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goo. When soluble phosphates cost 3$. per unit (t. e. for 

 I per cent, per ton), a ton of super containing 40 per cent, of 

 Ca 3 P 2 O 8 in the soluble form ( as CaH 4 P 2 O 8 ) would be 

 valued at 40 x 3 s. = 6. The price of insoluble phosphates 

 in bones is 5 to 10 per ton, of soluble phosphates 15 

 per ton and of reduced or dicalcic phosphate (Ca2H 2 P2O 8 ) 

 10 to 12 per ton. 



901. Super kept for a long time is reduced in its solubi- 

 lity in water by 5 per cent, or more of the total phosphates. 

 This reduction takes place chiefly in supers containing A1 2 O 8 

 and Fe2O 3 . The ferric and alluminic phosphates and the 

 tricalcic phosphates formed are insoluble in water. Dical- 

 cic phosphate ( Ca 3 P 2 O 8 + CaH 4 P 2 O 8 = 2Ca 2 H 2 P 2 O 8 ) is 

 not altogether insoluble in water. In the precipitated phos- 

 phates of glue manufactures, gelatine works and those manu- 

 factured from basic iron slag of iron foundries, the phosphates 

 occur as dicalciurn phosphate. This form of phosphate occurs 

 in some guanos also. It is more soluble in water charged 

 with C0 2 and in saline solutions. Dicalcic phosphate threfore 

 is found to be of equal manurial value with monocalcic 

 phosphates in certain soils. In sandy soils and soils contain- 

 ing little lime, dicalcic phosphate (called also ' reduced' or 

 f retrograde' phosphate) gives better result than monocalcic 

 phosphate, and even tricalcic phosphate in a finely divided 

 state is sometimes found to give better result in such soils 

 than the soluble monocalcic phosphate. The reason for this 

 is, that when soluble phosphate comes in contact with soil, the 

 phosphate is immediately precipitated (but not in sandy soils 

 or in soils deficient in lime) in a gelatinous form, in which 

 state it is extremely soluble though not easily washed out, 

 and it gets diffused through the soil very easily and quickly. 

 It gets gradually reduced and converted into dicalcic phos- 

 phate, afterwards into tricalcic phosphate and eventually into 

 phosphates of iron and aluminum. But in sandy soils and those 

 containing little lirne, the gelatinous precipitation does not take 



