SUPERPHOSPHATE. 39 



Superphosphate. An abundance of mineral phos- 

 phates (phosphates of calcium) occur in nature ; many of 

 these are so little soluble that their effect as manure is 

 but slight; by treating them with sulphuric acid (sp. 

 gr. 1*55) the sparingly soluble tricalcic phosphate is con- 

 verted into phosphoric acid, or into soluble monocalcic 

 phosphate, sulphate of calcium being at the same time 

 produced. Superphosphate is thuy a mixture of phos- 

 phoric acid and monocalcic phosphate, with gypsum, and 

 various impurities (as sand, and compounds of iron and 

 aluminium), derived from the original mineral. A super- 

 phosphate will generally contain more or less of un- 

 dissolved phosphate ; this amount will be more consider- 

 able if the manure is badly made. 



The value of a superphosphate chiefly depends on the 

 percentage of "soluble phosphate " present. By this 

 tsrm analysts do not mean monocalcic phosphate, but 

 the quantity of tricalcic phosphate rendered soluble. 



Besides the soluble phosphate, and the undissolved 

 phosphate, a superphosphate will frequently contain what 

 is known as "reduced phosphate," that is, phosphate 

 which was once soluble but has now lost this character. 

 The diminution of soluble phosphate during the storing 

 of superphosphates chiefly occurs when the manure has 

 been made from materials containing ferric oxide and 

 alumina, and is due to the formation of ferric and 

 aluminic phosphate. The proportion of reduced phos- 

 phate present in a superphosphate is estimated from its 

 insolubility in water, but solubility in a solution of citrate 

 of ammonium. Eeduced phosphate has an agricultural 

 value between soluble and undissolved phosphate. 



The best mineral phosphate found in England is Cam- 



