In a pie pan, mix a few small copper BBs (available in a sporting goods or discount store) with a handful of glass beads of about the same diameter (approximately 5 mm, available in an art supply store). Add two or three cups of water. Over a colander, swish the mixture in a circle, sloshing some over the side each time. Ask students: Do the glass beads or the copper BBs rinse out more easily with the water? Why is this happening?

Ask the following questions to prompt discussion before watching the segment: How do prospectors and geologists find gold? How is it mined? What are those small bits of yellow metal you sometimes see in rocks? Why are people so crazy about gold?

When you think of the gold rush days in the Old West, you probably picture the miner as a grizzled prospector with his mule and pickax. Gold mining today, however, is a scientific process that uses computers, geologic data, chemistry, microbiology, and sophisticated refining equipment to extract trace amounts of gold from rock blasted out from deep underground. If you look at a map of an underground mine, such as the one that the Homestake Mining Company operates in South Dakota, it looks like a very orderly ant farm, with rooms carved out of the solid rock for machine shops, laboratories, and other facilities.

Data from geological core samples goes into a computer that makes a drawing of an area (like a connect-the-dot map in three dimensions) and tells the miners where to find gold-bearing rock. The miners then drill a series of precision holes into the rock face, pack in explosives, clear everyone out, and blast. After checking for gas leaks, workers reinforce the walls and ceiling to prevent cave-ins and then hoist the ore out of the mine through the vertical shafts.

Gold, a pure element identified with the chemical symbol Au, exists in nature. (Fool's gold, a compound of iron and sulfur called a pyrite, looks a little like gold.) Tiny gold particles are encased in tons of rock, so the ore first goes to a mill where it is crushed very fine. The larger particles separate from the ground rock on a vibrating table that works on the same principle as panning, in which substances of different densities separate from each other.

The smallest particles of gold then are dissolved (leached) out of the ground rock with a weak cyanide solution. This still doesn't get all the gold, and it leaves some very toxic wastewater behind, but miners have some valuable new helpers for both of these problems: bacteria. Some bacteria chew up the cyanide in wastewater. Others chemically alter stubborn rocks so that the cyanide treatment can be more effective. This is called bioleaching. A side benefit of bioleaching is the fast production (and consequent treatment) of acids that would otherwise leak slowly from the sludge into the environment.

1. Mining can have significant environmental effects, particularly in developing countries without strict environmental monitoring. For example, cyanide and acid wastes from mines contaminate streams. How do you think we can help these developing countries, who may be rich in raw materials but poor in cash and education, protect their environment from being degraded by mining operations?

2. We use gold not only for jewelry, money, and high-tech products but also as a part of our language. How many ways do you use the word "gold"? How is this word used in the books you read and in products you buy?

Cave-ins have always been a serious danger in mines. As the depth increases, the pressure on the walls and ceiling of the mine tunnel becomes enormous. Engineers have worked to design new methods of reinforcing tunnels so they will not collapse. Using just paper as a reinforcement material, can you design a good, safe tunnel? Try this as a contest among several groups of students.

Materials

1. Place the smaller box, with the open top facing up, inside the larger box. (The larger box is just there to catch spilled sand.)

2. Design a tunnel, using only paper with enough tape to hold it together. Start with something simple like a long, narrow box or tube.

3. Place the tunnel through the holes in the small box. (The tunnel must be long enough to fit through both holes with an inch or so protruding from each side.)

4. Pour wet sand into the small box in measured quantities (scoops or cups), covering the tunnel. Record how much sand is required to make the tunnel collapse. (You will have to look through the tunnel from one end to determine when it collapses.)

5. Design some reinforcements or cross braces for your tunnel, still using just paper. You might try folding, twisting, tightly rolling, or braiding pieces of paper to obtain supports with different strength characteristics.

6. Repeat steps 3 and 4 to test your design again.

Questions

1. Which design best resisted collapse? Was any particular cross section-triangular, circular, rectangular-unusually good at withstanding pressure from all sides? Was any design better at withstanding pressure from the side than from the top or vice versa? What kind of cross braces were most effective?

2. Miners usually don't build tunnels and then bury them-they dig tunnels underground. Which tunnel reinforcement design would be easiest to install if you packed the small box firmly with sand and then dug a tunnel? Can you think of a way to dig and reinforce at the same time?

Find out what properties gold has that make it essential in some industries and medical procedures. Find five uses of gold in the world of high technology, two uses in art or architecture, and three uses in health care. Gold is sometimes used in food-do you know why?

The weight of an object divided by its volume will give you its density. How can you measure weight and volume? Find out if different metals have different densities and how metal densities differ from nonmetals such as glass.