the prevention of the smoke nuisance by electric deposition;
the purification of sewage; the manufacture of soaps; the manufacture of
butter, cheese, and ice cream; fruit jellies, salad dressings, etc. This
list could be extended to a great length, but is already long enough to
emphasize the very great importance and practical value of colloidal
phenomena in daily life.

NATURAL COLLOIDAL PHENOMENA

Many of the phenomena of nature are colloidal in character. These may be
observed in the mineral, the animal, and the vegetable kingdoms. Here,
again, a lengthy discussion of the nature of these phenomena would be out
of place in this connection, and a few typical examples will serve to
illustrate the general importance in nature of this property of matter.

In the soil, the following properties are easily recognizable as definite
colloidal phenomena: water-holding capacity of clays, silts, loams, etc.;
adsorption (or "fixation") of soluble plant foods so that they are not
readily leached out of the soil by drainage; flocculation and
deflocculation of clay, etc.

In the animal body; the contraction of muscles, the conveyance of nerve
stimuli, etc., are undoubtedly accomplished by colloidal changes; and the
existence of insoluble casein and fat in colloidal form in milk insures the
proper nourishment of the young of nearly all species of animals.

In both plants and animals, as will be pointed out in the following
chapter, practically all the vital activities of the cell protoplasm are
definite manifestations of colloidal phenomena. Enzymes perform their
catalytic functions by reason of their colloidal form. Proteins exist in
colloidal form and are the seat of all vital functions. The regulation of
the passage of materials into and out of the cell is governed by minute
changes in the electrolyte concentration, etc., which produce enormous
changes in the colloidal character of the protoplasm.

It is apparent, therefore, that the study of the colloidal condition of
matter and of the properties arising out of it is of immense importance to
the biochemist. No other single field is capable of yielding more fruitful
results to the plant physiologist, in his studies of the response of plants
to changes in their environment, or of the mechanism by which plants
perform their internal functions.

References

BECHHOLD, H., trans. by BULLOWA, J. G. M.--"Colloids in Biology