1. Gases and the Atmosphere
Understanding Atmospheric Pressure

2. Water Pumps
16th century miners noticed that water could only be pumped to a height of 32 feet (~10 meters)

3. Galileo Galilei ( )
Miners asked Galileo why water could only be pumped 32 feet
Galileo proposed that the “resistance to a vacuum” or “suction” is what draws the water up
He said “it seems that nature abhors a vacuum, but only to a height of 32 feet."

4. Gasparo Berti’s Experiment
1638, Galileo’s famous book Discorsi reaches Rome. In it he describes the miners’ problem.
Rafael Magiotti designed an experiment to study “the force of a vacuum”.
Berti’s Vacuum Experiment

5. Gasparo Berti’s Experiment
Gasparo Berti carried out the experiment with a long tube filled with water and inverted in a basin of water.
He noticed that some water flows out of the end of the tube, then stops.
There is an “empty” space is above the water.

6. Gasparo Berti’s Experiment
What holds up the water?
What is in the space above the water?

7. Popular Explanations
Berti and others claimed that there was nothing above the water…a vacuum or a void.
Others said there can’t be “nothing” so the water gave off “spirits” which pushed water down.
So what holds the water up?
Berti agreed with Galileo – resistance to vacuum

8. Evangelista Torricelli (1608-1647)
Torricelli believed the water was held up by the weight of the atmosphere on the reservoir below the tube.
He designed an experiment to measure the weight of the atmosphere that used mercury in a tube rather than water.
Mercury is 14 times more dense than water so the tube would only need to be one metre long.
Was likely aware of Berti’s experiment and the writings of Galileo but came up with a different hypothesis to the “what holds the water up issue” Believed we lived at the bottom of a sea of air. And it was this sea of air pushing against the water that held the water in the column to 32 feet

9. Torricelli’s Barometer 1643
Torricelli filled a tube with mercury and inverted it into a bowl of mercury.
The mercury flowed out like the water, but rested at a height of 76 cm or 760 mm.
From this he was able to measure the weight of the atmosphere, we call it atmospheric pressure.

10. What’s in the Space?
Torricelli wanted to show that the space above the mercury was empty.
He added water to the mercury in the bowl and began to raise the tube slowly. Hg
Space
water
Toricelli believed there was a vacuum in the empty space above the fluid - controversial

11. What’s in the Space?
When the mouth of the tube rose to the surface of the water, the mercury in the vessel poured out, and the water rushed in to fill the tube to its top.
What does this suggest?
water
Showed that nothing was above the surface of the liquid – if there was something in the tube it would have “stayed”

12. Torricelli’s Interpretation
In 1644, he famously wrote in a letter:
"We live submerged at the bottom of an ocean of air."
and the weight of the air supports the column of mercury.
Torricelli thought his experiment failed since the height of mercury fluctuated with changes in the weather.
He calculated kg per m squared. Which would be approx kg on your shoulders if assuming 0.1 m

13. Blaise Pascal (1623 – 1662)
Pascal did not believe Torricelli’s interpretation of the “sea of air”, nor the existence of a vacuum.
Pascal (along with Pierre Petit) repeated and perfected Torricelli’s experiments.
Pascal predicted if the earth is surrounded by a “sea” of air, and if the air has weight, then the pressure of the air will be less on top of the mountain. Why?

14. Puy-de-Dome Experiment (1648)
In one of the first controlled experiments, Pascal instructed his brother-in-law, Florin Perier, to take the barometer up the mountain Puy de Dome and take measurement of the height of the column of mercury along the way.

15. Puy-de-Dome Experiment (1648)
He found that the column of mercury did drop as he moved up the mountain.
Pascal’s hypothesis was correct and he was convinced of Torricelli’s hypothesis.

16. Otto von Guericke ( )
Von Guericke developed the first air pump in 1650.
He used it draw air out of barrels and other objects.
In his most famous demonstration, he used the pump to draw the air out of two 36 cm copper bowls.
The hemispheres could not be pulled apart by 2 teams of 8 horses each!

17. Von Guericke’s Experiment
Magdeburg hemispheres 1654

18. von Guericke’s Demonstration
What held the 2 hemispheres together?
von Guericke proposed the weight of the air, or atmospheric pressure, pushed the 2 bowls together.
von Guericke calculated the weight of the atmosphere on the 2 hemispheres was about 1200 kg!
Von Guericke provided evidence that atmospheric pressure is not just downward, but in ALL directions.

19. Robert Boyle Robert Boyle (1627 – 1691)
Heard of pascals experiment and of the pump made by Otto von Guericke. Wanted a more practical apparatus to run tests in a “vacuum”
Robert Boyle (1627 – 1691)

20. Boyle’s (Hooke’s) Air Pump

21. Boyle’s (Hooke’s) Air Pump

22. Experiment
Boyle designed the pump so he could test Torricelli's hypothesis that if the air is removed from above the mercury reservoir of a barometer, the mercury column would fall.

23. Experiment
Placed a mercury barometer into a container and attached the air pump.
When the air in the container was removed, the column of mercury fell. h
air
Evacuate h

24. Experiment
He was frustrated that he could not lower the pressure to the level of the mercury in the bowl.
He blamed this on leaks in the system.
One example of Boyle recognizing limitations to his experiments…what we call “sources of error”.

25. What is pressure? Pressure: force per unit volume
Ex. high heels vs flat shoes
The high heels exert more pressure on the ground as there is less area over which the force is applied

26. What is pressure? P atm < P fresh water < P salt water
This is why you are more buoyant in salt water, it is more dense (more mass per volume)
For every 33 feet (10.06 meters) you go down, the pressure increase is 1 atm
Archimedes principle