On Wed, 14 Jun 2006 09:20:22 -0500, Del Cecchi
wrote:

Your physics class covered "vapor" where "Technically, gases expand and
contract under pressure and spread uniformly through a cylinder. Vapors
don't do that."?

I think the concept that is being struggled with is something called
"particulate matter (PM), aerosols or fine particles".

http://en.wikipedia.org/wiki/Particulate

As a mixed phase substance, the gaseous portion is compressible but
the particles themselves are not.