ntain less than 1 per cent of the solid dispersed
through the liquid. In fact, extreme dilution is one of the necessary
conditions for suspensoid-formation.

Emulsoids are much more easily produced than are suspensoids. The property
of forming an emulsoid seems to be much more definitely a characteristic of
the substance in question than does the formation of sols from solids
which, under other conditions, may form true solutions. This difference may
be due to the fact that the liquids which easily form emulsoids (usually
those of organic origin) have very large molecules, so that the transfer
from molecular to colloidal condition involves much less change in such
cases than it does in the case of solid (inorganic) substances of
relatively low molecular weight. This view of the matter is further borne
out by the fact that solids which have very large molecules (generally of
organic origin) take on the colloidal form much more readily than do those
of small molecular size.

At the same time, a given liquid may form a true emulsoid when introduced
into one other liquid and a true solution when introduced into another.
Thus, soaps form emulsoids with water (true hydrosols); but dissolve in
alcohol to true solutions, in which they affect the osmotic pressure, the
boiling point of the liquid, etc., in exactly the same way that the
dissolving of other crystalloids in water affects the properties of true
aqueous solutions. Again, ordinary "tannin," when dissolved in water,
produces a sol, which froths easily, is non-diffusible, etc.; but when
dissolved in glacial acetic acid, it produces a true solution.

The concentration of the disperse phase may be much greater in the case of
emulsoids than it can be in suspensoids. This is probably because the
dispersed particles do not carry so large an electric charge and are not in
such violent motion.

GEL-FORMATION

The one property which most sharply distinguishes sols from true solutions
is their ability to "set" into a jelly-like, or gelatinous semi-solid,
mass, known as a "gel," without any change in chemical composition, or
proportions, of the two components of the system. In the gel, the two
components are still present in the same proportions as in the original
sol; but the mixture becomes semi-solid instead of fluid in character.
Thus, an agar-agar sol containing 98 per cent of water sets into a stiff
gel; while many other gels which contai