Man's physical universe

xanabras

FOODS 729

to particles which are visible under the microscope without the use

of the Tyndall effect, it would appear that they could be prepared by

combining many molecules or by breaking up larger particles.

When kerosene and a soap solution in water are violently shaken,

the kerosene divides into colloidal-sized particles. The whipping of

cream, beating of egg whites, or mixing of mayonnaise is the same in

principle. This process of preparing emulsoids is called emulsification.

Suspensoids may be prepared from coarser particles by grinding

them. Special grinding machines called colloid mills are available for

this purpose. A cement mill is somewhat of a colloid mill ; many of the

cement particles are colloidal in size, none of them being much larger.

Metallic suspensoids are sometimes prepared by forming an electric

arc between two electrodes of the metal in question.

Small particles of

the electrodes are violently disrupted and go into the water as colloidal

particles. This is called Bredig's electric-arc method.

When food is digested, the coarse particles are reduced to a colloidal

size or smaller. This is a chemical process aided by the pepsin and other

digestive enzymes in the digestive juices. Any chemical process by

which larger particles are reduced to colloidal-sized particles has thus

come to be known as peptization. Thus water is said to peptize glue

when it dissolves it.

Colloidal particles may be prepared from smaller particles {i.e., the

molecules in a solution) by chemical methods; for example, colloidal

silver or gold may be prepared by mixing soluble silver or gold compounds

with a reducing agent such as tannic acid ; iron, aluminum, and

chromium salts react with hot water to form insoluble colloidal particles.

In fact, almost any insoluble precipitate formed by the reaction

between soluble

substances may be produced as a colloid by careful

control of the conditions under which it is produced.

The Study of CoUoids Is Vital.

In the preparation of India ink it is desired to keep the colloidal

particles of carbon in suspension, while in abating smoke or producing

cheese from milk, it is desired to precipitate colloidal particles. The

tanning of leather, baking of bread, dyeing of textiles, action of soap,

purification of water, and the preparation of foods are but a few of the

operations in which a knowledge of colloids is invaluable. Inasmuch as

living protoplasm is colloidal in nature, the study of colloidal phenomena

is very important in medicine, biochemistry, and the life

sciences. For example, plants obtain minerals from solutions by

dialysis (see page 246), which is the separation of molecules or ions

from larger particles by means of a colloidal membrane.

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