In 1644 Evangelista Torricelli filled a glass tube with mercury, turned it upside down in a basin, and watched the metal fall until it stood about seventy six centimetres above the surface in the dish. Above it, in the sealed end of the tube, was a space containing nothing at all.
Two things had just been demonstrated, both of which contradicted more than twenty centuries of accepted physics. A vacuum can exist. And we are all living at the bottom of an ocean of air that presses on everything in it.
The Problem with the Pumps
The experiment began as a practical complaint. The gardeners and well diggers of Florence knew that a suction pump cannot lift water higher than about ten metres, no matter how well it is built. Beyond that height the column simply breaks and the pump draws air.
The accepted explanation went back to Aristotle. Nature abhors a vacuum, and when a piston is drawn up, water follows because empty space cannot be permitted to form. The doctrine was known as horror vacui, and it had the useful property of explaining suction without any need for a mechanism.
It also had no explanation for the ten metre limit. If nature genuinely refuses to allow empty space, why should it give up at a particular height?
The Grand Duke of Tuscany put the question to Galileo, who discussed it in the Discorsi of 1638, his last book. Galileo suggested that the resistance to a vacuum is real but finite, a measurable force that can be overcome, and he described it as the strength of a column of water breaking under its own weight. He was moving in the right direction without arriving.
Our edition of the Discorsi contains the discussion, written while Galileo was under house arrest and smuggled out to be printed in Leiden.
Torricelli’s Reversal
Torricelli was born in 1608, studied in Rome, and came to Florence in 1641 to work with Galileo, serving as his secretary during the last three months of his life. On Galileo’s death he was appointed mathematician to the Grand Duke.
His insight was to invert the whole question. The pump does not pull the water up. The atmosphere pushes it up, and the atmosphere can only push so hard.
Air has weight. A column of air extending from the ground to the top of the atmosphere presses down on every surface, including the surface of the water in a well. When a pump removes the air above a column of water, the outside pressure drives water up the pipe. Ten metres of water is simply the height whose weight balances the weight of the air. Nothing mysterious is holding it up and nothing is refusing to be empty.
The reasoning produced an immediate quantitative test. Mercury is about thirteen and a half times denser than water, so the same air pressure should support a column of mercury about thirteen and a half times shorter: roughly seventy six centimetres instead of ten metres. A tube of that length is a piece of laboratory glassware. A ten metre water column is a building project.
The Experiment
In 1644, with the assistance of Vincenzo Viviani, Torricelli filled a metre long glass tube, sealed at one end, with mercury. He covered the open end, inverted it into a dish of mercury, and uncovered it.
The mercury dropped and settled at about seventy six centimetres, leaving a gap at the closed top. The predicted height appeared.
The space above the mercury is now called the Torricellian vacuum. It is not perfect by modern standards, since a little mercury vapour is present, but nothing else could have entered: the tube had been full and the top was sealed.
Torricelli described the result in a letter to Michelangelo Ricci in 1644 with a sentence that has outlived most of his mathematics. We live, he wrote, submerged at the bottom of an ocean of the element air, which by experiment undoubtedly has weight.
He also noticed something that turned the apparatus into an instrument. The height of the column was not constant. It varied slightly from day to day, and the variation correlated with changes in the weather. He had built the first barometer without setting out to.
The Objections
The claim was resisted, and not only by Aristotelians.
Descartes held that extension is the essence of matter, so a space with nothing in it is a contradiction in terms: if there is a distance between the top of the mercury and the glass, something must be occupying it. Descartes and others therefore proposed that the apparently empty space was filled with a subtle matter capable of passing through glass.
This is not a foolish position. It cannot be refuted by looking at the space, since the whole point is that the substance is undetectable. What can be tested is Torricelli’s positive claim about air pressure, and that is where the argument was decided.
Pascal and the Mountain
Blaise Pascal took up the question in France and saw the decisive test. If the mercury is held up by the weight of the air above it, then carrying the apparatus up a mountain, where less atmosphere remains overhead, must lower the column.
In September 1648 his brother in law Florin Périer carried a barometer up the Puy de Dôme in the Auvergne, roughly a thousand metres of ascent, while a second barometer was watched continuously at the bottom as a control. The mercury at the summit stood about eight centimetres lower than at the base, and it rose again on the descent. The control did not move.
No version of horror vacui predicts that nature’s abhorrence weakens with altitude. Air pressure predicts it exactly. Pascal followed up with further experiments, including carrying a barometer up a church tower in Paris to measure the smaller effect there.
What Followed
Once the vacuum was accepted as producible, it became a laboratory tool.
Otto von Guericke in Magdeburg built air pumps and staged the most theatrical demonstration of the century in 1654, evacuating two copper hemispheres pressed together and harnessing teams of horses that could not pull them apart. Nothing held the hemispheres together except the air outside.
Robert Boyle, working with Robert Hooke as his assistant, built an improved pump in Oxford and used it for a systematic programme: showing that sound does not travel through a vacuum, that a flame goes out in it, that animals cannot survive in it, and that a feather and a coin fall at the same rate. Out of that work came the relationship between the pressure and volume of a gas that carries Boyle’s name.
The barometer itself became the first instrument of quantitative meteorology, and remained the basis of weather forecasting for three centuries. It belongs on any short list of the instruments that changed what humans could see.
Torricelli’s Other Work
He died of typhoid fever in 1647, aged thirty nine, three years after the experiment.
His mathematics was substantial. He worked on the cycloide, on the area under curves using early infinitesimal methods that fed into the development of the calculus, and produced Torricelli’s law, which gives the speed at which fluid escapes from a hole in a container in terms of the depth of the fluid above it. He was also an exceptional lens grinder, and telescopes made by him were regarded as among the best available.
The unit of pressure called the torr is named after him, and a barometric pressure of one atmosphere is still commonly quoted as 760 millimetres of mercury, which is the height of his column.
The Shape of the Argument
The episode is a compact example of how a physical theory gets replaced.
The old doctrine explained the phenomenon qualitatively and could not account for the numbers. The new proposal explained the same phenomenon with a mechanism, air has weight and presses, and immediately generated a prediction that was specific, quantitative, and easy to check: use mercury and the column should be about seventy six centimetres.
When the prediction held, opponents shifted to an alternative that could not be tested directly, and the question moved to a second prediction, altitude, which decided it. At no point did anyone have to see a vacuum or prove that nothing is there. They only had to show that the amount of air above the apparatus determines what happens inside it.
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