Conceptual Activity: Atmospheric Pressure and the Nature of Science

Note: The following three pages should be read in conjunction with the section The Development of the Concept of Atmospheric Pressure.

We can use our observations on collapsing cans as a window into the nature of science. Scientists, like curious kids, begin with simple observations. They note, for example, that for some mysterious reason water pumps are effective in 20- and 30 ft.-deep wells (see your Reader, p. 19), but not for 40- and 50 ft.-deep wells. They tinker with soda cans, and notice that these cans are fairly stable under most situations, but not under others. They poke a hole at the bottom of a can and notice that the water pours down in the same way it pours from a shower or a waterfall. They block this hole, and the water flow ceases, as one might expect. But then, perhaps by chance, they cover the top opening and to their astonishment discover that this is almost as effective in stopping the water flow as closing the bottom opening. They turn the can upside down, with neither hole covered, and the water flows freely. They cover the side hole, and the water still comes out, but by fits and starts.

Most likely, you too would have liked to know what was going on+finding an explanation that would throw light on your observations, an explanation that might, perhaps, enable you to predict other situations and bring a sense of order and predictability into all these scattered observations. For instance, when you invert a can full of water, the water gurgles. Would it gurgle on the surface of the moon (which doesn't have an atmosphere)? Would it gurgle on the surface of the planet Venus (which has a much heavier atmosphere than earth)? Would other liquids, e.g., mercury, behave like water?

The first important breakthrough on record in this area was made by Evangelista Torricelli (1608-1647). Torricelli didn't start from scratch, for he could resort to Aristotle's hypothesis that "nature abhors vacuum." Note that Aristotle's explanation is not as silly as it is sometimes made out to be. It would explain very well the collapse of the canthis collapse is nature's way of dealing with the vacuum it abhors. Similarly, the water's rise in the suction pumpaccording to this hypothesisis seen as nature's way of eliminating the vacuum it despises. The notion that nature abhors vacuum (or that the universe is full), begins to break down when you try to apply it to specific situations. Why does nature dislikes vacuum up to 34 ft. of water, but tolerates it beyond this point? Why is nature's tolerance for mercury-related vacuum higher, so that mercury in the suction pump only rises to some 2.5 ft.? How does nature go about fighting vacuum? Why did the water gurgle when you held the side hole, and freely flow when you didn't? And what about space—we now think that it is mostly vacuum, yet mother nature seems to tolerate it fairly well. And what about matter itself? Our own bodies, for instance, are made of atoms, but these atoms are for the most part empty space. If you could expand a single atom to the size of an orange, you would see nothing but empty spaceyou would need a microscope to see its nucleus, which is (besides its far smaller electrons) the only "thing" that's really there. Thus, a theory which does have some explanatory power gradually begins to lose its appeal. As we observe nature more carefully, the old hypothesis that nature hates vacuum explains less and less, and it often leads to wrong predictions. Note that we can still adhere to it by inventing some kind of an ad hoc explanation, e.g., nature is limited in its ability to fight vacuum. So, because a vacuum of 35 ft. is just too great to eliminate; nature does the best it can.

A scientist's first reaction is to try to save his time-honored theory by explaining away discrediting evidence. This conservatism throws perhaps some light on the fact that Aristotle's theory survived unchallenged for 2000 years. It takes self-confidence to think that all those bright people who lived before you, including your own teachers, could be mistaken. "If our civilization is to survive," says philosopher Karl Popper, "we must break with the habit of deference to great men. Great men may make great mistakes." The same could be said about scientific progress. The first step to scientific breakthroughs is breaking away from the tutelage of the past, from the shackles of authority and prejudice. Aristotle, Galileo, Kepler, Marx, and Einstein were human beings like you and I. Although a combination of luck, perseverance, curiosity, and talent brought them to the cutting edge of intellectual progress, it didn't confer upon them special wisdom, sainthood, or infallibility.

Once you have broken away from the misleading image of the "founding fathers," you use your imagination to come up with an alternative explanation to the observed phenomena. You now survey all the known facts on the subject and see whether your guess explains them better than the rival hypothesis. For instance, if you attributed the collapse of the can to a mere temperature shift, not to air pressure, would this explain the behavior of a water-filled soda can with two holes? Would it tell you why a glass immersed in water and then partially raised above the waterline retains the water?

If your guess fits in reasonably well with the known facts, you proceed to test itthis takes discipline and a willingness to listen to nature, not to your own preconceptions. For example, you predict the behavior of a water suction pump on the moon or of a mercury suction pump on Mt. Whitney. If your prediction comes true, you go on testing it through other means until you are fairly confident about it.

You may be motivated by such things as insatiable curiosity, perhaps also by vanity, the wish to give meaning to your life, or the need to solve some practical problems. But regardless of your original intentions, science often leads, unexpectedly, to useful applications. Torricelli had no idea that he was inventing a barometerhe was only concerned with the solution of a mystery. Yet, thanks to his efforts in pure science, it became eventually easier to forecast weather. It was soon discovered that even when Torricelli's column was left in the same place (and thus in the exact same altitude above sea level), the height of mercury fluctuated, meaning that air pressure is not constant. This led to the realization that these fluctuations are related to weather and to the widespread use of the barometer. Another practical result was one type of an altimeter (height gauge): if changes in the height of a mercury column are determined for the most part by altitude, then these changes could tell you how much lower or higher than sea level you are.

To sum up. Your experiment with the collapsing can provides you with a hands-on demonstration on the nature of science. You started with some observations, e.g., under certain conditions, aluminum cans collapse. This arouses your curiosity, e.g., Why do they collapse? You come up with a few guesses, e.g., (i) the inability of aluminum to stand sudden temperature shifts, (ii) condensation of the steam inside the can, the creation of partial vacuum, and collapse of the can by external air pressure, (iii) nature abhors vacuum. You now rely on your imagination to devise ways of testing these guesses, one at a time. You gradually eliminate some guesses. If you are lucky, you come up with a tentative answer to your original question. But then you begin to wonder about a related puzzle, e.g., Would a glass bottle collapse too? Does the collapse of the can have anything to do with the behavior of water in a suction pump? With a mercury barometer? You publish your results, other people all over the world check them, replicate them, teach them to others, and raise more questions. Gradually, the specialized scientific community generates a considerable body of facts and explanations about the atmosphere and the behavior of gases.

At times, the language scientists use is unfamiliar. Sometimes scientists use mathematical expressions. It often takes intellectual effort to understand a particular observation or explanation. But all this should not blind us to the simple, commonsense quality of natural science, the essence of which you have just experienced first hand. Science is simple, fun, and universal+we don't ordinarily think of ourselves in such terms; nevertheless, we are all scientists.

Discussion Questions

  1. We constructed a mercury barometer in class. Can a water barometer be constructed? How long would it have to be? Why didn't we use water+which is much safer and cheaper+to construct our barometer?
  2. Imagine that a cup of water is poured into a one-gallon aluminum can. The can is then placed on a stove burner until some of the water has boiled away. The can's only opening is then screwed on tightly with a lid and the can is placed outside, where the temperature is 20¦F. (At this temperature, most of the steam inside the can would be quickly converted to water droplets.) Recalling now that the same amount of steam occupies much more space than water, try to predict what may happen to the can and why.
  3. The moonour nearest neighbor in the vast universehas no atmosphere. A long time ago a fellow claimed that he actually visited the moon, that it had an advanced civilization, and that, among other things, these aliens were excellent at predicting the weather by means of fabulous mercury barometers. Is this possible? Why?
  4. Venus, on the other hand, has a dense atmosphere100 times denser, in fact, than ours. Ignoring for the present the sizzling temperatures, and the fact the all objects on Venus weigh a bit less than they weigh on Earth, can a suction pump be constructed on Venus? If such a pump can raise liquid mercury approximately 2.5 feet on earth, to what height would it raise mercury on Venus? Why?

    Source:  Moti Nissani.  Permission for the free use of this material is hereby granted.

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