TES:

[6] 1 mu is one-thousandth of a millimeter; 1 mu mu is one-thousandth of a mu, or
one millionth of a millimeter.

It is recognized by all students of these matters that it is not possible
to draw a sharp dividing line between these three types of conditions, and
that they shade into each other, in many cases; but in general it may be
said that a colloidal solution is one in which the dispersed particles are
usually between 5 mu mu and 200 mu mu in diameter, are difficultly or not at all
diffusible through the membrane of a simple dialyzer, cannot be filtered
out of solution, do not settle out under the action of gravitation, and are
visible only under the "ultramicroscope"; and one which has certain
peculiar optical, osmotic, and other physical and chemical properties.
Since colloidal particles are very minute in size, they possess very large
relative surface areas as compared with their total mass or volume, very
high surface tension, and a relatively high surface energy as compared with
their total, or molecular, energy. These properties bring into play, in a
substance which is in the colloidal condition, in a remarkable degree, all
the phenomena which are associated with surface boundaries between solids
and liquids, liquids and gases, etc.

The properties arising out of the colloidal condition are of such
tremendous importance in connection with the vital phenomena exhibited by
cell protoplasm that it is necessary to give some detailed consideration to
them here. Many large volumes dealing with this condition of matter have
been written, and it is very difficult to condense even the most important
facts concerning it into a few pages, but an attempt has been made to
present in this brief summary the most essential facts and principles
involved in the colloidal phenomena.

NOMENCLATURE AND CLASSIFICATION

Colloidal mixtures may exist in two different forms: one, in which the
mixture is fluid and mobile, like a true solution, is known as a "sol"; and
the other, which is a semi-solid, or jelly-like, form, is known as a "gel."
Sols may be easily converted (or "set") into gels, by changes of
temperature or of the electrolyte content, or by changes in the
concentration of the mixture, etc., and in most cases gels can be converted
again into sols. In some cases, however, gel-formation is irreversible, the
gels are permanent and cannot be changed back again into sols by any known
change in