked it may not be out of place to give at some length the conclusion 

 arrived at. 



In the first place the author states, " that of the phosphates contained 

 in basic slag the most soluble consists of a chemical union of tetra- 

 calcium phosphate and mono-calcium silicate. The more insoluble 

 phosphates are in the form of hexagonal needles and flat plates and 

 appear to consist essentially of tetra-calcium phosphate, which however 

 varies in solubility in different specimens. Some varieties are as insoluble 

 as coprolites and nearly as insoluble as apatite." 



The above appears very much at variance with the usual opinion 

 of the solubility of tetra-calcium phosphate. 



What really is of still greater importance is the fact that in the 

 large number of slags which Stead examined, "there was an entire 

 absence of tetra-basic calcium phosphate crystals and a constant 

 recurrence of blue crystals " the composition of which he states to be 

 (CaO)4P206, CaO . SiOa containing 



Ca 56-578 7o, SiO, 10-791 %, PA 29146 7o. 



Several attempts were made to obtain some crystalline specimens 

 of slag. These however proved difficult to obtain, the makers stating 

 that crystals were by no means common, and only occurred in certain 

 balls of slag. 



Finally, Messrs Albert were able to send a crystalline sample, 

 in which however there was no sign of the presence of crystals 

 of tetra-calcium phosphate, those present being apparently the blue 

 crystals described by Stead, in a more or less pure condition. A sample 

 obtained by the author in Berlin showed the same composition. 



The pure blue crystals being very minute it was not easy to obtain 

 sufficient for analysis. The time occupied in picking them out was 



» Trans. Chem. Soc. 1887, 601. 



^ Proceedings of the Cleveland Institute of Engineers. 



