Michelle Toomey, age 12, of Wilmington, Del., for her question:

JUST WHAT IS SOOT?

Soot is a black substance that is found in smoke. Soot is made up of tiny particles of fuel, such as coal, wood or oil, that have not been burned. These particles are usually considered harmful to man and plant life.

Soot can have certain value. Soot contains a great deal of carbon and ammonium salts. The ammonium salts contain a large amount of nitrogen, which is an excellent fertilizer. Soot is, therefore, useful as a fertilizer.

But soot in the air isn't good. It sticks to anything it touches. Smoke blowing through a city leaves its soot on buildings and makes them look dingy. In some cities, about 3 percent of the coal that is burned is converted to soot.

Soot found nearest the fire is usually a shining brown powder containing dried tar. This soot is used as a pigment called bister. The soot found farther from the fire and up higher in the chimney is usually black. It is used as a pigment called lampblack.