IV.] COMPOSITION OF SUPERPHOSPHATE 125 



a really dry product that would run easily through a 

 drill, but all necessity for that process has passed away 

 since high grade phosphates containing but little iron 

 or alumina have been available. The more gypsum the 

 finished " super " contains, the drier and more friable 

 the powder, hence the former value of Somme phosphate 

 for mixing purposes, because its impurity was almost 

 wholly calcium carbonate. The objection of the super- 

 phosphate maker to oxides of iron and aluminium in 

 the raw material arises from two sources — the bad 

 mechanical condition of the resulting compound and 

 its tendency to revert. In calculating the amount of 

 sulphuric acid to use, a little calcium phosphate is 

 always left undecomposed, because free phosphoric 

 and sulphuric acids would injure the mechanical con- 

 dition of the fertiliser. This phosphate of lime left 

 unattacked will always slowly combine with some of 

 the acid phosphate to form two molecules of the 

 intermediate di-calcium phosphate, which is thus known 

 as reverted or retrograde phosphate — 



CaH^PaOg + CagPgOg = 2Ca2H2P208. 



Since this last compound is insoluble in water, freshly 

 made superphosphate always contains a little more 

 phosphoric acid soluble in water than it does after it 

 has been stored for some time, and as in England this 

 fertiliser is valued only on the basis of its water soluble 

 phosphoric acid, to this extent it deteriorates on storage. 

 The deterioration is more pronounced when the 

 raw material contains oxides of iron and aluminium, 

 because both of these substances will slowly react with 

 acid phosphate to form insoluble phosphates of iron or 

 aluminium. Even if these oxides have been attacked 

 by the sulphuric acid to form ferric or aluminium 

 sulphates, or if the iron and aluminium were originally 



