E. J. Russell and J. A. Prescott 



10^ 



them to go entirely: not till the soil is ignited is the property lost 

 altogether, and the whole of the added P2O5 recovered : 



The acids adsorbed. Although we have confined ourselves hitherto 

 to PgOg we have evidence that the phenomena are general and that other 

 acids are adsorbed in the same manner, though in varying degrees. 

 Thus oxalic acid is adsorbed in the presence of nitric acid, giving a 

 curve of exactly the same nature as that for phosphoric acid. (See p. 124.) 



Citric acid is also adsorbed but the analytical difficulties proved 

 so considerable that we have been unable to obtain sufficiently reliable 

 results to allow of the drawing of a curve. 



On the other hand, we could find no evidence that either hydro- 

 chloric or nitric acid is adsorbed to any notable extent. 



Thus, it appears that the acids most diminishing the adsorption 

 of phosphoric acid, viz. citric and oxalic acids, are themselves freely 

 adsorbed : while the acids which do not so greatly diminish the adsorp- 

 tion, viz. hydrochloric and nitric acids are not adsorbed. 



We have examined various possibilities, but the simplest and most 

 in accordance with the facts is that citric and oxalic acids satisfy the 

 adsorption capacity of the soil better than P2O5 and thus displace some 

 of it into the solution, while dilute hydrochloric and nitric acids will 

 not, so that much of the P2O5 still remains adsorbed. We therefore 

 enquired whether the addition to nitric or hydrochloric acids of sub- 

 stances known to be adsorbed by the soil would increase the net solvent 

 action of these acids by displacing the adsorbed P2O5. The results 

 of numerous trials are recorded in Table IX. None of them was 

 successful. 



