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.)
FOODS 727 charged plates which cause the colloidal smoke particles of opposite charge to precipitate out. The source of the charge on colloids is known to be due, in some cases at least, to the selective adsorption of ions; if only positively charged ions are adsorbed, the colloid will then be positively charged. The Outstanding Property of Colloids Is Their Adsorptive Power. The distinctive characteristic of colloids is their very small size. particles are subdivided, the exposed surface is greatly increased in size. It has been calculated thcrt a cubic centimeter of material if ground into a powder of colloidal dimensions would expose a surface of sixty square meters. Every solid or liquid surface exerts a pull upon molecules that come near it As and tends to hold them as a permanent film only a few (and perhaps just one) molecules deep. Thus activated charcoal adsorbs so much air that when it is heated it fairly boils as the gases are driven off. This adhesion of one material to the surface of another is called adsorption. (Note that absorption is not a surface phenomenon but takes place within the liquid or solid, as the case may be.) It is to be expected, then, that colloids, with their great surfaces, possess very great adsorptive powers. Activated charcoal was used in gas masks during the World War to remove poison gases from the air before it reached the lungs. It is now used to remove objectionable odors and tastes from drinking water and to decolorize sugar solutions. Fuller's earth, a porous clay, is used to decolorize oils. Silica gel adsorbs gases and is widely used in adsorbing gasoline from natural gas and in household adsorbent-type refrigerators. Colloids Remain in Suspension. Colloidal particles never settle out unless something is done to cause them to become larger in size. There are several reasons why colloids remain in suspension. In the first place, the Brownian movement tends to prevent the colloidal particles from settling out. Then again the fact that every colloidal particle of a given colloid carries the same electrical charge causes the particles to be repelled as far apart as possible, according to the familiar law of repulsion of like electrical charges. Some colloids are also kept in suspension by protective films. Thus a kerosene and water colloid quickly separates into two layers unless it is emulsified with soap which forms a protective film around each kerosene particle and thus prevents it from making contact with other kerosene particles to form larger particles.