Air Pressure Experiments
By the late 1500s, scholars knew and were elaborating Archimedes’s system of hydrostatics, which depends on how the weight of water exerts pressure uniformly over a surface. Several scholars extended an analogy to air in the early 1600s, to ask if air “pressure” might be due to weight. The practical dimension of this lay in the working of pumps and siphons, limited to 34 feet of lift. Galileo, among others, tried empirical investigations of siphons and columns of water. His assistant (briefly) and successor at court, Evangelista Torricelli, elaborated the technique of filling a glass tube, closing one end, then inverting it into a vessel of liquid.
The question was why there was a regularity in this phenomenon. And what was the “empty” space at the top of the tube, once it settled? Torricelli accepted that it is a vacuum. By 1643 he was arguing that the height was due to the equilibrium between the the weight of the water in the tube and the pressing weight of the air (14.7 lb/sq in) pushing it up the column. In 1644 he built a tube using mercury instead of water. Being 13.6 times heavier, a mercury column should be 1/13.6 as high, or about 30 in, which could be demonstrated. Torricelli also argued that his instrument should function as a barometer: “an instrument which will exhibit changes in the atmosphere, which is sometimes heavier and denser and at other times lighter and thinner.”
Blaise Pascal, another member of the Mersenne circle, did two telling experiments to address the two questions at issue. Some argued that the space atop the closed column was full of vapor. Pascal compared tubes filled with wine with those filled with water. Wine is more vaporous and so should push the column down more. But wine is lighter, so the air would push the column up higher. A public experiment showed the wine to rise higher.
Pascal’s public demonstration in 1648 at Puy-de-Dôme of the effects of altitude on a column of mercury was particularly effective. Quoting from I.H.B. Spiers (ed.), 1937, The Physical Treatises of Pascal, New York: Columbia University Press:
“The weather on Saturday last, the 19th of [September], was very unsettled. At about five o’clock in the morning, however, it seemed sufficiently clear; and since the summit of the Puy de Dôme was then visible, I decided to go there to make the attempt. To that end I notified several people of standing in this town of Clermont, who had asked me to let them know when I would make the ascent. Of this company were some clerics, others laymen…. All these men are very able, not only in the practice of their professions, but also in every field of intellectual interest. It was a delight to have them with me in this fine work.
On that day, therefore, at eight o’clock in the morning, we started off together for the garden of the Minim Fathers, which is almost the lowest spot in the town, and there began the experiment in this manner.
First, I poured into a vessel six pounds of quicksilver [mercury] which I had rectified during the three days preceding; and having taken glass tubes of the same size, each four feet long and hermetically sealed at one end but open at the other, I placed them in the same vessel and carried out with each of them the usual vacuum experiment. Then, having set them up side by side without lifting them out of the vessel, I found that the quicksilver left in each of them stood at the same level, which was twenty-six inches and three-and-one-half lines above the surface of the quicksilver in the vessel. I repeated this experiment twice at this same spot, in the same tubes, with the same quicksilver, and in the same vessel; and found in each case that the quicksilver in the two tubes stood at the same horizontal level, and at the same height as in the first trial.
That done, I fixed one of the tubes permanently in its vessel for continuous experiment. I marked on the glass the height of the quicksilver, and leaving that tube where it stood, I requested Father Chastin…, a man as pious as he is capable, and who reasons very well upon these matters, to be so good as to observe from time to time all day any changes that might occur. With the other tube and a portion of the same quicksilver, I then proceeded with all these gentlemen to the top of the Puy de Dôme, some 500 fathoms [3000′] above the Convent.There, after I had made the same experiments in the same way that I had made them at the Minims, we found that there remained in the tube a height of only twenty-three inches and two lines of quicksilver; whereas in the same tube, at the Minims we had found a height of twenty-six inches and three-and-one-half lines. Thus between the heights of the quicksilver in the two experiments there proved to be a difference of three inches one line and a half. We were so carried away with wonder and delight, and our surprise was so great that we wished, for our own satisfaction, to repeat the experiment. So I carried it out with the greatest care five times more at different points on the summit of the mountain, once in the shelter of the little chapel that stands there, once in the open, once shielded from the wind, once in the wind, once in fine weather, once in the rain and fog which visited us occasionally. Each time I most carefully rid the tube of air; and in all these experiments we invariably found the same height of quicksilver…. This satisfied us fully.”
From Pascal’s treatise “On the Weight of the Air”:
" If one takes a balloon half-filled with air, shrunken and flabby, and carries it by a thread to the top of a mountain 500 fathoms high, it will expand of its own accord as it rises, until at the top it will be fully inflated as if more air had been blown into it. As it is brought down it will gradually shrink by the same degrees, until at the foot of the mountain it has resumed its former condition.
This experiment proves all that I have said of the mass of the air, with wholly convincing force; but it must be fully confirmed, since the whole of my discourse rests on this foundation. Meanwhile it remains to be pointed out only that the mass of the air weighs more or less at different times, according as it is more charged with vapor or more contracted by cold.
Let it then be set down: 1) that the mass of the air has weight; 2) that its weight is limited; 3) that it is heavier at some times than at others; 4) that its weight is greater in some places than in others, as in mountains and lowlands; 5) that by its weight it presses all the bodies it surrounds, the more strongly when its weight is greater.”
Evidence from these experiments showed the reality of air’s weight and pressure, and fostering acceptance of the reality of vacuum. That meant a new explanation to replace Aristotle’s on the action of pumps. Pascal argued for the mass of the air and the action of vacuums, contrary to the teachings of Aristotle. These famous experiments were rich propaganda for the new science, as Pascal recognized:
”Let all the disciples of Aristotle . . . learn that experiment is the true master that one must follow in Physics; that the experiment made on the mountains has overthrown the universal belief in nature’s abhorrence of a vacuum; . . . and that the weight of the mass of the air is the true cause of all the effects hitherto ascribed to that imaginary cause.”
Air pressure experiments became popular among the promoters of the new philosophy, especially as an easily repeatable critique of Aristotle’s physics. Otto von Guericke invented in 1650 an air pump to evacuate chambers. His Experimenta nova Magdeburgica de vacuo spatio [New Magdeburg Experiments Concerning Empty Space] of 1672 described experiments and illustrated the equipment.
He performed public demonstrations with an evacuated ball, showing the power of the air pressure and rarefaction. This one took place in 1654 in front of the Emperor Ferdinand III. The drawings of such demonstration were widely published.
Another patron and experimenter who began investigating the phenomena of air was Robert Boyle, who developed new equipment with the assistance of his hired experimenter Robert Hooke. This is an old engraving of one of Boyle’s vacuum-chamber flasks, showing his experiment on the sound of bell in a vacuum.
After 1660, Boyle did experiments with his air-pump driven vacuum flasks to demonstrate that Mersenne was wrong to declare that sound was unaffected in a vacuum. Notice the anecdotal style, the careful and precise description, and the clever experimental technique, in this short passage from a much longer 1669 publication on “physico-mathematical” experiments:
“The event of our trial was, that, when the receiver was well emptied, it sometimes seemed doubtful, especially to some of the by-standers, whether any sound were produced or no; but to me, for the most part, it seemed, that after much attention I heard a sound, that I could but just hear; and yet, which is odd, methought it had somewhat of the nature of shrillness in it, but seemed (which is not strange) to come from a good way off…. To discover what interest the presence or absence of the air might have in the loudness or lowness of the sound, I caused the air to be let into the receiver, not all at once, but at several times, with competent intervals between them; by which expedient it was easy to observe, when a little air was let in, the stroke of the hammer upon the bell (that before could now and then not be heard, and for the most part be but very scarcely heard) began to be easily heard; and when a little more air was let in, the sound grew more and more audible, and so increased, until the receiver was again replenished with air.”
The text goes on to describe several versions of the experiment, as he steadily modified it to take care of one objection or design flaw after another. For example, he pursued the question of mechanical transmission of the sound not through the air but up through the apparatus that holds the bell (or watch, in some other experiments). He tried several techniques for suspending the sound-producer, so that only the effect of the air was being measured.
Experiments on air pressure and vacuum jars, showing the properties of air, continued to be popular as an avenue into the new science. This 18th century painting by Joseph Wright shows the public interest in air pressure experiments, as the gentleman and his colleagues and his family gather around an air pump experiment.
