MODERN HIGH FARMING. 83 



The transformation undergone by the mineral will be understood, 

 by calling to mind that phosphates of lime are tribasic, that is to 

 say, composed of three parts of lime to one part of phosphoric 

 acid, and that in this state they are totally insoluble in pure 

 water. 



When the sulphuric, which is the strongest acid, is brought into 

 contact with the powder, it drives away all the carbonic, fluoric and 

 silicic acids, and, depriving the phosphoric acid of two of its equiv- 

 alents of lime, forms them into sulphate (gypsum). The phosphoric 

 acid, thus left with only one equivalent, becomes mono, or rneta- 

 phosphate, which is soluble in water. It follows from this that 

 unless a sufficiently large proportion of sulphuric acid be used to 

 completely effect this transformation, a certain quantity of the phos- 

 phates will retain their primitive tribasic state, and be shown as 

 insoluble when the manufactured article is submitted to analysis. 



As a final result of the above chemical decomposition, the differ- 

 ence in the weight of the raw materials and the manufactured 

 product will be considerable, but should, nevertheless, not exceed 

 G^ or 7 per cent. This loss, however, chiefly arises from the 

 disappearance of the gaseous elements driven away by the sul- 

 phuric acid, and in no way affects the quantity of phosphoric 

 acid. Thus, for example, presuming the proportions used, to have 

 been 200 pounds each of sulphuric acid and tribasic phosphate of 

 lime, we should find the weight of the superphosphate to be 372 

 pounds, and that of the mono-phosphate to be equivalent to 120 

 pounds of tribasic phosphate of lime, or rather more than 30 per 

 cent. 



Farmers believe the word " soluble " to mean that the acid phos- 

 phates, when put into the ground, at once dissolve, and hence that 

 their use is much more advantageous than that of phosphates in 

 their raw state. 



The time has come for a correction of this misapprehension and 

 for a recognition of the true phenomenon ; because, if farmers are to 

 be scientific, they must at least understand something of the elemen- 

 tary chemistry of their operations. 



