Spot the Frippet: vacua.

American cockroach. Photo by Gary Alpert

Nature may abhor vacua, but on the whole they're an inoffensive sort of thing.

Yes, vacua is the plural - one of the plurals - of vacuum. The most widely-used and sensible plural of vacuum is vacuums, of course, but vacua is a pleasing conceit for private moments.

But how on earth can we spot vacua? Aren't they invisible?

Well, spotting a vacuum flask is easy enough, and there are vacuum cleaners all over the place. It's admittedly unusual to find either of these devices nowadays with any sort of a vacuum anywhere about its innards, but you might have some food about the place that's been vacuum-packed.

On the whole the best way to see a vacuum is to do a simple but satisfying experiment. First, find a thin empty plastic bottle such as a water bottle. Fill it up with hot water (but not boiling water, or you might melt the thing); after a few moments empty the water out and quickly screw the cap on tight. Wait.

Soon the sides of the bottle will begin to make nice cracking sounds, and soon after that they'll collapse inwards quite spectacularly. 

Why?

Well, the air inside your container was quite warm when you put the top on, because it had been heated up by the container's water-heated surfaces. Soon, though, the air cools down, and cool air takes up less space than hot air. This means you have a lower-than-normal volume of stuff in your bottle, which is what a vacuum is, and so the bottle collapses until it's the right size for the amount of stuff in it.

(Some people will insist that a vacuum is somewhere with absolutely nothing at all in it, but without the use of a brain-scanner and a career politician that's much harder to demonstrate.)

If you are a cockroach, all such experiments are unnecessary because you have a vacuum-detector in your bottom. This detects any sort of pressure wave (such as is generated by a descending foot) and allows you to take instant evasive action. 

Unfortunately the same principle encourages you to run straight into the nozzle of any vacuum cleaner. 

Spot the Frippet: vacua. This word comes to us from vacuum, which is Latin for empty space, from vacuus, empty.