l substance which 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 solution. (_b_) _Adsorption out of different liquids_, by the same
adsorbing body, is different in amount. It is usually greatest out of
water. Hence, many dyes may be adsorbed out of water by charcoal, 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 laboratory 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 "capillary
analysis" to separate different dyes, or other colloidal materials which
have been stained different colors, into alternate layer by reason of the
different rate at which the paper adsorbs the different 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