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Boyle's law, PV = constant at fixed temperature, is one of the first gas laws in Class 11 chemistry and physics. It is usually taught as one line. The story behind it is more useful: a new machine, a suggestion from a colleague, a careful measurement, and a scientist who refused to claim more than his data showed.
Who was Robert Boyle?
Boyle was born at Lismore, Ireland, on 25 January 1627 and died in London on 30 December 1691 (MacTutor, University of St Andrews). He was the 14th child of the Earl of Cork, the Science History Institute notes, and was tutored at home and abroad. In the 1650s he worked at Oxford alongside a group committed to observation and experiment.
He helped found the Royal Society in 1660. The Stanford Encyclopedia of Philosophy records that in its list of founding members he comes second only to the President, Viscount Brouncker.
What did the air pump let Boyle do?
His first scientific book was *New Experiments Physico-Mechanicall, Touching the Spring of the Air, and Its Effects* (1660). It came from three years of work with an air pump designed by Robert Hooke, whom Boyle employed as his assistant, MacTutor says. With it Boyle showed, among other things, that sound did not travel through the evacuated chamber and that a flame, like an animal, needed air.
"Spring" was his word for what we call pressure: air pushes back when squeezed, like a spring.
How did he find the pressure-volume law?
According to the Stanford Encyclopedia, it was Richard Towneley who suggested, from Boyle's air-pump results, that the spring and the volume of air might be inversely related. Boyle tested this with a long J-shaped glass tube, sealed at the short end. Pouring mercury into the long arm trapped air in the short arm. As he added mercury, the trapped air was squeezed, and he recorded its length against the height of mercury.
He reported, in the 1662 second edition, that air "reduced to half its wonted extent, obtained near about twice as forcible a spring as it had before", and that halving it again made the spring about four times that of ordinary air.
Worked example: a J-tube you can check
Here is a version with round numbers, not Boyle's own data.
The short arm traps a column of air 12 cm long. At the start the mercury is level in both arms, so the trapped air is at atmospheric pressure, 76 cm of mercury.
Mercury is added until the air column is 6 cm long. What is the height difference between the two mercury surfaces?
Since the tube has the same cross-section, volume is proportional to length. So P₁L₁ = P₂L₂.
76 × 12 = P₂ × 6, so P₂ = 152 cm of mercury.
The trapped air's pressure equals the atmosphere plus the extra mercury in the long arm: 152 = 76 + h, so h = 76 cm.
Halving the volume doubled the pressure, exactly what Boyle reported.
Common mistakes:
Forgetting the atmosphere. The pressure on the gas is atmospheric pressure plus the height difference, not the height difference alone.
Changing temperature. Boyle's law needs a fixed amount of gas at a fixed temperature. If the gas warms, use the full ideal gas equation.
Using volumes and lengths in different units on each side. Any unit works as long as both sides match.
For more practice, our guide to ideal gas problems, Dalton's law and units has a full set.
What else did Boyle do?
In *The Sceptical Chymist* (1661) he argued against the four classical elements and in favour of matter made of small particles, or corpuscles. The Science History Institute adds a caution worth keeping: his idea of an element was not the modern one, and he practised alchemy all his life, believing that he had seen metals transformed. Great scientists can be right about method and wrong about particular ideas.
In some countries the same relation is also called Mariotte's law, MacTutor notes.
What can you learn from Boyle's way of working?
Let a new tool open new questions. The air pump made the "spring" of air something he could measure.
Take a colleague's idea seriously and test it. Towneley suggested the pattern. Boyle designed the experiment.
Claim only what your data shows. His caution about whether the law held "universally" is good practice in any lab report.
If the link between particles and pressure is still hazy, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can go from Boyle's law to the kinetic theory of gases one lesson at a time.
Your next step: redo the J-tube example with an air column of 20 cm squeezed to 16 cm, and find the extra height of mercury.
