s being bent
more than are those in the other part of the spectrum. The Tyndall
phenomenon is similar in its effect in making the tiny particles of the sol
visible to the illumination of the dust particles in the air of a darkened
room when a ray or narrow beam of light passes through it. In a true
molecular solution, the particles are too small to be visible by this mode
of illumination.

=Other Optical Properties.=--Sols are generally translucent and opalescent;
many of them are highly colored, some of the sols of gold, platinum and
other heavy metals possessing particularly brilliant colors. In general,
metallic suspensoids are red, violet, or some other brilliant color; while
inorganic suspensoids are bluish white, and emulsoids generally blue to
bluish white.

=Formation of Froth, or Foam.=--Colloidal solutions, especially those of
the natural proteins, fats, glucosides, gums, and the artificial soaps,
have a strong tendency to produce froth, or foam, when shaken; this being
due to the enormous surface tension resulting from the finely divided
condition of the dispersed material.

=Low Osmotic Pressure.=--All colloidal solutions exhibit a very low osmotic
pressure; the freezing point of the dispersion medium is lowered only very
slightly and its boiling point is only very slightly raised by the presence
of the dispersed particles in it.

=Precipitation by Electrolytes.=--Sols of all kinds are precipitated, or
caused to form gels, by the addition of electrolytes, since these cause a
disturbance of the electric charge on the dispersed particles, to which the
colloidal condition is due. In the case of most emulsoids and of a few of
the suspensoids, this change converts the mass into a stiff gel; but in
that of many of the metallic suspensoids, the dispersed particles are
gathered together into larger aggregates, which settle out of the liquid in
the form of a gelatinous precipitate. In the latter case, the effect is
usually spoken of as "precipitation" by electrolytes; while in the former,
it is called "coagulation," or "gelation."

The effectiveness of the various electrolytes in bringing about this change
is proportional to their valency; bivalent ions are from 70 to 80 times,
and trivalent ions about 600 times as effective as monovalent ions.

Further, all sols in which the dispersed particles carry a charge of the
opposite sign likewise precipitate both suspensoids and emulsoids.

A demonstration of the pre