THE CELL 19 
object of which the form can be seen with the aid of a microscope. 
These limits are arbitrary, as there can be no sharp division be- 
tween a colloidal dispersion and a suspension. Likewise, there 
is no sharp distinction between a colloidal dispersion and a true 
solution, as some complex molecules are large enough to be 
within the limits of size given above for colloidal particles. 
A suspension is a dispersion of particles which are large 
enough to be visible with the aid of a microscope. A true 
solution is a dispersion of molecules or parts of molecules. A 
colloidal dispersion is a dispersion of particles which are larger 
than most molecules and yet too small to be seen even with 
the aid of a microscope. 
- Colloidal dispersions are not confined to dispersions of solids 
in liquids. Just as we may have an emulsion (which is a sus- 
pension of a liquid in another liquid, as oil in water), so we 
may have a colloidal dispersion of a liquid in another liquid. 
This condition is known as an emulsoid. Smoke is a disper- 
sion of a solid in a gas; a cloud is a dispersion of a liquid in 
a gas; meerschaum is a dispersion of a gas in a solid; ruby 
glass is a dispersion of a solid (gold) in a solid (glass). Proto- 
plasm is an emulsoid in which proteins are dispersed in water. 
Importance of colloidal state. Much of the importance of the 
colloidal state arises from the fact that the dispersed substances 
have enormous surfaces for the play of surface forces and for 
chemical reactions. The increase in surface when a substance 
becomes finely divided can be illustrated by the following ex- 
ample: A cubic centimeter of material in the form of a cube 
would have six sides, each with an area of 1 square centimeter. 
The cube would therefore have a surface of 6 square centimeters. 
If this cube were divided into two parts by a cut in a plane 
parallel with two sides, the surface would be increased by the 
area of two sides. The two parts would then have a total area 
of 8 square centimeters. Subdividing these parts would, of course, 
increase the surface area. If the original cube were divided into 
cubes with edges 1 millimeter long, there would be one thousand 
cubes and they would have a total area of 60 square centimeters. 

