ARTIFICIAL AND CONCENTRATED MANURES. 91 



mixing them with sulphuric acid, which changes the 

 tricalcic — three-lime — into the monocalcic — one-lime 

 — form, or the insoluble into the soluble form. In this 

 process, two of the three parts of the lime, combined 

 with the phosphoric acid to form the insoluble phos- 

 phate, are removed and united to sulphuric acid, forming 

 sulphate of lime, leaving one part of lime combined with 

 phosphoric acid, which is the "monocalcic" or "super- 

 phosphate." 



A pure superphosphate is, therefore, a mixture of a 

 soluble phosphate, and of sulphate of lime, or gypsum. 



Soluble Phosphoric Acid. — Nearly all workable pro- 

 ducts containing phosphate of lime are capable of being 

 converted into an " acid phosphate " or a " superphos- 

 phate." The soluble phosphoric acid thus obtained is a 

 definite compound, and is identical in composition, from 

 whatever source derived. 



The term "phosphate" is applied to any material con- 

 taining, as its chief constituent, phosphoric acid. The 

 term " superphosphate " is applied to any material con- 

 taining soluble phosphoric acid as its chief constituent. 



Thus we have the phosphates already described, which 

 when treated with sulphuric acid are converted into 

 superphosphates, as bone superphosphate, South Carolina 

 rock superphosphate, bone-black superphosphate, bone- 

 ash superphosphate, and Florida rock superphosphate. 

 Care should be taken not to confound the terms "phos- 

 phate " and " superphosphate." They are, as we have 

 seen, very different both in composition and character. 



Composition of Superphosphates. — Superphosphates 

 differ in their content of phosphoric acid according to the 



