60 THE CHEMISTRY OF THE EAKM 



of white clover. For pasture the inferior quahties of 

 slag may be employed. Much of the special action of 

 basic slag depends on the large amount of lime which 

 it contains. 



Superphosphate.— An abundance of mineral phos- 

 phates (phosphates of calcium) occur in Nature ; many 

 of these are so Httle 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 converted into phosphoric acid, or into soluble 

 monocalcic phosphate, sulphate of calcium being at 

 the same time produced. Superphosphate is thus- a 

 mixture of phosphoric acid and monocalcic phosphate, 

 with gypsum, and various impurities (as sand, and 

 compounds of iron and aluminium), derived from the 

 original mineral. A superphosphate will generally 

 contain a small proportion of undissolved phosphate ; 

 this amount will be more considerable if the manure 

 has been badly made. 



The value of a superphosphate chiefly depends on 

 the percentage of " soluble phosphate " present. By 

 this term analysts do not mean monocalcic phosphate, 

 but the quantity of tricalcic phosphate which has 

 been rendered soluble. 



Besides the soluble phosphate, and the undissolved 

 phosphate, a superphosphate will sometimes contain 

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

 phosphate which was once soluble but has now lost 

 its character. The diminution of soluble phosphate 

 during the storing of superphosphates chiefly occurs 

 when the m<inure has been made from materials con- 

 jiaining ferric oxide and alumina^ and is due to the 



