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From: drw@culdev1.UUCP (Dale Worley)
Newsgroups: sci.bio,sci.physics
Subject: check your figures
Message-ID: <1628@culdev1.UUCP>
Date: Fri, 9-Oct-87 12:23:25 EDT
Article-I.D.: culdev1.1628
Posted: Fri Oct  9 12:23:25 1987
Date-Received: Sun, 11-Oct-87 14:52:02 EDT
Organization: Cullinet Software, Westwood, MA, USA
Lines: 33
Xref: mnetor sci.bio:708 sci.physics:2459

doug@ndcheg.UUCP (Doug Price) writes:
>   What I meant was that since xenon is more dense than oxygen, it would
> settle towards the bottom of the storage vessel when gravity is present.
> It is known that mixtures of gases will separate if their densities vary
> by a large amount and if there is no strong intermolecular forces to keep
> the mixture homogeneous.  Linde (producer of laboratory gases) recommends
> that cylinders containing such mixtures be rolled around prior to use so 
> the gases are well mixed. > 

Yes.  And what I was pointing out was that (assuming you know the
temperature) the *equilibrium* separation is *determined* by the fact
that it all equilibrates to the Maxwell-Boltzman distribution, and
that on laboratory scales at room temperature, the separation is very
small.  The "mixtures" that separate in a laboratory setting are most
likely not mixed on a molecular scale, but rather were made by
injecting certain amounts of two gasses into a bottle, leaving a gas
which is a mosaic of macroscopic-scale regions of the two components.

Compare to pouring one liquid into another.  The two naturally form
two layers, which diffusion will slowly mix completely.

One can apply this method to the atmosphere.  The M-B distribution
gives you the fall-off in air density with altitude.  But note, the
e-folding height differs for each molecular species (being smaller for
heavier species), so once you get up real high, hydrogen and helium
predominate.

Dale
-- 
Dale Worley    Cullinet Software      ARPA: culdev1!drw@eddie.mit.edu
UUCP: ...!seismo!harvard!mit-eddie!culdev1!drw
Give me money or kill me!
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