ADSORPTION COMPOUNDS. 41
The laws of their motion have been studied and found to correspond to that
expected from the kinetic molecular theory. Colloidal chemistry, therefore,
has served to bring the molecular hypothesis out of the realms of theory into
the field of positive fact.
Many irreversible colloids separate out from their solutions as volumi-
nous precipitates containing a large amount of water (hydrogels). Such
hydrogels as, for example, those of ferric oxide, aluminium oxide, and silicon
oxide, can also be obtained directly by chemical precipitation from solution.
These precipitates contain a great deal more water than would correspond to
their hydroxide formulas (Fe(OH) 3 , A1(OH) 3 , Si(OH) 4 , etc.), and since the
water thus contained does not show the characteristics of chemically com-
bined water (compare the theoretical section on Hydrates preceding No. 144),
these precipitates should be designated, according to van Bemmelen, as oxide
hydrogels, or by their old name of hydrated oxides.
The change from the hydrosol to the hydrogel state is most simply brought
about by the addition of an electrolyte. Many colloidal solutions are ex-
ceedingly sensitive to electrolytes (gold solution, No. 25); the most essential
condition for the preparation of the solution must therefore be the absence
of any unnecessary electrolyte. Even reversible colloids can be separated
from their solutions by the addition of large amounts of an electrolyte; the
process is termed "salting out," and is employed in the precipitation of pro-
teins and dyestuffs (No. 24).ADSORPTION COMPOUNDS.
Adsorption is the phenomenon shown by certain substances having a
large surface (for example, wood-charcoal) of condensing gaseous or dis-
solved substances upon themselves. Adsorption is a special kind of natural
phenomenon which is quite distinct from the process of solution.
It has been shown, especially by van Bemmelen, that the typical col-
loids exhibit a particularly high power of adsorption; and also that organized
matter, such as plant fibers and decayed material like humus, possesses the
same power. The latter classes of substances are, therefore, included among
colloids, especially since in contrast to crystalloids they lack a definite form,
bounded by rigid surfaces.
Among other instances in which adsorption may occur, the following are
of especial importance. It may take place: (1) between liquids and solid
substances (as when moisture is retained by many solid substances and par-
ticularly by hydrogels); (2) between dissolved substances and solids (as in
the dragging down of dissolved salts by precipitates and in the adhering of
fertilizer salts in the soil; cf. Nos. 22 and 23); (3) between dissolved reversible
colloids and solid materials (as in numerous dyeing processes; No. 24).
The combinations so produced are known as adsorption compounds.
For many adsorption processes, definite relations exist between the con-
centration in the solution of the substance adsorbed and the composition of
the adsorption compound. Adsorption acts relatively more strongly the
less concentrated the solution (cf. No. 22). Many adsorption compounds