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Thanks to Bill Noyce and Jared Sherman for useful information about Mercury
and its vapour.

The answer seems to be that the equilibrium vapour pressure is .00185 torr,
at 25 deg Celsius, increasing by 8% for each extra degree C.

Jared asked what a "torr" is. It's a unit of pressure equivalent to 1mm
head of Mercury, or 1/760 of the pressure of a standard atmosphere.

That allows us to calculate what the concentration of Mercury in the air
would eventually reach, if it were sealed up over a pool of Mercury for
long enough, though it provides no clue as to how long that would take. I
will explain my reasoning, which may bore many, in order to encourage those
who know more than I do to correct me if I'm wrong.

Going back to what I remember of school physical-chemistry, 22.5 litres of
any gas weighs its molecular-weight, in grams, at normal temperature and
pressure. Mercury has an atomic weight of about 200, and presumably the gas
molecules are atom-pairs weighing 400, so 22.5 litres would contain 400
grams. However, we know that the partial pressure is not 760 mm but
.00185mm, or .00185/760 atmospheres. And we are looking for the total
Mercury content, not in 22.5 litres, but in 1000 litres (1 cubic meter), to
compare with the maximum permissible 8-hour limit of .025 milligrams per
cubic metre.

At 25 deg C that would then be 400 x (.00185 / 760) x (1000 /22.5), or .043
grams per cubic meter. I suggested in an earlier mailing that the
equilibrium vapour concentration would be greater than the maximum
permissible level, but I admit to being rather surprised to discover that
it's so much greater, by a factor of 1730!

At higher temperatures the equilibrium Mercury vapour concentration would
be greater still, about 3 times higher at 40 deg C.

That tells us how dangerous it would be, to be confined in a sealed volume
for a long time with a large open tray of Mercury, such that the vapour
pressure could approach equilibrium, at which Mercury was evaporating off
at the same rate that it was condensing back. That is far different from
the real-life situation that faces an observer using an artificial horizon
outdoors, in which the slightest breeze will sweep away all Mercury vapour
except for that which has evaporated over the last second or so.

That is the situation I am searching for a way to evaluate, or to discover
if others have done so. Any ideas would be most welcome.

Finally, many thanks to Clive Sutherland for a really useful collection of
thoughts. I don't remember having seen his level-with-prisms but will
certainly arrange to do so. (He and I live in adjacent villages, and meet
up often).

George Huxtable.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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