PHOSPHATIC MANURES 143 



the Continent the substance is generally 

 bought on the basis of a certain price per 

 pound of phosphoric acid, in this country 

 valuation is invariably made on the system 

 of units. In superphosphate there is gener- 

 ally a small percentage of phosphate that 

 has not been dissolved by acid, but this is 

 disregarded for the purposes of valuation. 

 It is only the phosphate which is soluble in 

 water that is taken account of, the percentage 

 of such soluble phosphate usually lying 

 between 25 and 30 per cent., though it may 

 run both higher and lower. Assuming a 

 sample containing 30 per cent, of soluble 

 phosphate to be offered at 2, 15s. a ton, 

 the value of a unit will be found, by dividing 

 2, 15s. by 30, to be Is. lOd. Utilizing 

 this unit-value we thus find that a super- 

 phosphate containing 28 per cent, of soluble 

 phosphate will be worth 2, 11s. 4d. a ton, 

 while a superphosphate containing 35 per 

 cent, of similar phosphate will have a value 

 of 3, 4s. 2d. per ton. If the slag is bought 

 upon a basis of citric solubility, it is a simple 

 matter to determine the corresponding value 

 per unit, and such value can be similarly 

 utilized in determining the price that should 

 be paid for varying qualities bought on 

 this basis. 



