Chemistry 1662

New Experiments Physico-Mechanical, Touching the Spring of the Air

Robert Boyle

Squeeze a gas into half the space and it pushes back twice as hard — pressure times volume stays fixed.

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In depth · the introduction

Put your thumb over the hole of a bicycle pump and press: the harder you squeeze, the harder the trapped air shoves back. Boyle found the exact rule behind that shove.

Squeeze it, and it pushes back — by the numbers

Air has a springiness Boyle called the spring of the air: it presses outward, and the more you compress it, the more strongly it presses. His discovery was how much more.

Squeeze a sealed pocket of air into half its space, and it pushes back exactly twice as hard. Into a third of its space, three times as hard. Volume down, pressure up, in perfect step — so that volume times pressure always comes to the same number. That is Boyle's law.

A tube, a critic, and a hypothesis borrowed by name

Robert Boyle was a wealthy Anglo-Irish gentleman and a founder of the Royal Society, working in Oxford with a young assistant, Robert Hooke, who built him an air-pump. In 1660 he published experiments arguing that air is elastic. A Jesuit named Francis Linus disagreed, claiming an invisible thread, not a spring, held things up.

To settle it, Boyle bent a glass tube into a J, trapped air in the sealed short arm, and poured mercury down the long one to crush it — reading off the volume and the pressure space by space. The neat inverse rule he found, he honestly attributed to another man, calling it “Mr Towneley's hypothesis,” after Richard Towneley, who had proposed it. Boyle's gift was to measure it cleanly enough to prove it.

Why it mattered

It was the first time anyone had pinned a formless gas to a hard number. Air, which seems like nothing, turned out to obey a law as exact as a clock. That single reproducible relationship — and the public, instrument-and-table way Boyle argued for it — helped show the world what the new experimental science could do.

A spring you can weigh

Think of the trapped air as a spring inside a bicycle pump. Push the plunger halfway down and the spring resists with twice the force; push it to a third and it resists threefold. Boyle's law says a pocket of gas is exactly that kind of spring — and unlike a metal coil, every gas, anywhere, obeys the same rule.

Its place in the story

Boyle's law is the first of the gas laws. A century later Charles tied volume to temperature, and Gay-Lussac tied pressure to temperature; Avogadro (see avogadro-1811) tied volume to the number of molecules. Knit together, the four become the ideal gas law, PV = nRT. And the deeper “why” — why a gas springs back at all — arrived only with the kinetic theory, when pressure was revealed as the ceaseless hammering of molecules too small to see, the same molecules Dalton (see dalton-1808) and Avogadro were learning to count.

The original document

Original source text

Robert Boyle · New Experiments Physico-Mechanical, Touching the Spring of the Air · Oxford, 1660 · the law in the “Defence…” appendix, 2nd edition 1662

The spring of the air

In 1660, working with an air-pump that his assistant Robert Hooke had designed and built, Boyle argued that air is elastic — that it behaves like a fleece of tiny coiled springs, pressing outward and pressing harder the more it is compressed. He named this elasticity the spring of the air.

Linus's objection

The Jesuit Francis Linus rejected the idea, holding that an invisible thread-like pull — his “funicular hypothesis” — held up the mercury, rather than any spring. In the second edition of his book (1662) Boyle added an appendix, A Defence of the Doctrine Touching the Spring and Weight of the Air, to answer him with measurement instead of argument.

The J-tube experiment

Boyle bent a glass tube into the shape of a J, sealed at the short end. Pouring mercury into the long open arm trapped a column of air at the top of the short one. He marked the trapped air in equal spaces and read the height of mercury that balanced it. As more mercury was added the air shrank and the pressure on it rose — from 48 equal spaces at 29 1/8 inches of mercury down to 12 spaces at 117 9/16 inches, a fourfold squeeze answered by a fourfold pressure.

the same air being brought to a degree of density about twice as great as it had before, obtains a spring twice as strong as formerly.

The relation — that pressure and volume stand in reciprocal proportion — Boyle credited not to himself but to Richard Towneley, calling it “Mr Towneley's hypothesis.” He set his measured pressures beside the values the hypothesis predicted, column against column, and left the reader to judge how closely they agreed.

[ … ]

Boyle never wrote an equation, nor used “pressure,” “volume,” and “temperature” in their modern senses; he reasoned with the spring and the density of the air, keeping the warmth of the room steady without isolating it. In France the same law is named for Edme Mariotte, who published it independently in 1679 and added the condition Boyle had left tacit — that the temperature must be held constant.

Oxford · 1662