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    Re: Artificial Horizons and Tea
    From: Fred Hebard
    Date: 2003 Jul 12, 10:20 -0400

    A few questions and comments,
    Geoffrey, Why did you not just use the surface of the mercury itself as
    a reflector, rather than floating a glass on top?
    I believe the high toxicity of mercury is in some of the complexes of
    mercury with organic compounds, such as dimethyl mercury.  Some of
    those are among the most toxic substances known, with LD50s on the
    order of picograms per kilogram.  When the soon-to-be-mad hatters were
    soaking their felt in mercury, undoubtedly a few of these
    organomercuries were being synthesized in low quantities.  Elemental
    mercury itself is not toxic, to my understanding.  If it's kept in
    glass away from organic materials, few of the organomercuries would be
    synthesized.  Add a bit of dust and dirt and the hazard increases, but
    the tendency of elemental mercury to keep dust at its surface keeps
    most of it away from organics.  This may account for Geoffrey and
    George's longevity, although mercury also may be a cause of their
    interest in our obscure art!
    The other problem with mercury is that, being a metal, it stays around
    virtually forever, not being broken down by microbial and chemical
    actions like organic compounds.  It's high specific gravity makes it an
    ideal pollutant of rivers and streams, moving up the food chain until
    rather toxic levels can accumulate in fish and people.  Here in
    Virginia we have two streams that are still polluted with elemental
    mercury from industrial activities that ended in the early 1970s.  This
    latter problem may be some of the motivation behind the strictures
    against it.  I'm not sure I believe all the warnings about the direct
    health hazards of elemental mercury.
    The great drawback of all reflective artificial horizons is the
    limitation to altitudes less than 60 or 70 degrees with modern
    sextants.  Thus the great appeal of eyepiece levels such as the C.
    Plath.  Unfortunately, only that one seems to be accurate to better
    than a minute or tenth of a minute, and it is quite expensive.  I am
    still attracted to the idea of a clothes line level or similar
    arrangement, but it's impractical for many of my sights.
    On Saturday, Jul 12, 2003, at 03:41 US/Eastern, Dr. Geoffrey Kolbe
    > George Huxtable wrote,
    >> Later, at university, we found a similar environment. We would use
    >> mercury
    >> in pint quantities for diffusion-pumps in high-vacuum systems.
    >> I suspect many physics students from my generation, the world over,
    >> could
    >> tell a similar story.
    > Quite so. When they pulled down the old Royal College of Science in the
    > early '60's (to build what is now Imperial College, London), the
    > mercury
    > vapour in the Spectroscopy lab had reached the level where absorption
    > lines
    > of mercury were always present in any spectra taken in the lab. A lake
    > of
    > mercury was found under the floor boards when they pulled them up! My
    > ten
    > years in the Spectroscopy group was spent when it had moved to the new
    > physics department of Imperial College, but the researchers who worked
    > in
    > old Spectroscopy lab were still alive and working when I was there -
    > and it
    > goes without saying that they were hale and hearty and lived to a ripe
    > old
    > age...
    >> I'm not convinced about the virtues of floating a solid mirror on a
    >> disc-raft on liquid. The liquid and the solid would need to have a
    >> repulsive surface tension between them to ensure blobs wouldn't
    >> gather up
    >> the sides of the raft. That surface tension would require to be
    >> exactly
    >> even around the edges of the disc or the raft would be unbalanced. How
    >> would one prevent the raft from nearing the edges of the container,
    >> which
    >> would unbalance the surface-tension forces or give rise to friction
    >> which
    >> would constrain the self-levelling? There are serious problems here
    >> which
    >> would need resolving.
    > I am not so sure that this is as much of a problem as you paint it
    > George.
    > It is quite easy to work a glass disc so that it is flat and the two
    > sides
    > parallel to a micron or so, and with a sharp uniform edge. In my
    > experience, the main problem was making sure the surface of the
    > mercury was
    > absolutely clean, or the glass would sit on top of a spec of dust and
    > the
    > glass would not be level.
    > The surface tension forces between mercury and glass are repulsive, so
    > the
    > problems of blobs of mercury adhering to or sitting on the glass raft
    > disappear.
    > Geoffrey Kolbe.

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