28 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
Acid phosphate is sold on the basis of its so-called available phos- 
phoric acid, and has a value of 40 to 56 cents per unit. The marketed 
product contains from 14 to 21 per cent of phosphoric acid, depending 
on the raw material used in its manufacture. 
Double acid phosphate is made by treating phosphate rock with 
sufficient diluted sulphuric acid to produce free phosphoric acid, and 
then using the phosphoric acid thus obtained to decompose a fresh 
batch of rock. The final product contains from two to three times 
as much phosphoric acid as ordinary acid phosphate, and is very 
useful in the making of concentrated fertilizers. 
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WASHINGTON : GOVERNMENT PRINTING OFFICE : 1914 
