ually
exists in a different form than originally, having undergone some chemical
change during the peptization, and the coagulation and peptization cannot
be repeated, that is, the process is not a definitely reversible one.

=Importance of Gel-formation.=--From the physiological point of view,
gel-formation is undoubtedly the most important aspect of colloidal
phenomena. In the first place, the ability to absorb and hold as much as 80
to 90 per cent of water in a semi-solid structure is of immense
physiological importance. In no other condition can so large a proportion
of water, with its consequent effect upon chemical reactivity, be held in a
structural, or semi-solid, mass. But a vastly more significant feature of
the conditions supplied by the gel lies in the fact that the non-water
phase, or phases, of the system are spread out in a thin film, or membrane,
thus giving it enormous surface as compared with its total volume. This
effect is easily apparent if one thinks of the enormous surface which is
exposed when a tiny portion of colloidal soap is blown out into a
"soap-bubble" several inches in diameter. This condition brings into play
all the phenomena resulting from surface boundaries between solids and
liquids, liquids and liquids, liquids and gases, etc., from surface
tension, surface energy, etc. Among these effects may be cited those of
adsorption, increased chemical reactivity due to enlarged areas of contact,
permeability and diffusion, etc., the importance of which in the vital
phenomena of cell-protoplasm will be discussed in detail in the following
chapter.

GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS

=Non-diffusibility.=--The most characteristic property of all sols is the
failure of the suspended particles to pass through a parchment, or any
similar dialyzing membrane.

=Visibility under the "Ultramicroscope."=--The particles of a sol, in
contrast with the molecules of a true solution, are visible as bright
scintillating points under the ultramicroscope. This is a modification of
the type of dark-field illumination of the ordinary microscope, as applied
to microscopic studies, in which the solution to be studied is contained in
a small tube or box of clear glass which is mounted on the stage of an
ordinary microscope and instead of being illuminated from below by
transmitted light is illuminated by focusing upon it the image of the sun,
or of some other brilliant source of light s