Joey Biernacki, age 12, of Niagara Falls, Ont., Canada, for his question:     

WHICH IS THE EARTH'S HEAVIEST METAL?

A pound of aluminum is almost three times larger than a pound of iron. So when we think of weight we must compare it with size. It is usual to compare the weights of different metals with an equal volume of water. This gives specific gravity, or relative density.

We might guess that lead is the heaviest of the earth's metals. Not so. Its specific gravity is 11.3, which means it is 11.3 times heavier than an equal volume of water. The specific gravity of gold is 19.3. Of the earth's metallic elements, the heaviest is osmium. Its specific gravity is 22.48, which makes it almost twice as heavy as lead.