Henry Cavendish

Who Made Acid Out of Air · Natural philosopher · 1731–1810

Who sparked ordinary air over an alkali for weeks together and got an acid out of it — showing that the two chief airs of the atmosphere combine under a strong enough spark, a century before there were engines to do it.

Who was Cavendish?

Cavendish was the grandson of two dukes, immensely rich, and so shy that he had a separate staircase built at his Clapham house to avoid meeting his own servants; he communicated with them by notes. The house was given over to apparatus. He published sparingly and left a great deal of first-rate work in drawers, including results on electricity that Maxwell edited a century later and found had anticipated Ohm and Coulomb. What he did publish is exact to a degree that was unusual then and would be respectable now. In 1766 he isolated and weighed inflammable air, which we call hydrogen, and treated it as a distinct substance. In 1784 he reported that when inflammable air is fired with dephlogisticated air, the entire product is water and nothing else — a result that entangled him in a long priority dispute with Watt and with Lavoisier, and which he described in the phlogiston language he never abandoned. The experiment that matters for the chemistry of the air came in 1785. He took common air, added more dephlogisticated air, and passed electric sparks through the mixture over a solution of alkali, continuing for weeks. The volume of air slowly diminished, and what the alkali absorbed was nitrous acid: the nitrogen and oxygen of ordinary air had combined under the spark and gone on to make an acid in water. He was measuring rather than moralising, and the significance for a world of hot engines and acid rain was a century and a half away. He also noticed that after all the phlogisticated air had been made to react, a small bubble remained — not more than a hundred and twentieth part of the whole — which would not combine with anything. He recorded the fraction exactly and left it. In 1894 Rayleigh and Ramsay showed that it was argon, and used Cavendish's own numbers. He also weighed the Earth, with a torsion balance, to within a fraction of a per cent. He died in 1810, alone, having asked to be left so.

Major achievements

• Isolated and weighed inflammable air as a distinct substance in 1766
• Showed that firing it with dephlogisticated air produces water and nothing else
• Sparked common air over alkali in 1785 and obtained nitrous acid from the air itself
• Recorded exactly the small unreactive residue that proved, a century later, to be argon
• Determined the density of the Earth with a torsion balance

Life in brief

• 1731 — Born at Nice: Grandson of two dukes.
• 1766 — Inflammable air: Isolated, weighed, and treated as a distinct substance.
• 1784 — The composition of water: Two airs fired together, and nothing but water.
• 1785 — Sparks over alkali: Nitrous acid made from common air, and a residue left over.
• 1798 — The density of the Earth: Weighed with a torsion balance.
• 1810 — Dies at Clapham: Alone, at his own request.

Explore the life of Cavendish — five chapters

1. The Back Staircase — A shyness that has no comparison
2. Inflammable Air — Hydrogen isolated, weighed, and treated as a substance
3. Weeks of Sparks Over a Solution of Alkali — 1785, and a bubble that would not react
4. Weighing the Earth — A torsion balance in a sealed shed, 1798
5. The Drawers — What Maxwell found a century later

Read the five-chapter life of Henry Cavendish →

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