E. J. Russell and J. A. Prescott 



93 



24 hour extract to the instantaneous nitric acid extract, which contained 

 a high amount of P2O5; no precipitation occurred. That the soil and 

 not the extract was responsible for the removal of the P2O5 was demon- 

 strated by adding sodium phosphate to the mixture of soil and nitric 

 acid; in spite of the excess of acid some of the added P2O5 was with- 

 drawn from the solution by the soil (Table IV). 



Table IV. Amounts of P^O^ removed by soil from sodium and calcium 

 2)Jiosphates in free N/10 nitric acid 



In all cases excess of acid remained at the end of the experiment. 



In view of the excess of acid invariably present the removal of the 

 P2O5 from the solution cannot be regarded as simple chemical precipita- 

 tion. 



There remained the possibility that the action might belong to the 

 remarkable physical effects classed vaguely as absorptions in the older 

 days, and now called by the more definite name of adsorptions. 



It is well known that charcoal has the power of withdrawing certain 

 dissolved substances from their solutions. There is no evidence of any 

 chemical change in the ordinary sense of the term, and indeed the 

 quantitative relationships are quite different from those of any ordinary 

 reaction. A considerable amount of experimental work has shown 

 that the quantity of substance adsorbed by a given amount of the 

 adsorbent is not proportional to the actual amount present in the 

 solution, but to some power of this quantity : so that if 



