144 PHOSPHATIC FERTILISERS [chap. 



phosphoric acid will come into solution ; in fact, all the 

 phosphates can be eventually completely dissolved 

 away by the solvents in question. The following table 

 (XXXVII.) shows the amount of phosphoric acid 

 extracted by a i per cent, solution of citric acid from 

 one of the Broadbalk soils manured for fifty years with 

 superphosphate, the extraction being repeated with fresh 

 solvent as soon as one portion had been saturated and 

 then removed : — 



Table XXXVII.— ioo Grms. Broadbalk Soil (Plot 7), wrrH 

 I Litre i per cent. Solution of Citric Acid. 



Very similar results have been obtained when 

 manures are treated in the same manner, and they 

 may be taken to show that a single extraction of any 

 solvent of the kind proposed does not dissolve the 

 whole of a particular compound of phosphoric acid, 

 which may be thereupon reckoned as distinct in kind 

 from the rest of the phosphates left unattacked. This 

 mode of attack with weak solvents should be regarded 

 as affording only empirical figures to assist the analyst 

 in forming a judgment of the manure; and the condi- 

 tions of making the solution, such as time, shaking, 

 relative amounts of solvent and substance, must be 

 strictly defined. Furthermore, the only solvent which 

 has any a priori justification is a solution of carbon 

 dioxide, such as does the work in the soil ; the acids of 

 the cell sap, to resemble which citric acid was taken, 

 have been shown to experience no direct contact with 

 the soil particles. 



