Robert Romano, age 14, of Staten Island, New York, for his question:

Exactly how do we get mercury?

This marvelous metallic element is as rare as silver and its deposits are few and far between. Thin films and fine droplets of pure mercury may be deposited by seething underground water, around the rims of hot springs. But most of the earth's stingy supplies are combined with other minerals in fiery volcanic activity. Extracting it from these ores is well worth while because the silvery, liquid metal has more than 3,000 uses in our industrial world.

The most useful mercury ore is a handsome, rusty red clock called cinnabar. Deposits of cinnabar almost always occur close to the surface. Mining it is merely a matter of blasting it into moveable sized chunks and toting them away. Post cinnabar mines go no deeper than 500 feet and some are very old. A deposit in Spain has been mined since 800 B.C. The United States has valuable cinnabar mines in several western and southwestern states. But they do not yield enough mercury to supply our yearly needs of around 2,000 tons. More must be imported, most of it from Spain and Italy.

Cinnabar is a red sulphide, a compound of mercury and sulphur. It is worth mining when the ore yields only ten pounds per ton. A prospector may test his find by tossing a few red, rocky crumbs on a bed of hot.charcoal. If the sample vaporises and disappears, it is indeed cinnabar. Amore revealing lab test may be made by heating the cinnabar in a flask. This separates the sulphur from the rocky compound and changes it to vapor. A black ring and fine droplets of mercury form on the glass inside the flask. When scraped with a needle, the film and droplets congeal into large drops of the liquid metal.

This lab experiment suggests how mercury can be extracted from cinnabar on a larger scale. The ore is heated high enough to turn the sulphur into vapor and the heavy mercury stays behind. At one time, line was often used to heat the cinnabar. Nowadays, the job usually is done with currents of hot air. It is economical to heat the cinnabar in flasks because the sulphur also is a valuable industrial element.

When the sulphur departs in the hot air, the mercury content congeals in a silvery, liquid metal. The sulphur fumes are piped away to cool. Instead of changing to solid or liquid form, they sublime to form powdery grains called flowers of sulphur. This product is shipped away for industrial uses. The liquid mercury is washed and strained to sift out impurities and less valuable metals. Then it is poured into sturdy flasks. Each flask contains 75 pounds of valuable mercury    the only metallic element that is liquid at normal temperatures.

The atomic number of mercury is 80 and its atomic weight is 200.59. It is about 13 1/2 times heavier than water and a liquid until the temperature drops to minus 38.7 degrees centigrade. Then it freezes solid. When heated to 356.58 degrees centigrade, the metallic liquid boils and becomes gaseous mercury vapor. All mercury compounds are poisonous. So it is unwise to touch them or the liquid metal with bare hands.