218 SOILS AND MANURES 



apart from gypsum, may amount in some cases to from 

 10 to 20 per cent. 



The value of superphosphates obviously depends upon 

 the proportion of soluble phosphates they contain. The 

 standard quality is one which contains about 11*5 per 

 cent, of phosphoric acid in a soluble state, equal to 

 25 per cent, of phosphate of lime rendered soluble. 

 Analyses of superphosphates are generally expressed in 

 this form. The phosphate of lime here referred to is, 

 of course, tricalcic phosphate, and the expression " phos- 

 phoric acid" means the oxide (P 2 5 ), not the hydrate 

 to which the name is more properly applied. It is 

 an unfortunate misnomer, and occasionally leads to 

 some confusion, but as it is in general use it must be 

 accepted. It is noteworthy that neither the P 2 05 nor 

 the tricalcic phosphate are present in the superphosphate 

 as such. The former occurs as a compound, and the 

 latter not at all, but when the quantity of P 2 5 has been 

 found it is easy to calculate how much tricalcic phosphate 

 it is equivalent to. 



Superphosphates of higher grade, i.e., containing up 

 to 35 per cent, or even more, of soluble phosphates are 

 sometimes prepared. Probably owing to the competition 

 of other manures rich in phosphates, they have become 

 much more common of late years. A very high grade 

 product called "double superphosphate," which contains 

 about 80 per cent, of soluble phosphate, has been manu- 

 factured chiefly in Germany and the United States. 

 Apart from practical difficulties it is obvious that such a 

 product could not be obtained, even theoretically, in the 

 ordinary way. In the special process employed for this 

 purpose the purest native phosphates are selected, and 

 treated with excess of sulphuric acid so as to liberate 

 the phosphoric acid ; the calcium sulphate is removed, 

 and more tricalcic phosphate added to the free phosphoric 



