126 PHOSPHATIC MANURES [chaP. 



present as phosphates, which by treatment with the 

 acid give rise to sulphates of these metals and free 

 phosphoric acid, reversion will still take place at the 

 lower temperatures. A mixture of ferric sulphate and 

 phosphoric acid is not stable, but will always partly go 

 back to ferric phosphate and sulphuric acid, the final 

 state of equilibrium which is attained being one with a 

 large proportion of insoluble ferric phosphate. It is 

 for this reason that so much stress is laid in the United 

 Kingdom on raw phosphates being free from iron and 

 aluminium ; on the Continent, where reverted phosphate 

 (estimated by solubility in ammonium citrate or 2 per 

 cent, solution of citric acid) is ranked as almost the 

 equal of water soluble phosphate, there is not the same 

 objection to the use of such materials. 



Superphosphate as manufactured nowadays is a grey 

 friable powder, which is made in various grades contain- 

 ing 26, 30, 35 and 40 per cent, of phosphate made soluble 

 or 12, 14, 16, and 18 respectively of phosphoric acid, 

 together with about 2 per cent, of insoluble phosphate. 

 The 26 per cent, super has for long been the standard 

 article and is still the most generally manufactured, 

 because a good dry product can be made from the 

 raw phosphates most readily available, whereas the 

 higher grades require selected materials if the result 

 is to be dry. They were, in fact, chiefly made for 

 export, but since they are now, owing to the better 

 quality of the raw phosphates available, just as cheap 

 per unit of phosphoric acid as the lower grades, there is 

 every reason for saving carriage by their purchase. 



The chief phosphates at present employed for 

 superphosphate-making are those from North Africa 

 — Tocqueville, Gafsa, Tebessa, and Algerian — Florida 

 hard rock and pebble, Christmas Island phosphate, and 

 the Aruba and Carolina deposits. The material is ground 



