COMMERCIAL MANURES. 205 



To be entitled a really good article, it should coutain not less than 

 thirty per cent, of phosphates, and about one half this amount in 

 a condition to be dissolved at once, and the other half as fast as it 

 may be required by the plants. Such an article manufactured on 

 a large scale can be afibrded at the present time for about three 

 cents per pound, at wholesale. At this price, and also with the 

 additional cost of a pretty long transportation, it will, in a great 

 majority of cases, where judiciously used, pay a large profit, both 

 in increased quantity, better quality," and earlier maturity of the 

 crops grown. 



The aim of the manufacturer ought to be, to give the largest 

 amount possible of fertilizing constituents, in the best relative pro- 

 portions to each other, at the smallest cost. Whether all do so is 

 quite another matter. 



Let me here offer a word regarding the chemistry of the manu- 

 facture of superphosphates. What is commonly called phosphate 

 of lime — or as chemists term it, tribasic phosphate of lime, or more 

 recently, tricalcic phosphate, — is a combination of three equiva- 

 lents of lime with one of phosphoric acid, hence the name 

 "tribasic," — three of base with one of acid. This is the form in 

 which it commonly occurs in nature. It dissolves in water very 

 slowly and with difficulty, and hence is called insoluble. To con- 

 vert this into soluble phosphate, or as known to chemists, mono- 

 phosphate of lime, or monocalcic phosphate, it is needful to take 

 away from tribasic phosphate tivo of its three equivalents of base, 

 (that is to say, of lime,) leaving one of lime in combination with 

 one of phosphoric acid. Combined in this proportion it is very 

 easily soluble. Indeed, if made dry, by artificial means, it has 

 such an attraction for water that it will take it from the air and 

 become wet again. This change from a tribasic to a monobasic 

 phosphate, or from insoluble to soluble, is effected by the agency 

 of sulphuric acid, which has a stronger affinity for lime than phos- 

 phoric acid has, "and so the sulphuric acid takes away lime and 

 combines with it, forming sulphate of lime, which is the same 

 thing as gypsum. We do not put sulphuric acid with tricalcic 

 phosphate for the sake of making gypsum. By no means ; for 

 we can get enough of this ready made from sources in nature 

 cheaper than we can make it. But we do it for the sake of the 

 other result, viz., the conversion of insoluble phosphate to soluble. 



Thus you see that when only pure tricalcic phosphate and sul- 

 phuric acid are put together you have a mixed result, a compost, 



