Joseph Black & “Fixed Air”The gas that changed everything — and the birth of quantitative chemistry

A Scot Born in Bordeaux

Black was born on 16 April 1728 in Bordeaux, the fourth of twelve children of John Black, a Belfast-born wine merchant of Scottish descent, and Margaret Gordon, the Aberdeen-born daughter of another Scots wine factor. Though born on French soil he was, in his biographers' phrase, “by blood a pure Scot.”

Schooled in Belfast from about 1740, he entered the University of Glasgow around 1746 and fell under the influence of William Cullen, who in 1747 had founded the first independent chemistry lectureship in the British Isles — the first man in Britain to treat chemistry as an important science rather than an appendage to medicine. Cullen promoted Black from lecture room to laboratory and made him his assistant.

In 1752 Black moved to Edinburgh to complete his medical studies under the physiologist Robert Whytt and the botanist-chemist Charles Alston. There he did the work that earned his MD in 1754. His other towering achievement — latent heat — came later, during his Glasgow years.

Chemistry Before Black

The reigning theory of combustion was phlogiston, developed from Johann Joachim Becher's 1667 “combustible earth” and renamed around 1700 by Georg Ernst Stahl. Anything combustible was said to contain phlogiston; burning released it, leaving a calx behind. Elegant, all-purpose — and entirely wrong.

Equally limiting was the inherited idea of air. Since Aristotle, air was one of the four elements: a single, uniform substance. A few pioneers had glimpsed the edges of the truth.

The gap was glaring: nobody had proved, by controlled reactions and careful weighing, that a particular gas was a distinct chemical entity separable from common air by repeatable tests.

The 1754 Thesis

It began as medicine. Black set out to study antacids and “lithontriptic” remedies for bladder and kidney stones. He had meant to work on lime-water from quicklime — but Alston and Whytt were locked in a public feud over it, so the prudent young graduand chose the less contested magnesia alba instead. It proved a fateful choice.

His experiments ran as a planned, quantitative cycle, with the balance used at every stage:

• Heating

  Strongly heated, magnesia alba lost a large proportion of its weight and gave off a gas, leaving magnesia usta (magnesium oxide) that no longer effervesced with acid.

• Acid release

  The same gas came off, vigorously, when acids were poured on magnesia alba or on chalk.

• Testing the gas

  It would not support combustion — “a piece of burning paper, immersed in it, was put out as effectually as if it had been dipped in water” — was heavier than air, was fatal to animal life, and turned lime-water milky.

• Breath

  The same gas was in exhaled breath. The gas driven from cold stone by fierce heat was the gas a living animal breathed out.

• Fermentation

  The same gas again appeared in fermentation.

• Reversibility

  Adding potash solution to the heated magnesia usta produced a solid weighing the same as the original magnesia alba. The gas had been put back.

He set out the full cycle: limestone → (heat) → quicklime + fixed air; quicklime + water → slaked lime; slaked lime + fixed air → limestone again.

“To this I have given the name fixed air, and perhaps quite improperly; but I thought it better to use a word already known in philosophy than to invent a new name.”Joseph Black

The name reflected the insight: the gas had been fixed — chemically locked — inside the solid, and heat or acid unfixed it.

The 1756 Paper

Black read a fuller account to the Philosophical Society of Edinburgh in 1755, published the following year as “Experiments upon Magnesia Alba, Quick-Lime, and Some Other Alcaline Substances” in the Society's Essays and Observations, Physical and Literary (1756).

It broadened the work from magnesium to calcium salts, laid out the lime-water test and the reversible chemistry in full, and settled a hotly contested Edinburgh argument by demonstrating that the gas released when chalk is burned to quicklime comes from the chalk, not from the fire. Historians regard it as the founding document of pneumatic chemistry — and it was essentially his only major chemical publication.

Why It Changed Chemistry

The bombshell was conceptual: there is more than one kind of air. Gases can be chemically distinct, and a gas can combine with a solid. The ancient single-element notion of air collapsed.

Henry Cavendish isolated “inflammable air” — hydrogen — in his 1766 paper Three Papers Containing Experiments on Factitious Airs, winning the Royal Society's Copley Medal; his second paper in that series dealt with Black's fixed air. Joseph Priestley isolated oxygen in 1774, working squarely in the tradition Black had opened, and credited him in print in 1775: the gas from fermenting liquors “obtained the name of fixed air, especially after it had been discovered by Dr. Black of Edinburgh.”

Antoine Lavoisier then used those new gases to demolish phlogiston and found modern chemistry.

The Lime-Water Test

Bubble a gas through lime-water — a clear solution of calcium hydroxide — and if it contains carbon dioxide the liquid turns milky as insoluble calcium carbonate precipitates:

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Simple, visual, unmistakable — and still the standard schoolroom test for carbon dioxide nearly 270 years on. Few discoveries leave a legacy you can repeat with a straw and a glass of cloudy water.

From Antacids to Fizzy Water

Savour the irony. The whole investigation began as medicine: magnesia alba and chalk were antacids, lime-water a remedy for stones. The chemistry of stomach alkalis led Black to the gas locked inside them.

A few years later, in 1767, Joseph Priestley — living next to a Leeds brewery, experimenting with the fixed air bubbling off the fermenting vats — worked out how to dissolve that gas in water and so invented carbonated water. He published the method in 1772 as Directions for Impregnating Water with Fixed Air, and related Royal Society work won him the Copley Medal in 1773. The chemistry that began as an inquiry into an antacid is the chemistry that puts the fizz in every sparkling drink.

Edinburgh and the Enlightenment

Black was no isolated genius. He was a luminary of the Scottish Enlightenment, counting among his close friends the philosopher David Hume, the economist Adam Smith (Black was one of Smith's literary executors), the geologist James Hutton and the engineer James Watt, whom he mentored. Black, Smith and Hutton together founded Edinburgh's Oyster Club.

And he taught. Edinburgh's medical school was the finest in the English-speaking world, drawing students from Ireland, the Continent and America who carried his ideas home. The “Edinburgh Model” was emulated as far afield as the University of Pennsylvania and McGill.

Timeline

1. c. 1609–1640s

   Jan Baptist van Helmont describes “gas sylvestre” and coins the word “gas”

   The first sighting of CO₂ — qualitative, not quantitative

2. c. 1700

   Georg Ernst Stahl names phlogiston

   The theory that hid gases from view for a century

3. 1727

   Stephen Hales pioneers collecting gases over water

   The pneumatic trough: the hardware, but not the concept

4. 1747

   William Cullen founds the first independent chemistry lectureship in the British Isles

   Black's teacher makes chemistry a science in its own right

5. 1754

   Black presents his MD thesis on magnesia alba, 11 June

   First quantitative, reversible characterisation of a distinct gas

6. 1756

   “Experiments upon Magnesia Alba, Quick-Lime, and Some Other Alcaline Substances” published

   The founding document of pneumatic chemistry

7. 1766

   Henry Cavendish isolates “inflammable air” (hydrogen)

   The chain Black started begins to run

8. 1767

   Joseph Priestley dissolves fixed air in water

   Carbonated water — published as a method in 1772

9. 1774

   Priestley isolates oxygen

   Explicitly crediting “Dr. Black of Edinburgh”

10. 1789

    Lavoisier publishes his Traité élémentaire de chimie

    Phlogiston falls; modern chemistry begins

11. Today

    CO₂ sits at the centre of the carbon cycle and climate science

    The gas Black named now shapes the planet's future

Why It Matters Today

Every modern analytical laboratory works the way Black's did: measure the mass, follow the substance, close the loop. Quantitative method is his deepest bequest.

The gas itself became one of the most consequential molecules on Earth — central to photosynthesis, the carbon cycle, and, as the work of John Tyndall in the 1860s and Svante Arrhenius in 1896 later revealed, the greenhouse effect. Black identified CO₂; he could not have dreamed what it would come to mean.

And in a school laboratory this week, somewhere, lime-water is turning cloudy.

The Chain of Influence

1. Van Helmont (c. 1609) — Names “gas”; describes gas sylvestre↓
2. Hales (1727) — Collects gases over water — the pneumatic trough↓
3. Black (1754–1756) — Proves fixed air is a distinct chemical substance, by weight↓
4. Cavendish (1766) — Isolates hydrogen↓
5. Priestley (1774) — Isolates oxygen, crediting Black↓
6. Lavoisier (1789) — Overthrows phlogiston; founds modern chemistry↓
7. Tyndall & Arrhenius (1860s–1896) — CO₂ identified as a greenhouse gas↓
8. Today — Carbon cycle, climate science, and every school chemistry lab

Did You Know?

• Black discovered carbon dioxide while investigating stomach antacids — one of the great serendipities of science.
• He chose magnesia alba partly to avoid a feud between two Edinburgh professors over lime-water.
• The lime-water test he established is still taught in chemistry classrooms worldwide, nearly three centuries on.
• Van Helmont noticed the same gas 150 years earlier and even coined the word “gas” — but Black proved it was a distinct chemical entity.
• Sir William Osler wrote that there is “perhaps no other instance of a graduation thesis so weighted with significant novelty.”
• The same work made Black the first to recognise magnesia — and so magnesium — as distinct from lime.
• Priestley used Black's fixed air to invent carbonated water in 1767.
• Black's student Daniel Rutherford went on to isolate nitrogen.

Honest Caveats

“Discovery” must be qualified. Van Helmont described and named a CO₂-like gas first. Black's priority is specifically the rigorous, quantitative characterisation — not the first sighting.

Dates vary across sources. The thesis was presented on 11 June 1754; the expanded paper was read in 1755 and published in 1756. Popular accounts often collapse these into a single year.

He did not invent the analytical balance. Black made no explicit mention of a special balance; what was new was his use of weighing to probe reactions.

The climate link is retrospective. Black, Tyndall and Arrhenius were not working toward climate science. This is a chain of knowledge, not a prophecy.