156 Analysis and Valuation of Manures [263 



but attacks any silica which may be present, forming SiF 4 , and 

 this in its turn may be acted upon by water, forming Si0 2 and 

 H 2 SiF 6 . 



Thus it will be seen that the exact valuation of a mineral 

 phosphate by analysis would entail a very complex calculation. 

 A rough method is very often employed based upon the per- 

 centages of Ca 3 P 2 8 , Fe 2 3 , and A1 2 3 , the other impurities 

 being neglected. When less than 3 per cent, of the mixed 

 oxides of iron and aluminium is present, the valuation is calcu- 

 lated entirely from the percentage of Ca 3 P 2 8 . When more 

 than 3 per cent, is present, the excess is multiplied by 2 

 and subtracted from the percentage of phosphate. Thus, sup- 

 posing that analysis showed a mineral to contain 80 per cent, 

 of Ca 3 P 2 8 and 5 per cent, of oxides of iron and aluminium, 

 the excess of 2 per cent, over the 3 allowed would be 

 doubled and subtracted from the 80. The mineral would 

 be then valued as containing 76 per cent. Ca 3 P 2 8 . In fact, 

 many vendors would sell it as guaranteed to contain 76 per 

 cent phosphate of lime. 



