193 



composition. To overcome this slow action and render it immediatelj 

 available, it is treated with sulphuric acid, and the resulting product is 

 " Dissolved Bone Black," a most valuable and available form of phos- 

 phoric acid, CDntaining usually about 16 per cent, to 18 per cent, solu- 

 ble phosphoric acid. 



Acid Phosphate. — Formerly Bone Black, Bone Ash, or Ground 

 Bones were used for treatment with sulphuric acid, but the demand be- 

 coming greater than the supply, recourse was had to the newly dis- 

 covered beds of mineral phosphates and to-day much the larger quan- 

 tity of soluble phosphates are manufactured from the mineral phosphates 

 obtained in various parts of the world ; South Carolina, Florida and 

 Tennessee each furnishing large quantities annually. These mineral 

 phosphates, after being cleaned from adhering dirt, sand, etc., are 

 ground to a fine powder by the " roller" process, and sold on the market 

 as floats," or are treated with sulphuric acid and converted into solu- 

 ble phosphates which are called " Acid Phosphates," " Superphosphates," 

 or " Dissolved Bones." T he amount of soluble phosphoric acid which 

 they carry varies from 10 per cent, to 20 per cent., according to the 

 purity of the phosphate from which they were made. If phosphoric 

 acid is used to dissolve the bone black or mineral phosphates, the re- 

 sulting product will be much higher in soluble phosphoric acid. In 

 this way are made the so-called Double Superphosphates" which are 

 sometimes found on our markets, and which contain from 40 per cent, 

 to 50 per cent, of soluble phosphoric acid. 



In the treatment of either bones, bone black, or mineral phosphates, 

 with sulphuric acid, if enough of the latter be used, all of the sulphuric 

 acid will become soluble in water. The above substances are tricalcic 

 phosphates, containing three molecules of lime to one of phosphoric acid, 

 and are soluble only in acids. They are, therefore, slowly soluble in 

 soil water, and are not a readily available form for plant food. 



Treated with sulphuric acid, they lose two molecules of lime, which 

 are appropriated by the acid, and recover in exchange two molecules of 

 water, leaving one molecule of lime combined with one molecule of 

 phosphoric acid and two molecules of water. The resulting products 

 are now monocalcic phosphates, and are very soluble in water, and there- 

 fore readily available to plants. 



Should a deficiency of sulphuric acid be used in the manufacture of 

 soluble phosphates, a quantity of the tricalcic phosphates will be left 

 unacted upon by the acid while another portion will be converted into 

 monocalcic or soluble phosphate. 



If this mixture be permitted to remain in bulk any length of time, a che- 

 mical reaction takes place, by which a dicalcic phosphate is formed, which 

 is insoluble in water, but soluble in certain salts, notably the citrate of am- 

 monia, and is called reduced," or " reverted," phosphates. The change 

 is as follows : — One part of insoluble phosphate, containing three molecu- 

 les of lime, and one part of phosphoric acid, reacts upon another part of 

 soluble phosphate containing one molecule of lime and one of phosphoric 

 acid to form two parts of "reduced," or " reverted," phosphate, which con- 

 tains two molecules of lime and one of phosphoric acid. These three forms 

 of phosphoric acid are found in every " Acid Phosphate," or fertiliser, con- 

 taining acid phosphate. They are usually denominated as " soluble," 



reverted," and "insoluble/^ and to the Chemist are known as water 



