ABSORPTION BY SOILS 67 



The phenomena of selective absorption suggest the important 

 part which surfaces play in modifying and changing chemical 

 reactions. 1 For instance, Becquerel 2 observed that a solution of 

 copper nitrate or cobalt chloride diffusing from a cracked test- 

 tube placed in a solution of sodium sulphide, led to the form- 

 ation of the corresponding sulphide, but in the crack the metal 

 itself was precipitated. Experiments of Graham 3 show that 

 when a solution of silver nitrate is percolated through charcoal, 

 not only is there a selective absorption as is shown by the perco- 

 late containing free acid, but there is a chemical reaction involved, 

 since the silver is deposited in metallic spangles in the interstices 

 of the absorbent. Graham has shown, and since his time others, 

 that often metals can be separated from solutions of their salts 

 by such absorbents as charcoal. Spring 4 has shown that at 

 bounding surfaces of dilute solutions, chemical action is 

 increased. 



It has been shown that the amount and kind of surface has a 

 marked influence on the decomposition of hypochlorous acid, 

 carbon dioxide, phosphine, arsine, and other compounds. Meyer 

 and his associates, as well as a number of other investigators, 

 have shown that the character of the surface of the containing 

 vessel greatly affects the combination of hydrogen and oxygen. 

 Many reactions have been investigated by van't Hoff, who con- 

 cludes that both the nature and amount of surface exposed have 

 an influence. The inversion of sugar is affected by the nature 

 of the walls of the containing vessel, and its reduction by Fehl- 

 ing's solution is affected both by the walls of the vessel and the 

 amount of cuprous oxide formed in the reaction. Alteration in 

 the character as well as degree of a number of reactions by having 

 them take place in capillary spaces has been observed by 



1 For references to the literature see, Bull. No. 30, Bureau of Soils, 

 U. S. Dept. Agriculture, p. 61 et seq. 



2 Note sur les reductions metalliques produites dans les espaces capil 

 laires, par M. Becquerel, Comptes rendus, 82, 354-356 (1876). 



3 Effects of animal charcoal on solutions, by T. Graham, Quart. Jour. 

 Sci., 1, 120-125 (1830). 



4 Uber eine Zunahme chemischer Energie an der f reien Oberflache 

 fliissiger Korper, von W. Spring, Zeit. physik. Chem., 4, 658-662 (li 



