October, 1644: Torricelli Demonstrates the Existence of a Vacuum

Elegant physics experiment; enduring practical invention

 

Evangelista Torricelli

Ed. Note: This month’s column was written by guest author Richard Williams.

Evangelista Torricelli, born of a humble family, eventually rose to the top of the Italian intellectual community. He led Italy, and then the world beyond, to resolve a two-thousand-year-old philosophical debate about vacuum and the nature of space. He did this by performing and understanding a single elegant physics experiment. The apparatus he used was also a practical invention–the mercury barometer.

Torricelli was born at Faenza, Italy on October 15, 1608. “Left fatherless at an early age” he was sent to Rome for his education. His achievements there brought him to the attention of Galileo in Florence. He came there and lived with Galileo. Galileo was preoccupied with a problem of Tuscan well diggers who were frustrated in their attempts to raise water more than about ten meters with lift pumps. When they tried to raise it higher, the water separated from the pump plunger and would go no farther. Could this be due to a vacuum forming under the plunger? They asked Galileo why the water could not be pumped higher. He considered the problem seriously, but died in 1642 with it still unresolved.

Then, in 1644, Torricelli took up the problem. After some study of earlier experiments he did one of his own. The apparatus was a glass tube about a meter long, sealed at one end. He filled it with mercury, covered the open end, and inverted it over a dish of mercury. This was not as easy as it sounds today. Glass tubes at the time were fragile and hard to come by. They often broke when filled with a kilogram of mercury. But with the help of a skilled assistant the experiment was done. The mercury in the tube fell and stabilized at a level about 76 centimeters above the level in the dish. Torricelli surmised correctly that the mercury rose in the tube because of the weight of the atmosphere pressing down on the mercury in the dish, and that the space above the mercury column was a vacuum. It was the first time that a vacuum had been created in the laboratory, and understood as such.

The concept of a vacuum had been contentious since antiquity. Both Plato and Aristotle thought the existence of a vacuum to be impossible, against Nature. In medieval Europe, this was summed up by the expression: “Nature abhors a vacuum.” To discuss a vacuum became heretical and dangerous.

The word “vacuum” first appeared in the English language in 1550, introduced by Thomas Cranmer, the Archbishop of Canterbury, who composed the Book of Common Prayer, the central document of the Church of England. The phrase he used, as part of a theological argument, is cited in the Oxford English Dictionary:  “Naturall reason abhorreth vacuum, that is to say, that there should be any emptye place, wherein no substance shoulde be.” This was the sanctioned view, but, with the accession of the Catholic Queen Mary in 1553, the winds of orthodoxy shifted. Cranmer was convicted of heresy in 1555, and was burned at the stake the following year.

Torricelli’s achievement brought the concept of vacuum from the dialectics of antiquity into experimental physics. Mindful of the contention around the idea of vacuum, he did not make his experiment public at first, but disclosed it only in letters to a friend, Michelangelo Ricci. In October, 1644, the French scientist Marin Mersenne visited Torricelli, who repeated the experiment for him and gave him copies of the letters to Ricci. Mersenne took these to Blaise Pascal and others in France, disclosing Torricelli’s work publicly for the first time.

Pascal immediately understood the meaning of the experiment, and repeated it in 1646. He believed that the atmospheric pressure should decrease with altitude, and engaged a relative and some friends to carry a barometer up a mountain in the south of France. They found the anticipated decrease of pressure with altitude, laying the foundation for the science of meteorology.

Pascal understood the pressure to be equal to the weight of the atmosphere per unit area. He combined this with the surface area of the earth and calculated the total mass of the atmosphere. His result differed by less than 30% from the currently accepted value as cited in The Handbook of Chemistry and Physics. About the calculation, Pascal noted that “a child who knows addition and subtraction could do it,” a strong endorsement of the French school system.

Torricelli’s apparatus was the first mercury barometer. Minor improvements were later made to increase the precision of the readings, but the basic design remained unchanged. In meteorological stations around the world it served as the reference standard for measuring atmospheric pressure for more than three centuries, perhaps a record time for an instrument to be used with the same design. Finally, in 1977, the US National Weather Service announced that the mercury barometer would be replaced as the reference standard by a recently developed piezoelectric quartz crystal pressure transducer.

Torricelli stood at the nexus where, with a single elegant experiment, vacuum and the nature of space, defined in philosophical terms for two thousand years, gave way to the modern view, defined by experimental physics. In the twentieth century, physicists went far beyond this. They found, not an “emptye place, wherein no substance shoulde be,” but rather a vacuum filled with wonders: electromagnetic radiation, including that from the last gasp of the Big Bang; a sea of virtual particle-antiparticle pairs; a space bent out of shape by gravitational warping–all unimaginable to earlier physicists.

Following his work on the barometer, Torricelli did research in mathematics and physics. His formula for the efflux of a liquid from a small orifice in a container is still known as Torricelli’s Theorem. He died in Florence in 1647, at the age of thirty nine. A commemorative statue of him was erected in Faenza in 1864.

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