VII. 



SPECIAL MANURES 



151 



main constituents are represented in the following analysis of a 

 specimen l : 



99-95 



The sulphur present seems to exist as a sulphide, probably of 

 calcium. The phosphoric acid is present as tetracalcium phosphate, 

 Ca 4 P 2 9 , a substance which sometimes occurs nearly pure as crystals 

 imbedded in the slag. 



According to Hoyermann and Wagner 2 the citrate-solubility of the 

 phosphoric acid in basic slag is greatly increased if the slag be fused 

 with sand. Wagner states that the citrate -solubility (by which he 

 judges of the availability) of the basic slag of commerce varies from 

 100 to 40 per cent, and that a high solubility depends upon the 

 presence of at least a certain percentage of silica. Eidsdale 3 denies 

 this, and states that the only function of the silica is the neutralising 

 of a certain proportion of the free lime of the slag and thus preventing 

 the action of this upon the citrate solution employed. 



According to Blome, 4 tetracalcium phosphate, 4CaO.P 2 O 5 , melts at 

 1870 C., while a compound 4CaO.P 2 O 5 + SiO 2 , melts at 1700 and is 

 totally soluble in 2 per cent solution of citric acid. Blue crystals 

 having this composition have been found in basic slag by Stead and 

 Eidsdale. 



Blome also obtained evidence of the existence of 



4CaO.P.p 5 + 2CaO.Si0 2 and of 4CaO.P,O 5 + 4(2CaO.SiO 2 ) 



the former melting at 1710, the latter at 1780. In both cases, the 

 melted mass contained about 4 per cent of free lime. By slow cooling 

 of the melt corresponding to 4 CaO.P 2 O 5 + 2CaO.Si0 2 , a large increase in 

 the amount of free lime was observed and the resulting product was 

 much more soluble in citric acid solution. He concludes that it is 

 probable that a double compound of tetracalcium phosphate and 

 calcium ortho-silicate is the combination, whose presence in Thomas' 

 slag conditions the citrate solubility of that material. 



Many attempts to improve basic slag as a manure have been 

 made, some directed to the removal of the iron, others the sulphur, 

 while -others have attempted to render the phosphoric acid more 

 soluble by treatment with sulphuric acid. Practically all these 



1 Stead and Ridsdale, Jour. Chem. Soc., 1877, Trans., 601. 



2 Chem. Zeitung, 1895, 1511. 3 Jour. Soc. Chem. Ind., 1895, 170. 



4 Metallurgie, 1910, 7, 659 ; also Jour. Soc. Chem. Ind., 1910, 1467. 



