Under natural conditions on earth, every bit of matter is in motion, either traveling or vibrating in place. The movement we're talking about is at the molecular level, so usually invisible to the eye. We sense this movement in another way, though, as heat. The faster the molecules of given material move, the higher the temperature. In gases or liquids, molecules careen freely about, resulting in a phenomenon known as diffusion.
Picture a room with blindfolded people clustered in a corner. Now set
each person to walking rapidly until running into something, and then continuing in
whatever direction the bounce takes them. Soon, instead of a corner clump, you have
people evenly scattered through the room. Change people to molecules, and you've
got diffusion. Open a vial of perfume, and soon that bottled cluster of aromatic
molecules spreads throughout the room. Or, more satisfying, venture out on a still
night after a rain, and bathe in the scent of the desert. The scent brought to you from
distant sources courtesy of the phenomenon of diffusion.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.