Man's physical universe

xanabras

726 MAN'S PHYSICAL WELFARE

becomes visible when passed through a colloid.

we have of recognizing the colloidal state.

This is the best method

Fig. 309. TheTyndall effect; under

King Solomon's Temple, Jerusalem.

(Courtesy of Ward Drury.)

Colloids cannot be identified

by chemical analysis, for the only relation between chemical constitution

and colloids is that the

more complex the compound is, the

more likely it is to occur in this

state. Qualitatively, colloids may

be recognized by their illumination

with a powerful beam of light.

The

tobacco smoke in the upper portion

of a motion-picture theater is thus

illuminated by the light from the

projection booth.

3. Electrical Charge. Colloidal

particles are electrically charged.

Some colloids, such as a ferric hydroxide

suspensoid or colloidal copper,

are positively charged, while

others, such as an arsenic sulfide

suspensoid or starch and oil

emulsions,

are negatively charged. This electrical charge on colloidal

particles may readily be shown by electrophoresis — i.e., the movement

of the colloidal particles to the oppositely charged pole. Electrophoresis

(sometimes called cataphoresis) has many useful applications;

for example, the Cottrell smoke-precipitation process employs two highly

Fig. 310. Left, electric precipitator at the plant of the Portland Gas and Coke

Company, Portland, Oregon; electrical current off. Right, electrical current

turned on. This precipitator collects lampblack, tar mist, and fumes. The estimated

cost of the smoke nuisance to the American people is $2,500,000,000. This

cost is divided between wastage of fuels (smoke is the result of incomplete combustion),

cleaning, laundering, dry cleaning, and depreciation of wearing apparel

and house furnishings. The effect on the health cannot be estimated. (Courtesy

of the Western Precipitation Company.)

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