Frizz Gerald, age 14, of Mullens, South Carolina, for his question:

What exactly is specific gravity?

We know, more or less, what gravity is. But the word "specific" means something in particular    or something special. Here it denotes a particular quality of gravity that can be used to grade the weight or density of different substances. The specific gravity of water is rated as one and the specific gravity of gold is 19.3. This means that under standard conditions, pure gold weighs 19.3 times more than an equal volume of pure water.

Objects on the surface of our planet get their weight from the earth's force of gravity. Their weight, of course, depends on mass    which is the amount of matter occupying a certain volume. We know from experience that some substances weigh more for their size. Cork is light; lead is heavy. This quality is called specific gravity or density. It denotes the precise ratio of a body's mass to its volume.

Geology books give the specific gravities of minerals and metals in figures and decimals. Just by skimming through the notes, we can conclude that the larger numbers go with heavier substances. But it's nice to know how the experts arrive at their figures. The scale is based on the weight of water, measured under one atmosphere of pressure at 4 degrees centigrade. This gives the scale unit of one. The weights of other substances are compared with equal volumes of water.

You may suspect that heavy lead tops the scale. Its specific gravity is 11.3, which makes it 11.3 times denser than water. But gold rates 19.3, which means it weighs 19.3 times more than an equal volume of water. Silver is rated 10.5 and copper is 8.9. These metals are lighter than lead, but still quite a bit heavier than water. Platinum, with a specific gravity of 14 is denser than lead but less dense than gold.

The quality of density depends on the particles of matter in a substance and how they are arranged. In heavy substances they are crowded close together. In light gases, the separate molecules are strewn apart. The specific gravity scale works fine for solids and liquids. It can work for gases, though it is more convenient to weigh these lighter than water substances by the liter.

The ratios of specific gravity are based on the Archimedes Principle. This is named for the Greek scholar who figured it out, way back about 200 B.C.

At that time, the king of Syracuse suspected that his new crown was not the pure gold he had paid for. Archimedes solved the problem when his tub slopped over and an idea popped into his head. He saw that his body displaced a certain volume of water. He tested various substances and figured a way to use water to test their different densities. The crown was dunked and its weight compared with an equal volume of gold. It weighed less than it should. The goldsmith had alloyed the king's gold with silver, which weighs less for its size.