Tom Erhard, age 14, of Atlantic City, N.J., for his question:

HOW WAS NATURAL GAS FORMED?

Most scientists agree that natural gas was formed billions of years ago when water covered much more of the earth's surface than it does today. Down through the ages, great quantities of tiny marine plants and animals called plankton died and settled to the ocean floors.

On the ocean floors, fine mud and sand drifted down over the plankton. Layer upon layer of these deposits piled up. Their great weight, plus bacteria, heat and other natural forces, changed the chemical compounds in the plankton into natural gas and oil.

The gas and oil flowed into the holes in limestone, sandstone and other porous rocks. Layers of solid rocks formed over the porous rocks and sealed the gas and oil beneath. Later, movements in the earth's crust caused the ancient seas to draw back and dry land appeared over many gas and oil deposits.

Pure natural gas is made up of chemical compounds of the elements hydrogen and carbon. These compounds are called hydrocarbons.

Some hydrocarbons are naturally gaseous, some are liquid and some are solid. A hydrocarbon's form depends on the number and arrangement of the hydrogen and carbon atoms in the hydrocarbon molecule.

Natural gas is made up chiefly of methane, the lightest hydrocarbon. In a methane molecule, one atom of carbon is bound together with four atoms of hydrogen.

Other gaseous hydrocarbons usually in natural gas include ethane, propane and butane. Impure natural gas may contain such gases as carbon dioxide, helium and nitrogen.

When natural gas burns, the hydrocarbon molecules break up into atoms of carbon and hydrogen. The atoms combine with the oxygen in the air and form new substances.

The carbon and oxygen form carbon dioxide, an odorless, colorless gas. The hydrogen and oxygen produce water vapor. As the molecules break up and recombine, heat is released.

Heat from natural gas as it burns is measured in B.T.U.s, or British thermal units. In the metric system, the heat is measured in calories. One cubic foot of burning gas releases about 1,000 B.T.U.s or  252,000 calories.

Gas is manufactured for its chemical byproducts and for use as fuel. In the United States, manufactured gas accounts for only about one percent of the total gas consumed.

There are several types of manufactured gas. The most important are coke oven gas and acetylene.

Coke oven gas is mace by roasting coal. As the coal turns into coke, vapors consisting of many chemicals escape from the coal. The vapors are sent through water, which absorbs some of the unwanted chemicals. The rest of the gas bubbles up through the water.

Coke oven gas has much lower heating value than natural gas.

Acetylene is produced chiefly by dropping water onto calcium carbide, a compound of calcium and carbon. It is also made by breaking apart methane molecules by heating them.

Acetylene has a higher heating value than natural gas. It produces a very hot flame. Acetylene is used in welding and cutting metals.