G. S. Robertson 25 



The substitution of fluorine or chlorine for the oxygen of one of the 

 CaO groups in each of the above rock phosphates does not interfere 

 with the ratio P2O5 : CaO. The combination of fluorine or chlorine in 

 this manner would account for the low solubility of rock phosphates 

 in citric acid compared with bone meal, whilst the higher lime content 

 of the rock phosphates accounts for their higher solubility in citric acid 

 compared with apatite^. ' 



The results of calcining the various mineral phosphates show that 

 a citric soluble sihca phosphate is formed. This phosphate goes into 

 solution in the 1st citric extract. In addition to the sihca phosphate 

 one or more phosphates with a lower lime content than the original 

 phosphate are produced by calcining. The longer the calcining continues 

 the greater the tendency to produce phosphates of low hme content, 

 and hence the lower the solubihty. 



It is of interest to note that with one exception (due probably to a 

 higher fluorine content) the higher the percentage of hme actually 

 combined with phosphoric acid the more soluble the mineral phosphate 

 is in citric acid. Colhns^ has shown this to hold true in the case of 

 basic slag. 



^ The Author, Journ. Soc. of Chem. Industry, 29 Feb. 1916, xxxv. 

 2 Journ. Soc. of Chem. Industry, 29 May 1915, xxxiv. 



{Received March ISth, 1916. 



