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History of Air Pressure and the Existence of a Vacuum
The brief survey below covers:
Artistotle's World View: "Nature Abhors a Vacuum"
The Greek philosopher Aristotle (350 B.C.) can be given much of the credit for the historic misunderstanding of air pressure and the existence of vacuums and their effect. The Aristotelian viewpoint did not allow for the existence of a vacuum in Nature. His concept was that matter "filled up" the world completely and that anything conceived as space was not empty, but an extension of the matter that surrounded it. Aristotle would argue that if a vacuum was produced, sound and light could not travel through it because they needed a medium to be transmitted. So, if a vacuum was produced you couldn't see or hear through it. (He was half right.)
In explaining, how water would go up a tube or straw if you sucked on one end, Aristotle might say: every thing moves by displacement. Since matter fills the world completely, matter that flows into one vessel must be displacing the matter that is already filling the space. It will not flow unless that matter can be displaced so other matter can be moved to fill in behind. He might also have used the idea that Nature could sense that you were trying to create a vacuum by expanding your lungs and the water and air in the tube was reacting by rushing in so the vacuum wouldn't happen, since "Nature abhors a vacuum".
Aristotle would not have employed the idea of air pressure because he did not recognize that it had weight in its "natural state" or exerted pressure on objects in it. The force moving the fluids up would have come from the fluids themselves and not from any external force outside the tube.
The Atomist World View: "Nothing Exists but Atoms and the Void"
The counterpoint to this world view was the Atomist conception of matter. The most notable was the Greek philosopher Democritus (400 B.C.). Basically, he viewed matter as being composed of minutely small, invisible spherical particles. If you image stacking spheres together you get a space where the spheres edges do not touch. This truly empty space is where nothing exists. This nothing is a vacuum. The phrase attributed to Democritus is that "Nothing exists but atoms and the void". It seems that the concept of Atoms and particles too small to see or sense did not appeal to many people. Hence, Aristotle's world view and the interpretations of it by religious institutions was the standard by which natural phenomena was explained and understood for some 2,000 years.
Historically, restructuring the Aristotelian world view and understanding the effects of air pressure relied on the acceptance of the possibility of the existence of continuous vacuum. In showing the possible fallibility of Aristotle's doctrines by producing a continuous vacuum, the groundwork for accepting alternative concepts of matter and causal relationships related to air pressure was laid.
The steps to restructuring Aristotelian World View
Studies in hydrostatics by Archimedes (250 B.C.) were the standard by which learned people of the late 1500's understood how water could exert pressure. Water was conceived to have weight and could exert pressure equally over a surface at the same depth. The relationship between pressure, depth, and density of a liquid would eventually be applied to air pressure and help solve the mystery of suction and vacuums.
It was not until the late 1500's when Simon Stevin published a reworking of Archimedes' Hydrostatics in 1575, that the analogy of water was used by others as a basis to explain the effects of air pressure. During this same period, a Treatise on Pneumatics by Hero of Alexander (100 A.D.) was being republished and added to the interest in the role of air in the movement of fluids in tubes.
Hero's work focused on the functioning of siphons. He was a follower the Atomist's concept of matter and believed that minute particles and voids did exist. However, he did not accept the idea that a continuous vacuum or void could be sustained. His work is mostly concerned with producing mechanical apparatus that function on air and water pressure to produce effects from opening doors to making fountains.
Influence of Economics, Industry and Technology in the Renaissance
During the late 1500's the mining industry in Europe had become very important. Industrial use of metals and wood for fuel, not to mention the great need for lumber for ship building created a need for mines to provide raw material and fuel. In order to reach the valuable ore, mines needed to be drained. Pumping out the water that would seep into the shafts was a terribly difficult and expensive activity. The pump technology of the times used hollowed out tree logs as pistons with wood shafts wrapped in leather as plungers. The energy was supplied by humans or horses running on treadmills, or water mills that turns cranks and gears to moves the pistons. The efficiency of these pumps was moderate and the best of designs could not "lift" water more than 34 feet. This limit caused them to place pumps in series or to use buckets on chains to haul it up. All of this at great cost in materials and energy.
The solution to this well known problem was of great importance to those involved in the economics of this industry. If water could be pumped higher with less energy and materials, then profits could be increased and more ore could be recovered. The limit to the height to which water could be suction pumped was thought to be caused by imperfections in the pumps themselves or by a semi-material fluid or ether seeping in from outside the pump. The belief Aristotle's conception of matter eliminated the possibility of using air pressure and vacuums to explain the limits to their pumps. They chose to create an alternative explanation for their experiences rather than seek a new conceptual framework to view the problem.
The Role Played by Galileo
Galileo was the court mathematician for the Duke of Tuscany during the early 1600s. When faced with the question from the Duke of Tuscany about why the pumps wouldn't "lift" water more than 34 feet; Galileo is supposed to have replied...."It seems that Nature abhors a vacuum only to a height of 34 feet." Galileo was already in trouble with the Church on the matter of Cosmology and World View. He may have been a bit reserved about plunging into another mess about the existence of vacuums because this topic was also in conflict with current Church doctrine about creation and the possibility of worlds beyond Earth.
It appears that Galileo never really understood about the effect of air pressure or even its existence. However, he did write about his belief in the possible existence of a vacuum. Galileo wrote: "If the vacuum cannot be recognized either by the senses or by intellect, how have you managed to find out that it does not exist?" Galileo's role seems to have been one of questioner. Because of his status in the scientific community or perhaps his smudged reputation by the Church, he was able to question the validity of Aristotle's logic and ideas. This was the spark that would ignite others.
The Torricellian Tube
The person who did come to understand the role of air pressure and the existence of a vacuum was Evanglista Torricelli. Torricelli was a pupil of Galileo during the last year of Galileo's life and also inherited his position as Court Mathematician for the Duke of Tuscany. It should be noted that other people in Europe where doing related experiments on creating a vacuum using water columns in tubes or siphons and demonstrating the elasticity of air (Isaac Beeckman, Giovanni Baliani, Rafael Magiotti, and Gasparo Berti). These experiments or demonstrations focused mainly on trying to produce a vacuum. If a vacuum could actually be continuously maintained then Aristotle's framework was fallible and new conceptual models could take its place.
Torricelli did not need to be convinced of the existence of a vacuum. It seems that from studying Galileo's work he accepted its existence and proceeded to create an experiment to measure the effect of air pressure on a column of mercury. Torricelli designed an experiment (he did not perform it) in which he would fill a glass tube four feet long and sealed at one end with mercury. Placing his finger over the opening he would invert the tube in a bowl of mercury with the sealed end up and measure the resulting height of the column. The mercury would fall to about 30 inches in height and an empty space or vacuum was created in the top of the tube.
He perceived that the weight of the column of mercury was equal to the weight of the air column pushing down on the bowl of mercury. The two columns were in equilibrium. If the weight of the column or pressure it exerted on the surface of the mercury were to increase the height of the column of mercury should also increase and vice versa. Torricelli envisioned that the air exerts pressure due to weight of the vast ocean of it over the surface of the earth. This pressure should be like the pressure you experience when diving into water. The deeper you go the greater the effect. He seems to have been one of the first people to see the world in this way.
In 1648, Blaise Pascal confirmed Torricelli's idea (1643) of the variability of air pressure. Pascal's experiment took a Torricellian Tube (barometer) from sea level to the top of a mountain and observed the predicted movement of the mercury in the tube as falling when it increase in altitude.
Conclusion and educational implications
In a short span of about 50 years, researchers understood the effect of air pressure on the movement of fluids in tubes, pumps, and siphons. By creating an experiment to measure the effect of air pressure on a column of mercury, Torricelli set the stage for an explosion of scientific thought by introducing a mechanical apparatus that could be used to measure pressure. By challenging the validity of a conceptual framework, he freed others to see the world in a new way.
The path of events culminating in Torricelli's experiment and Pascal's confirmation of the idea that air exerts pressure, provides a framework by which you can help your students experience the process of science and wrestle with restructuring their own world view. The basic notion is that there is a dynamic relationship between perception, experience, and explanation. Your students use their direct observations and common sense notions of the world to explain the events they encounter. What they can not see or experience directly (atmospheric pressure or vacuums) is very difficult for them to use in explaining the world around them. When faced with contradictory evidence, most students will try to "invent" a rationale to explain away the facts. The last thing that they will do is to restructure the way they conceptualize things in order to match the evidence collected.
The task of a teacher is to provide students with opportunities to challenge their ideas and allow them to participate in the scientific processes of collecting information, making arguments, and reworking their perception. The process is not swift or straight forward. In fact, it can be quite messy and confusing. The rewards come from the effort and ownership that students make in trying to convince others of their ideas and involve themselves in an activity that they help direct. Hopefully, they will gain an appreciation of the challenging work involved in explaining the simplest things and learn some science concepts, while having fun doing it.
James Bryant Conant, Case Studies in Experimental Science, vol.1,2, (Cambridge: Harvard University Press, 1952).
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