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In article <9505@nigel.udel.EDU> berryh@udel.edu (John Berryhill) writes:
>>In particular, 10E-7 particles per cm3 corresponds to which category?
>>This is said to be the density of an interstellar dust cloud.  What's
>>the best that can be achieved in the lab?
>
>I think the record is in the low 10^-9 mm of Hg range.

Not even close.  Glass apparatus peters out around 10^-10 mm because of
atmospheric helium seeping through the glass (!), metal goes on for
several more orders of magnitude with hydrogen seepage an increasing
nuisance.  With lots of liquid helium for cryogenic "pumping" (freezing
residual gas onto the walls), the theoretical limit is somewhere around
10^-21, although measuring a vacuum that hard is beyond current equipment.
This is from memory, so I may have botched details.  (I was interested in
ultrahigh-vacuum technology some years ago, but it's been a while.)

O'Hanlon's "A User's Guide to Vacuum Technology" (1980) gives the following
ranges of vacuum, pressure in Pascals (101323.2 Pa = 760mmHg):

	low			1e5 > P > 3.3e3
	medium			3.3e3 > P > 1e-1
	high			1e-1 > P > 1e-4
	very high		1e-4 > P > 1e-7
	ultrahigh		1e-7 > P > 1e-10
	extreme ultrahigh	1e-10 > P

and notes that "high" often is used to cover "very high" as well.

Those dust-cloud density numbers are talking about solid particles, not
gas molecules.  The old rule of thumb for interstellar gas is about one
hydrogen atom per cc, although I think that's been revised down a bit.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu