306 THE COLLOIDAL STATE 



these substances in a soil. For this purpose a quantity of the 

 soil is shaken up with the dye solution in a cylindrical vessel ; 

 on settling, the heavier particles sink to the bottom, and a 

 band of the dyed soil constituents is formed on the surface. 



Thermodynamical considerations, coupled with experi- 

 mental measurements, show the fact that true adsorption 

 takes place according to well-defined mathematical laws which 

 enable one to decide definitely whether a certain phenomenon 

 is due to physical adsorption or to chemical reaction ; thus, it 

 has been found that a relatively larger amount of the total 

 substance in solution is withdrawn from a dilute than from a 

 strong solution. 



ENZYME ACTION OF COLLOIDS. 



Associated with this enormous development of surface 

 there is, of course, a corresponding development of surface 

 energy,* which no doubt, in part, explains the remarkable 

 catalytic activity exhibited by colloidal solutions of the 

 metals. 



Bredig f and his collaborators have shown that a colloidal 

 solution of platinum containing 194 grams of metal (i.e. 

 I gram atom) in 70,000,000 litres of water, or a colloidal 

 solution of gold containing 197 grams of metal in 1,000,000 

 parts of water, are still able to produce a distinct accelerating 

 influence on the decomposition of hydrogen peroxide into 

 water and oxygen. 



It has long since been known that metallic platinum, more 

 especially the variety known as spongy platinum, when left 

 in contact with hydrogen peroxide induces the decomposition 

 of this substance into water and oxygen, and Berzelius,J as 

 long ago as 1836, pointed out an analogy between this cata- 

 lytic action of platinum and the action of an insoluble ferment, 

 such as yeast on sugar. 



This suggestion has since been borne out by a number of 

 examples of chemical changes which could be effected equally 

 well either by means of finely divided platinum or by a 

 ferment, e.g. the oxidation of alcohol to acetic acid by Myco- 



*Ostwald: " Z. physik. Chem.," 1897,23, 172. 



f Bredig: " Anorganische Fermente," Leipzig, 1901, p. 96. 



Berzelius : " Jahresber.," 1836, 13, 237. 



