MAJXLREti. 2yy 



ma -Hire containing its phosphoric acid in a readily avail- 

 able form. Tricalcic phosphate is practically insoluble in 

 water and only sparingly soluble in solutions of carbonic 

 acid gas : of such solutions the water of the soil may be 

 taken as a special case. By treatment with sulphuric 

 acid the phosphate Ca 3 (P0 4 )o is converted into mono- 

 calcif phosphate CaH, (PO,),, which is easily soluble in 

 water. 



Briefly stated the steps in the manufacture of superphos- 

 phate are as follows. The raw phosphate is first reduced 

 to powder by grinding. It is then mixed in special vessels 

 with a calculated amount of "chamber" sulphuric acid and 

 the pasty mass resulting is run into "pits" or "dens" to 

 "set." This setting or drying is brought about largely 

 through the union of the calcium sulphate with water 

 forming gypsum as in the setting of plaster of Paris. 

 When it is complete the mass is dug into, broken up, 

 pulverised in a suitable mill and put into bags for the 

 market. 



The quantity of acid used is regulated by the nature 

 and the amount of the impurity present in the phosphate. 

 ( Carbonates, chlorides and fluorides and the oxides of iron 

 and aluminium are more objectionable impurities than 

 silica and many of the silicates. They use up acid which 

 should go to attack phosphate, and so add to the expense 

 of manufacture, which silica and the more insoluble silicates 

 do not do. It is of course evident that, apart from the 

 nature of the impurity, the more tricalcic phosphate there 

 is present, the greater the amount of acid required to 

 convert it into monocalcic phosphate. A further objection 



