Topic 6

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Topic 6 Airs and The Chemical
Revolution Today Some properties of air
overcoming the horror vacui Dr. George
Lapennas Dept. of Biology
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• Studies of air and airs lead to revolution in
  the understanding of the nature of elements and
  matter
• - The weight and spring of air
• Different kinds of air (airs)
• Redefinition of elements
• Atoms and their properties

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Aristotles 4 earthly elements
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Aristotles 4 earthly elements natural motions
were up or down to their natural places, where
they came to rest
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• Aristotles 4 earthly elements
• element fire is absolutely light
• element air is relatively light
• element water is relatively heavy
• element earth is absolutely heavy

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• The horror vacui
• For Aristotle (and for most others until mid
  1600s, including Descartes)
• Space is defined by the matter that occupies it
• Empty space is a logical impossibility (void
  vacuum) matter is everywhere
• Nature abhors a vacuum (the horror vacui)
  and will do what is necessary to prevent
  formation of a vacuum
• (Contrast Democritus concept of atoms moving
  in the void)

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Examples of the power of the horror vacui
1. Draw water up a tube (soda straw water pump
syringe)
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Water pump
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Examples of the power of the horror vacui
1. Draw water up a tube (soda straw water pump
syringe) 2. Water does not drain from a vessel
unless air can enter to replace it
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Examples of the power of the horror vacui
1. Draw water up a tube (soda straw water pump
syringe) 2. Water does not drain from a vessel
unless air can enter to replace it 3. Water
siphon through a tube
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Siphon
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Examples of the power of the horror vacui
1. Draw water up a tube (soda straw water pump
syringe) 2. Water does not drain from a vessel
unless air can enter to replace it 3. Water
siphon through a tube 4. Glass bottle breaks
when water in it freezes (water presumed to
shrink upon freezing nature crushes the bottle
to prevent formation of a vacuum)
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Apparent limitations to the power of the horror
vacui
1. Water pumps cannot lift water more than 34
feet 2. Water siphon cannot carry water over a
hill more than 34 feet high 3. Behavior of water
in a tall, inverted, closed tube
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Gasparo Berti (1600-1643)

• water filled tube
• level of water inside tube stayed at 34 ft
• space left above water in tube
• Suggests vacuum existing in the space above the
  water.

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Water height is the same whatever the length of
the tube
Wouldnt nature more strongly abhor a larger void?
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Torricelli used mercury instead of water
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Torricelli used mercury instead of water same
pattern, except
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height of mercury columns were only 2½ feet,
or 1/13.6 the height of water column
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height of mercury columns were only 2½ feet,
or 1/13.6 the height of water column Note
Mercury is 13.6 times as heavy as the same volume
of water, so the weights of the mercury and water
columns were the same. Does mercury abhor a
vacuum less strongly than water does, and it is
merely a coincidence that its abhorrence is less
by the same factor that its weight per unit
volume is greater? or is there an underlying
explanation?
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Torricellis alternate hypothesis to the horror
vacui Perhaps something pushes the water or
mercury up the tubes, and could push up the same
weight of both liquids?
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Torricellis hypothesis Perhaps the weight of
the air (atmosphere) is doing the pushing.
(Galileo had already weighed air 1/800 as heavy
as same volume of water.) Pascals prediction If
so, then there should be less push as one moves
up through the atmosphere, because there would be
less air above the observer.
Blaise Pascal
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In 1648, Pascal sent his brother-in-law Florence
Périer up 3000-foot Mt. Puy-de-Dome with bowls,
tubes and mercury
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The mercury rose to 2 ½ feet minus 3
inches! These results proved that Torricellis
hypothesis was true.
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The mercury rose to 2 ½ feet minus 3
inches! These results proved that Torricellis
hypothesis was true.
NOT!
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The mercury rose to 2 ½ feet minus 3
inches! These results proved that Torricellis
hypothesis was true.
NOT!
These results supported Torricellis hypothesis.
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Today we use the barometer to measure changes
in atmospheric pressure to help predict weather
changes.
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Weather station atop Mt. Puy-de-Dome
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Another test of weight of air hypothesis
Predict that if a barometer is placed in a
chamber and the air pumped out, then the mercury
column will not be as high.
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Boyles/Hookes improved pump of 1660
von Guerickes original air pump
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When Boyle and Hooke pumped air out of a chamber
containing a barometer, the mercury dropped lower
and lower down to a small fraction of an
inch. This result lent further support to the
hypothesis that water and mercury columns were
pushed up to their heights by the weight of air,
rather than climbing up in attempts to eliminate
the vacuum.
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von Guerickes Magdeberg Hemispheres
demonstration - 1657
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Boyles experiments on the spring of air Air
resists compression like a spring does.
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Boyles experiments on the spring of air Air
resists compression like a spring
does. Explanation?
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Boyles experiments on the spring of air Air
resists compression like a spring
does. Explanation? Boyle Air consists of tiny
particles that are like springs, pressing against
each other, and resisting compression.
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Boyles experiments on the spring of air Air
resists compression like a spring
does. Explanation? Boyle Particles of air are
like springs, pressing against each other, and
resisting compression Newton Air particles
repel each other without contact, with a force
that decreases with distance.
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Boyles experiments on the spring of air Air
resists compression like a spring
does. Explanation? Boyle Particles of air are
like springs, pressing against each other, and
resisting compression Newton Air particles
repel each other without contact, with a force
that decreases with distance. Both of these
hypotheses ultimately proved incorrect. (Air
pressure results from the force of air molecules
colliding with surfaces and bouncing off them
exerting force on the surfaces that are equal and
opposite to the forces the surfaces are exerting
on them.)
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Further studies of air lead to the
realization that there are many different kinds
of air (airs), not just one.
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Further studies of air lead to the
realization that there are many different kinds
of air (airs), not just one. and to the
identification of elements as we know them today.
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Further studies of air lead to the
realization that there are many different kinds
of air (airs), not just one. and to the
identification of elements as we know them
today. and to the theory that matter consists
of atoms, to the understanding of the structure
and properties of atoms, and why they react the
way they do.
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but those are stories for other days.