Boyle's Law (1662): Robert Boyle, the Air Pump and Gas Pressure

Boyle’s law, published by the Anglo-Irish natural philosopher Robert Boyle in 1662, states that the pressure of a trapped quantity of air rises as its volume is squeezed down, and falls as it expands. It grew out of a famous series of air-pump experiments that Boyle carried out in Oxford with his assistant Robert Hooke. The law was one of the first quantitative rules in physics established by careful measurement, and the work behind it helped define what “experimental science” meant.

Background: the vacuum debate

For centuries many scholars had followed Aristotle in holding that nature could not tolerate empty space. That view was shaken in 1643, when Evangelista Torricelli in Florence showed that a column of mercury in a sealed glass tube would settle at around 76 centimetres, leaving an apparently empty space above it. The experiment, described in our article on the Torricelli barometer, suggested that the weight of the atmosphere held the mercury up.

In 1648 Blaise Pascal arranged for a barometer to be carried up the Puy de Dôme in central France, showing that the mercury fell at altitude where there was less air above it. In the German lands, Otto von Guericke, mayor of Magdeburg, built the first air pump in the 1650s and staged dramatic demonstrations with evacuated copper hemispheres that teams of horses could not pull apart. News of Guericke’s machine reached England in a 1657 book by the Jesuit Kaspar Schott.

Robert Boyle (1627–1691), a younger son of the wealthy Richard Boyle, 1st Earl of Cork, had settled in Oxford in the mid-1650s. He belonged to a circle of experimenters who met there and later formed the core of the Royal Society. Hearing of Guericke’s work, he set out to build a better machine.

The air pump of Boyle and Hooke

Boyle hired the young Robert Hooke, who had a gift for mechanics, and in 1658–1659 the two produced a new pump. Its key feature was a large glass globe, the “receiver”, which could be emptied of most of its air while objects inside it remained visible. A brass cylinder and piston below it, worked by a rack and handle, drew air out stroke by stroke.

The machine was expensive, temperamental and often leaked, and only a handful existed in the early 1660s. Even so, it opened up a range of experiments that no one had been able to try before. Boyle placed candles, burning coals, bells, barometers, feathers and small animals in the receiver and recorded what happened as the air was withdrawn.

He found that a flame went out and a ringing bell grew faint, which showed that sound needed air to travel. A barometer inside the receiver fell as air was removed. Boyle’s account of these trials appeared in 1660 as New Experiments Physico-Mechanicall, Touching the Spring of the Air, and its Effects. By “spring” he meant the air’s elastic tendency to push back when compressed, much like a coiled spring or a fleece of wool.

Critics and the J-tube experiment

The book drew sharp responses. The Jesuit scholar Franciscus Linus argued that the mercury in a barometer was held up not by air pressure but by an invisible thread, the funiculus, pulling it from above. The philosopher Thomas Hobbes attacked Boyle in 1661, doubting that an air pump could ever produce a real vacuum and questioning whether experiments witnessed by a few gentlemen could produce certain knowledge.

Boyle replied in a second edition published in 1662, which included A Defence of the Doctrine Touching the Spring and Weight of the Air. To answer Linus he described a new experiment. He took a long glass tube bent into a J shape, sealed the short leg, and poured mercury into the open long leg so that a pocket of air was trapped in the short end.

As he added more mercury, the extra weight squeezed the trapped air into a smaller space. Boyle measured the length of the air column and the difference in mercury height at each step, adding the pressure of the atmosphere read from a barometer. He extended the trial to pressures below atmospheric with a second apparatus. His tables showed that when the pressure was doubled the volume of air was roughly halved, and when it was tripled the volume fell to about a third.

Who discovered the law?

Boyle did not claim the rule as his own idea alone. He wrote that the Lancashire gentleman Richard Towneley, after reading the first edition and working with the physician Henry Power, had suggested that pressure and expansion were related in a simple proportion. Boyle’s contribution was to test this “hypothesis” systematically and publish the figures. Some historians also stress Hooke’s role in the measurements.

In France, Edme Mariotte reached the same conclusion independently and published it in 1676 in an essay on the nature of air. He noted more clearly than Boyle that temperature should be kept steady. For this reason the rule is called Mariotte’s law in much of continental Europe, while English-speaking countries use Boyle’s name.

In modern notation the law is written PV = k, meaning that pressure multiplied by volume stays constant for a fixed amount of gas at constant temperature. Real gases follow it closely at ordinary pressures and temperatures but depart from it when they are highly compressed or near the point of turning liquid.

Key people

• Robert Boyle (1627–1691): experimenter, chemist and writer, author of The Sceptical Chymist (1661) and a founding Fellow of the Royal Society.
• Robert Hooke (1635–1703): Boyle’s assistant, who built the pump and later became the Royal Society’s Curator of Experiments.
• Richard Towneley (1629–1707) and Henry Power (1623–1668): northern English experimenters whose barometer observations suggested the pressure-volume relation.
• Thomas Hobbes (1588–1679): philosopher who rejected Boyle’s experimental programme.
• Edme Mariotte (c. 1620–1684): French priest and physicist who published the law independently in 1676.

Consequences and legacy

Boyle’s law became one of the building blocks of the gas laws. In the late 18th and early 19th centuries Jacques Charles and Joseph Louis Gay-Lussac added the link between volume and temperature, and the combined rules eventually became the ideal gas law. Physicists in the 19th century explained the behaviour through the kinetic theory of gases, in which pressure comes from countless molecules striking the walls of a container.

The air pump itself had a long career as a teaching and display instrument. A famous painting by Joseph Wright of Derby, An Experiment on a Bird in the Air Pump (1768), shows how such demonstrations entertained and unsettled audiences a century later.

Historians have also paid attention to how Boyle presented his results. He wrote long, detailed reports so that readers could act as “virtual witnesses”, listed failures as well as successes, and stressed that experiments should be repeated before trusted observers. Steven Shapin and Simon Schaffer’s study Leviathan and the Air-Pump (1985) used the clash with Hobbes to examine how experimental science gained its authority.

Timeline

• 1643: Torricelli’s mercury tube experiment in Florence.
• 1648: Barometer carried up the Puy de Dôme for Pascal.
• 1657: Kaspar Schott publishes an account of Guericke’s air pump.
• 1658–1659: Boyle and Hooke build their air pump in Oxford.
• 1660: Boyle publishes New Experiments Physico-Mechanicall; the Royal Society is founded in London in November.
• 1661: Hobbes and Linus publish criticisms of Boyle’s work.
• 1662: Second edition with the J-tube results, the basis of Boyle’s law.
• 1676: Mariotte publishes the same relationship in France.

Why it matters

Boyle’s law was an early demonstration that nature could be described by simple mathematical rules derived from measurement. Together with the air-pump experiments, it helped establish the idea that knowledge about the physical world should rest on repeatable, publicly reported experiments, a principle at the heart of modern science.