216 CHEMISTRY OF PLANT LIFE 



may be introduced into the two solutions. In this, adsorption 

 differs from chemical action, as the latter is proportional to the 

 concentration of the reacting material which is present in the solu- 

 tion. (6) Adsorption out of different liquids, by the same adsorb- 

 ing body, is different in amount. It is usually greatest out of 

 water. Hence, many dyes may be adsorbed out of water by char- 

 coal, porous clay, etc., and if the latter be then introduced into 

 alcohol, or ether, the dye goes back into solution in these latter 

 liquids. This process is often used industrially and in the labora- 

 tory for the purification of such substances when they are present 

 in impure form in aqueous solutions, (c) Selective adsorption. 

 Different substances are not adsorbed out of the same solvent 

 to the same extent by the same adsorbing agent. Advantage is 

 taken of this fact when filter paper is used in the so-called " capil- 

 lary analysis " to separate different dyes, or other colloidal mate- 

 rials which have been stained different colors, into alternate layer 

 by reason of the different rate at which the paper adsorbs the dif- 

 ferent materials out of the solution in which they are present 

 together, (d) Similar relative adsorption by different adsorbing 

 agents. Although different adsorbing agents may possess varying 

 active surfaces and hence, variable adsorbing power, or rates of 

 adsorption, they adsorb the same relative amounts of different 

 materials; i.e., if substance A adsorbs more of X than it does of Z 

 out of any given solution, substance B will likewise adsorb more of 

 X than of Z out of the same solution; although the actual amounts 

 adsorbed by A may be quite different from those adsorbed by B. 



CATALYSIS AFFECTED BY THE COLLOIDAL CONDITION 



The velocity of a chemical reaction is the net result of opposing 

 influences. It is directly proportional to the chemical affinity of 

 the reacting bodies and inversely proportional to the so-called 

 " chemical resistance." The first factor, chemical affinity, is not 

 easily measured, as it depends upon both the mass of the reacting 

 molecules, atoms, or ions, and their attraction for each other. 

 But if, as the result of chemical affinity, a reaction takes place, it is 

 evident that the time required for its completion (which measures 

 the velocity of the reaction) is made up of two separate periods. 

 The first is the time required for the reacting molecules to come into 

 contact; and the second is that required for the molecular rear- 



