Joseph Louis Gay-Lussac

Chapter 1 · The Terror, the Polytechnique and Arcueil

A new institution for a new republic

1778 – 1804 · Saint-Léonard-de-Noblat · Paris · Arcueil

Joseph Louis Gay-Lussac was born in December 1778 at Saint-Léonard-de-Noblat in the Limousin. His father was a lawyer and royal official, and was imprisoned during the Terror; the family's position collapsed and Gay-Lussac's education was interrupted.

He entered the École Polytechnique in Paris in 1797. This institution matters. Founded in 1794 to train engineers and officers for the Republic, it was the most rigorous scientific and mathematical school in Europe: entry by competitive examination regardless of birth, a curriculum built on mathematics and laboratory work, and teaching by working scientists — Monge, Fourcroy, Berthollet, Laplace.

Gay-Lussac was noticed by Claude Louis Berthollet, who had worked with Lavoisier and who took him into his private circle at Arcueil, just outside Paris.

The *Société d'Arcueil* was an unusual arrangement: Berthollet and Laplace built laboratories at their neighbouring country houses and gathered a small group of young men — Gay-Lussac, Thénard, Humboldt, Biot, Arago — who worked, argued and published together. It was, for about fifteen years, the most productive concentration of physical science anywhere.

Gay-Lussac's first significant result came in 1802: all gases expand by the same fraction of their volume for the same rise in temperature.

He credited the unpublished work of Jacques Charles, done around 1787, which is why the relation is generally called Charles's law. Gay-Lussac established it properly and published it, and gave the credit away.

“Gases combine among themselves in very simple proportions by volume.”

— Joseph Louis Gay-Lussac, Mémoire sur la combinaison des substances gazeuses (1808)

Why this matters

Gay-Lussac established the expansion law now called Charles's law and credited it to Charles's unpublished earlier work, which is why his name is not on it.

You have the Polytechnique and the circle at Arcueil. What would you ask?

Ask Gay-Lussac

• “What made the École Polytechnique different?”
• “What was the Société d'Arcueil actually like?”
• “Why credit Charles for a law you established?”
• “What did the Terror do to your family?”
• “What does 'all gases expand alike' actually mean?”

Chapter 2 · Seven Thousand Metres, Alone

September 1804

1804 · Paris · The upper atmosphere

In 1804 the French Academy wanted to know whether the atmosphere changes with height: whether its composition alters, and whether the strength of terrestrial magnetism falls off with distance from the Earth.

The only way to find out was to go up.

Gay-Lussac made two ascents by hydrogen balloon. The first, in August, was with Jean-Baptiste Biot, and reached about four thousand metres.

The second, on 16 September 1804, he made *alone*, having jettisoned everything unnecessary — including, according to the account, a chair — to gain height. He reached approximately 7,016 metres, a record that stood for about half a century.

At that altitude the air pressure is under half that at sea level and the temperature was around minus nine degrees. He had no oxygen, no pressurised cabin and no protective clothing beyond ordinary winter dress. He reported difficulty breathing, a rapid pulse, a dry throat and numbed hands, and had trouble reading his instruments.

He made magnetic measurements, recorded temperature and pressure, and — the point of the exercise — filled evacuated glass flasks with air at altitude and sealed them.

Back on the ground he analysed them. The composition was the same as air at sea level: about one part oxygen to four of nitrogen. The atmosphere is *mixed*, not stratified by weight. And the magnetic intensity was unchanged within his ability to measure it.

Both results are negative, and both are important: they establish that the atmosphere is homogeneous through the region we live in, and that a scientist should go and get the sample rather than argue about it.

Why this matters

Gay-Lussac risked his life for two negative results — the air is the same up there, and so is the magnetism — and negative results of that quality are rare and valuable.

You have the balloon at seven thousand metres. What is your question?

Ask Gay-Lussac

• “What did it feel like at seven thousand metres?”
• “Why go up alone rather than with Biot?”
• “How do you collect a sample of air at altitude?”
• “Why does it matter that the air is the same up there?”
• “Was the risk justified?”

Chapter 3 · Simple Ratios by Volume

1808, and a result whose explanation he refused

1808 – 1811 · Paris

In 1808 Gay-Lussac announced a regularity: when gases react, the volumes that combine — and the volume of any gaseous product — stand to one another in *simple whole-number ratios*, measured at the same temperature and pressure.

Two volumes of hydrogen combine with one volume of oxygen to give two volumes of water vapour. One volume of nitrogen combines with three of hydrogen to give two of ammonia. One volume of hydrogen with one of chlorine gives two of hydrogen chloride.

These are not approximate. They are exact within the accuracy of measurement, and the whole numbers are small.

That is a very striking fact, and it is *begging* for an explanation in terms of particles — as Dalton's whole-number *mass* ratios had been.

Avogadro supplied it in 1811. Suppose equal volumes of any gas, at the same temperature and pressure, contain equal numbers of particles. Then the volume ratios *are* the particle ratios directly, and the formula of water follows at once. But it requires accepting that the particles of hydrogen and oxygen gas are *pairs* of atoms.

Gay-Lussac did not take it up. Neither did Dalton, who went further and disputed the accuracy of the volume measurements themselves.

Gay-Lussac's general position explains it. He regarded a law as a *regularity in the numbers*, and what lay underneath as speculation. He measured, published the regularity, and declined to theorise about invisible particles.

That position is defensible and it cost chemistry fifty years. The formula question was not settled until Cannizzaro pressed Avogadro's case at Karlsruhe in 1860, ten years after Gay-Lussac's death.

Why this matters

Gay-Lussac's combining volumes is exactly the evidence Avogadro's hypothesis explains, and his refusal to theorise meant the connection went unmade for fifty years.

You have the ratios and the explanation set aside. What would you ask?

Ask Gay-Lussac

• “Why should gases combine in whole numbers by volume?”
• “Why not accept Avogadro's explanation?”
• “Is a law just a regularity in the numbers?”
• “What did Dalton object to in your measurements?”
• “What would it have cost you to speculate?”

Chapter 4 · Iodine, Boron and a Race With Davy

Working under wartime blockade

1808 – 1815 · Paris

In 1808, working with Louis Jacques Thénard, Gay-Lussac isolated *boron* by heating boric acid with potassium — nine days before Humphry Davy reached it in London by electrolysis. Both are credited.

The iodine episode is sharper, and it happened in wartime.

In 1811 Bernard Courtois, a Parisian saltpetre manufacturer, noticed that adding too much sulphuric acid to seaweed ash produced clouds of a striking violet vapour that condensed to dark crystals. He had no facilities to investigate it and passed samples to others.

In 1813 both Gay-Lussac in Paris and Humphry Davy — travelling in France with a special passport granted by Napoleon despite the war, and with the young Faraday as his assistant — worked on it within weeks of each other.

Both concluded it was a new element. Gay-Lussac named it *iode*, from the Greek for violet, and published a substantial memoir. Davy sent a communication to the Royal Society. A sour priority dispute followed, sharpened by the fact that the two countries were at war and each man's supporters were disinclined to concede anything to the other.

Modern assessment credits Courtois with the discovery and both Gay-Lussac and Davy with independently establishing it as an element.

Gay-Lussac also prepared *cyanogen* in 1815, one of the first compound radicals identified — a group of atoms that passes intact through reactions as though it were an element — which fed directly into the radical theory of organic chemistry.

And he made extensive measurements of the *solubility* of salts across a range of temperatures, presenting the results as curves. That graph is the ancestor of the solubility curve every chemistry student reads today.

Why this matters

The iodine dispute shows two nations at war arguing over priority, and shows that a discovery often has three people in it and one name attached.

You have the violet vapour and the wartime race. What is your question?

Ask Gay-Lussac

• “Who actually discovered iodine?”
• “What was it like competing with Davy in wartime?”
• “Why does cyanogen matter?”
• “Why draw solubility as a curve rather than a table?”
• “How do you prove a new substance is an element?”

Chapter 5 · The Burette

The state assay office, and the apparatus on every school bench

1824 – 1850 · Paris · The Mint

Gay-Lussac's most widely used invention came from a bureaucratic problem.

The French state needed to determine the silver content of coin and bullion accurately, quickly and repeatedly. The traditional method — cupellation, in which the sample is melted with lead in a bone-ash dish — is slow, requires great skill, and loses some silver.

Gay-Lussac developed a *volumetric* method instead. Dissolve the silver in nitric acid. Then run in a solution of sodium chloride of accurately known strength from a graduated glass tube fitted with a tap, drop by drop, until no further precipitate of silver chloride forms. The *volume* of chloride solution used tells you how much silver was present.

The apparatus is the *burette*. The procedure is *titration*. It was adopted by the Paris Mint in 1832 and became the standard method across Europe.

Every school chemistry laboratory still has a rack of them, and the technique a pupil learns — fill the burette, run it in, swirl, watch for the indicator to change, read the volume, repeat until three results agree — is essentially the procedure Gay-Lussac established for weighing the King's silver.

What he did not have was the *mole*. He worked in equivalents and could not express a concentration as an amount of substance per unit volume; that had to wait for Avogadro's hypothesis to be accepted.

He held chairs at the Sorbonne, the Jardin des Plantes and the Polytechnique, sat in the Chamber of Deputies and later the Chamber of Peers, and advised on gunpowder and on the manufacture of sulphuric acid — the Gay-Lussac tower, for recovering nitrogen oxides, is his.

He died in Paris in May 1850, aged seventy-one.

Why this matters

The burette and the titration on every school bench were developed to assay the silver in French coinage, and the procedure has barely changed.

You have the burette, the mint and the drop-by-drop. What would you ask?

Ask Gay-Lussac

• “Why was volumetric analysis better than melting the sample?”
• “How do you know when to stop adding?”
• “What could you not calculate without the mole?”
• “Does it matter that the method came from a mint?”
• “Which of your results should carry your name?”

What Gay-Lussac changed

The law of combining volumes is the observation Avogadro's hypothesis was invented to explain and that the molar volume now makes routine. The burette and the titration Gay-Lussac developed for the silver assay are still on every school bench, and his solubility curves are the ancestor of the graph a student reads today. He also established the expansion law now credited to Charles, and isolated boron and established iodine as an element.

A debate that continues

Credit for iodine is genuinely divided between Courtois, who found it, and Gay-Lussac and Davy, who independently established it as an element in the middle of a war; and Gay-Lussac's refusal to accept Avogadro's explanation of his own law delayed the settling of chemical formulae for fifty years.

Keep exploring — ask Gay-Lussac

• “Would you go up in the balloon again?”
• “When is a scientist entitled to speculate?”
• “What would the mole have let you calculate?”

Related lives

• Amedeo Avogadro — Whose Idea Waited Fifty Years
• Humphry Davy — Who Prised New Metals Out of Salt
• John Dalton — Who Got Atoms Out of the Weather
• Justus von Liebig — Who Weighed What Burned Away

Related themes

Gases and volumes · Titration and concentration · Solubility