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Capillary Filter

One of the interesting things at St. Augustine, Florida's Oldest House is a large stone bowl with a bucket underneath. The stone bowl is half full of muddy water and there is a slow, steady drip of water from the bottom of the bowl to the bucket. Although the water in the bowl is a bit muddy, the water in the bucket is clear. Why?

To get an idea of how this works, you will need:

  • two glasses
  • water
  • dirt
  • paper towels
  • a box or stack of books about 3 inches high

Fill one of the glasses about 2/3 with water. Add a handful of dirt and stir. Now, does that look like something you would want to drink? No! But there is something we can do to help clean the water. Place the glass of muddy water on the phone book, at the edge. Place an empty glass beside it. The glass with the muddy water needs to be about 3 inches higher than the empty one. Roll the paper towel into a tube about an inch in diameter. Twist the tube until it looks like a rope. Put one end of the paper rope into the muddy water and the other end into the empty glass. Now go read a book for an hour or so.

When you come back, you should find that quite a bit of the water has moved to the empty glass, but it is now clear. The water in the original glass is still muddy. What happened?

Think about it, and when you think you know the answer, then continue.

First, the water soaked into the paper towel. It did this by something called capillary action. Water molecules are very sticky. They stick to the fibers in the paper. They also stick to each other. Water molecules move into the tiny spaces between the paper fibers, and they pull other water molecules with them. The water slowly soaks its way through the paper to the other glass. Once the water crosses the top of the glass, gravity helps to move it downwards, pulling more water up to take its place.

Because the spaces between the fibers are very small, the particles of dirt get left behind. They won't fit in between the fibers, so they can't move with the water. That does not mean the clear water is safe to drink! Although the filter will screen out the dirt, it does not remove germs or dissolved chemicals.

The same is true for the stone bowl we saw. The stone has tiny holes which let the water flow slowly through, but the mud is left behind. At the time the Oldest House was in use, it was a good way to get cleaner water, but with the levels of pollution we have today, it would not be enough to be sure the water was safe to drink. Instead of drinking the water yourself, give it to your houseplants or the plants outside.