Matthew Rowley, age 9, of Salt Lake City, Utah, for his question:

What is specific gravity?

Christmas comes on a specific day of the year and a thimble is a specific tool for sewing. The word "specific" means something special or something meant for a special kind of job. Government is a grave and serious business. The grave problem of war makes our hearts feel heavy and weighty. Young Isaac Newton borrowed the word "grave" to name the laws of gravity. It is the earth's gravity that hugs us to the ground, gives us weight and makes us feel heavy. So the concept. of specific gravity must have something to do with 'a special kind of weightiness.

A A shoebox of lead weighs more than a shoebox of feathers. The same box filled with almost any other substance would have a different weight. So a system was invented to compare the different weights of all substances with the special, or specific weight of water. The system is called specific gravity. It depends on the space an amount of material takes up. This is called its volume. Specific gravity compares the weight of any substance with the weight of an equal volume of water.

Thought of in a slightly different way, it would take 11 1/3 cans of water to equal the weight of one can of lead   all the cans being of equal size. The s.pocific gravity of lead is 11.34. Gold is a heavier metal with a specific gravity of 19.3. The specific gravity of the light metal alumi'na is 2.70. A cupful of aluminum weighs about as much as 2 3/4 cupsful of water.