environmental conditions.

Depending upon whether the liquid dispersion medium is water, alcohol,
ether, etc., sols are known as "hydrosols," "alcosols," "ethersols," etc.;
and gels as "hydrogels," "alcogels," etc.

Sols in which the disperse phase is a solid are known as "suspensoids";
while those in which it is a liquid are "emulsoids." Thus, sols of most
inorganic compounds, of dextrin, gelatin, and (probably) of casein, etc.,
are suspensoids; while sols of egg-albumin, of oils, etc., are emulsoids.
The classification of these substances into suspensoids and emulsoids is,
however, more a matter of convenience than of real difference in
composition, since it is practically impossible to say whether many of the
organic substances which normally exist in colloidal form are themselves
liquids or solids, when in the non-dispersed form.

CONDITIONS NECESSARY TO THE FORMATION OF SOLS

Suspensoids differ from mechanical suspension of solids in a liquid in that
in the latter the solid particles settle toward the bottom of the mixture,
because of the effect of the attraction of gravity upon them. The rate at
which such particles settle depends upon the size and density of the
particle and the viscosity of the liquid, and can be roughly calculated
from the formula for Stokes' law for the rate of falling of a spherical
body in a liquid. This formula is

_V_ = 2_r_^2(_s_ - _s_')_g_ / 9_n_;

_V_ = velocity of the falling body, in millimeters per second;

_r_ = radius of the particle, in millimeters;

_s_ = specific gravity of the solid;

_s_' = specific gravity of the liquid;

_g_ = the attraction of gravity, in dynes;

_n_ = the viscosity of the liquid.

For example, if this formula be applied to determine the rate at which the
particles of gold of the size of those in a red gold sol would settle, if
they were in mechanical suspension in water (_r_ = 10 mu mu, or
one-ten-thousandth of a millimeter; _s_ = 19.3; _s_' = 1; _g_ = 980, and
_n_ = 0.01), it will be found that such particles will settle at the rate
of approximately 0.0146 millimeter per hour, or a little over 10 mm. (0.4
inch) per month. Hence, the settling of such particles, if in mechanical
suspension, would be measurable, although very slow. Shaking up the
_suspension_ would cause the particles to rise through the liquid again.
But in a gold sol, or _suspensoid_, which contains particles of gold of the
size used in this cal