Air Space

There are only two more days before we head for New Zealand. I am looking forward to the farm, the beach, seeing friends, and the food. It will be a LONG flight, but I am even looking forward to that. Most of my packing is done, and I am down to the "what did I forget" stage.

During our shopping, we bought some of the storage bags that you fill with clothes and then squeeze the air out of so that your clothes take up much less room in your luggage. They really do work, and since we take LOTS off stuff, we need all the help we can get. This got me thinking about how much air space there is in other common substances.

To experiment with this, you will need:

  • a drinking glass
  • water
  • cotton balls

Fill the glass to the top with water, so that the water level is at the very top of the glass. Now, it would seem that you could not put anything else into the glass without spilling it.

Pick up a handful of cotton balls. Just by looking at them, you can tell that there is a lot of air in the spaces between the fibers. Take one of the cotton balls and squeeze it down as small as you can. Even so, it still takes up quite a bit of space. How many of them do you think you could put into the glass of water before the water spills over the side? Lets try and see what happens.

Don't just cram a handful of them into the glass. Instead, gently place one of the cotton balls in the water and watch as it soaks up the water and sinks. Then place another cotton ball in the water. Keep adding them. You will probably fill the glass with wet cotton long before the water level rises enough to cause it to run over the edge.

How can this be? Well, a cotton ball is made up of lots of cotton fibers, all tangled together. Most of the space taken up by the cotton ball is actually taken up by the air between the fibers. Only a small fraction of the space is filled with the stands. On top of that, the fibers are mostly air. Each strand of cotton is actually a long, hollow tube. These tubes are elongated cells which form on the surface of the cotton seed. When the cells die, the tough cell wall is left behind, forming the strand of cotton. Besides filling the spaces between the strands, the water also fills the hollow inside of the strand. That means that the water level rises only a tiny fraction with each cotton ball. Since the surface tension of the water lets it bow upwards quite a bit before it flows over the side, it is very easy to fill the glass with wet cotton without making a mess. Too bad it isn't that easy to fit all these clothes into the suitcases.