Earth 1

Chemist · Natural philosopher

Pastel portrait of Joseph Priestley in later life, grey hair, dark coat and white cravat

Joseph Priestley

1733 – 1804 · Birstall, Yorkshire → Northumberland, Pennsylvania

A self-taught minister who found oxygen, made the first fizzy water and showed that green plants remake the air we breathe.

Portrait: Ellen Sharples, c. 1794–97, pastel. Public domain, via Wikimedia Commons

The mouse and the mint

In a sealed jar a candle soon goes out and a mouse soon faints. Priestley found that a living sprig of mint, left in that spent air, put it right.

21.0 %

oxygen

0.0 %

carbon dioxide

0 min

elapsed

air good

jar

Each glowing dot is a share of oxygen; spent ones turn grey. Light the candle, let it die, then add the mint and watch the days pass.
“On the 17th of August, 1771, I put a sprig of mint into a quantity of air, in which a wax candle had burned out, and found that, on the 27th of the same month, another candle burned perfectly well in it.”
Priestley, Observations on Different Kinds of Air, Philosophical Transactions, 1772

He had seen the first half of photosynthesis. Jan Ingenhousz showed in 1779 that the plant only does it in light, which is why the sim stops at night. The full reaction:

  • CO₂: the carbon dioxide the candle and the mouse breathed out
  • H₂O: water the plant draws up
  • C₆H₁₂O₆: sugar the plant keeps as food
  • O₂: the oxygen it gives back, the good air Priestley measured

1 August 1774: oxygen

At Bowood he turned a 12-inch burning lens on red mercury calx sealed in a glass vessel standing in mercury. A gas came off. A candle in it blazed; a mouse lived far longer in it than in ordinary air.

20 °C

calx

0 mL

new air

×1

flame

Slide the lens until the sunlight meets in a point on the red calx. Above about 450 °C it splits: 2 HgO → 2 Hg + O₂.
“Hitherto only two mice and myself have had the privilege of breathing it.”
Priestley, Experiments and Observations on Different Kinds of Air, vol. II, 1775

  • HgO: mercury calx, the red powder
  • Hg: the beads of liquid mercury left behind
  • O₂: the new air, about 21 % of ordinary air, so a flame in pure oxygen burns nearly five times as hard

The first soda water

In Leeds his chapel stood next to a brewery. He hung bowls of water over the fermenting vats, poured the water back and forth, and it came out sparkling. In 1772 he published how to make it at home; in 1783 Johann Jacob Schweppe began selling it in Geneva.

1.0 atm

CO₂ pressure

2.8 g/L

dissolved

Sealed, the water takes up CO₂ in proportion to the pressure (Henry's law). Open it and the extra comes back out as bubbles.

  • c: how much CO₂ the water holds
  • k_H: a constant for CO₂ in water: about 2.8 g per litre per atmosphere when cold
  • p: the pressure of CO₂ above the water

William Henry wrote this law down in 1803; Priestley worked at one atmosphere.

The airs he named

Electricity, 1767

Franklin noticed that a cork inside a charged metal cup feels no force. Priestley saw what that meant: it only works if electric force falls off with the square of distance, like gravity. Coulomb measured it in 1785.

  • F: the push or pull between two charges
  • q₁, q₂: the two charges
  • r: the distance between them: double it and the force drops to a quarter
  • k: a fixed constant, 8.99 × 10⁹ N·m²/C²

Where he was wrong

He explained his gas with phlogiston, the old idea that burning releases a substance. Lavoisier, after hearing of the experiment, showed that burning takes oxygen in, named it in 1777, and founded modern chemistry. Carl Wilhelm Scheele in Sweden had made oxygen around 1771 but published only in 1777. Priestley never gave phlogiston up, and wrote a defence of it in 1800.

A free mind

He wrote on grammar, history, education, theology and politics as well as chemistry, argued for religious freedom and the American and French revolutions, and paid for it: the 1791 Birmingham riots burned his home and his laboratory. He rebuilt in Pennsylvania. Jefferson called him one of the few lives precious to mankind.

Timeline

  1. 13 Mar 1733

    Born at Fieldhead, Birstall, near Leeds, the eldest son of a cloth dresser. His mother dies when he is six; an aunt raises him.

  2. 1752 – 55

    Daventry Academy. As a dissenter he is barred from Oxford and Cambridge, so he teaches himself languages, maths and natural philosophy.

  3. 1761

    Tutor in languages and literature at Warrington Academy.

  4. 1765 – 66

    Meets Benjamin Franklin in London. Elected a Fellow of the Royal Society.

  5. 1767

    The History and Present State of Electricity. Minister at Mill Hill Chapel, Leeds, next door to a brewery, where he starts studying the gas over the vats.

  6. 1770

    Notes that a gum from the Indies rubs out pencil marks. The name rubber follows.

  7. 1771 – 72

    The mouse and the mint. Publishes Directions for Impregnating Water with Fixed Air and Observations on Different Kinds of Air.

  8. 1773

    Copley Medal, the Royal Society's highest honour. Becomes librarian and companion to the Earl of Shelburne at Bowood House, Wiltshire.

  9. 1 Aug 1774

    Heats red mercury calx with a burning lens. A candle in the gas burns with a remarkably vigorous flame. In October he describes it to Lavoisier over dinner in Paris.

  10. 1775

    Names it dephlogisticated air and breathes it himself.

  11. 1780

    Moves to Birmingham as minister of the New Meeting; joins Watt, Wedgwood, Boulton and Erasmus Darwin in the Lunar Society.

  12. 14 Jul 1791

    A Church-and-King mob burns his house, library and laboratory at Fair Hill for his support of the French Revolution.

  13. 1794

    Emigrates to Northumberland, Pennsylvania. Corresponds with Jefferson and Adams.

  14. 6 Feb 1804

    Dies at Northumberland, still defending phlogiston.

More minds on Thinkers and Become Legendary.

Sources