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‘Only a moment to cut off that head and a hundred years may not give
us another like it,’ lamented the 18th-century French mathematician Joseph
Louis Lagrange. The head in question had belonged to the French aristocrat
and chemist Antoine Laurent Lavoisier, who was executed at the hands of
French revolutionaries on 8 May 1794. Lavoisier’s ideas changed the face
of chemistry. He is best remembered for overthrowing the phlogiston theory,
but perhaps his greater and more lasting achievement was to impose order
on the language and symbolism that have shaped the thoughts of chemists.

By Lavoisier’s time, the edifice of alchemy was crumbling, but chemistry
as we now know it had yet to come of age. Alchemy encompassed the belief
that it should be possible to change or transmute one metal into another
– most famously, lead into gold. But by the early 16th century the itinerant
Swiss physician Para-celsus, who had become interested in the possibility
of curing diseases by chemical means, was putting forward alternative ideas
that combined medicine and alchemy.

A century later, in 1661, the Irish-born scientist Robert Boyle had
gone much further. Despite his own belief in transmutation, he had written
a critique of existing theories of chemistry, The Sceptical Chymist. In
it he introduced the concept of elements as substances which could not be
split up into constituent parts, replacing such earlier ideas as the Greek
notion of the four elements air, water, fire and earth, or the Arab concept
of the tria prima of sulphur, mercury and salt.

But alchemy’s surviving legacy of mystical language and poetic symbolism
still stood in the way of any attempts to rationalise the chemical world.
What chemists would now recognise as the simple salt magnesium carbonate,
for example, was known to Lavoisier under the guise of no fewer than nine
aliases: magnesie blanche, magnesie aere de Bergman, magnesie crayeuse,
craie magnesienne, magnesie effervescente, mephite de magnesie, terre muriatique
de Kirvan, poudre du comte de Palme, and poudre du comte de Santinelli.
And compounding this problem of language, the development of chemistry was
being held back by a widespread and erroneous belief in the phlogiston theory.

This theory, and the name phlogiston, was coined in 1718 by Georg Stahl,
a Bavarian professor of medicine. The essence of it was that combustible
substances contain a curious substance called phlogiston, which they lose
when they burn. Because balances were inaccurate, the fact that substances
often gained weight when they burnt was often missed. And so dogged was
the belief in phlogiston that when a gain in weight was recorded, it was
explained away – for example, by saying that the phlogiston was an incorporeal,
ethereal fire that was lighter than any other known substance and buoyed
up heavier ones, or even a substance with negative weight.

By the time Lavoisier was born in Paris on 26 August 1743, the phlogiston
theory was well established. Lavoisier’s later interest in chemistry began
from a much wider base, however, for he grew up in an era of critical examination
and rationalism in both science and philosophy. He began by following in
his father’s footsteps, graduating in law in 1763 from the College des Quatre
Nations. His interests included geology, meteorology, astronomy, botany
and anatomy, and his wealth gave him the means to pursue them. The numerous
papers that Lavoisier published during his lifetime covered subjects as
diverse as prisons, the Montgolfier hydrogen balloon, hypnotism, the rusting
of iron and the respiration of insects.

Lavoisier spent his early twenties helping a family friend to map the
geology of France. But at demonstration lectures at the Jardin du Roi in
Paris, Lavoisier became interested in chemistry, and at the age of 22 he
published his first chemical paper, on the transformation of gypsum to plaster
by heating.

Lavoisier was a dedicated student. He was able to devote only one day
a week exclusively to science, but on the others he rose at six, studied
until eight, and again in the evening from seven until ten. In between he
had public duties, as a tax official and financier and spokesman for the
French Academy of Sciences, to fulfil.

In his scientific work, Lavoisier was strongly supported by his wife.
Marie Anne Paulze had married him in 1771, when she was 14 and he was 28.
They had no children and she became not only his social companion but also
his personal assistant. She applied expertise gained in lessons with the
great artist Jacques Louis David to make engravings of her husband’s extensive
chemical apparatus, and worked in his laboratory, recording the results
of his experiments. And she learnt English, which her husband never mastered,
and translated scientific papers by the eminent English chemists Joseph
Priestley and Henry Cavendish as well as an influential essay on phlogiston
written by the Irish chemist Richard Kirwan.

Burning enthuasiasm

By 1772 Lavoisier had begun experiments that were to change the face
of chemistry. He was helped by a large burning lens that belonged to the
French Academy of Sciences. Arthur Donovan, author of a recent biography
of Lavoisier, pictures his delight in having access to the lens as being
‘exactly like that of 20th-century physicists who run experiments on gigantic
particle accelerators – it was a machine capable of blasting apart substances
previously considered immutable’. Lavoisier deposited his findings for safety
in a sealed note with the secretary of the academy on 1 November that year.
Within two months he was able to state that phosphorus and sulphur gained
weight on burning, due to their combination with air.

In 1774, Lavoisier extended his experiments to metals, improving on
experiments previously carried out by Boyle. Boyle had burnt metals in sealed
vessels and had weighed the metal before and after. He attributed the gain
in weight that he observed to material particles of fire passing through
the pores of the glass and combining with the metals. Lavoisier, however,
weighed the vessels – still sealed – with tin and lead inside them. He found
that they weighed the same before and after burning. But then came the crucial
observation: when he broke the seal, and air rushed in, Lavoisier noticed
that the weight of the air that entered was about the same as the difference
between the weight of the metal and its combustion product (an oxide, then
known as a calx).

Lavoisier published his find-ings, and in the same year Joseph Priestley
discovered ‘dephlogisti-cated air’, ‘in which a candle burnt much better
than in normal air’. The two eminent scientists met in Paris. Lavoisier
realised that Priestley’s dephlogisticated air, now better known as oxygen,
was an active constituent of the atmosphere and was absorbed by metals on
burning.

During the winter of 1774-75, Lavoisier repeated and elaborated on Priestley’s
experiments. He gave his findings to the Academy in April 1775. Although
his results were the same as Priestley’s, he drew very different conclusions.
Priestley, a firm believer in the phlogiston theory, assumed that the gas
produced by heating mercury calx supported combustion much better than air
because it was a better receptacle for phlogiston. Lavoisier argued that
air consists of at least two gases: ’eminently breathable air’ (Priestley’s
dephlogisticated air) and a residual air, now known to be mainly nitrogen.

Lavoisier went on to say that the eminently breathable air was the active
agent in combustion. ‘Fixed air’ (carbon dioxide) was a compound of charcoal
with this air. Metallic calxes were not elements as had been previously
thought, but compounds of elementary metals with eminently breathable air.
These views challenged the phlogiston theory, and at first were resisted,
often fiercely. Priestley was one of the most vocal in his dissent. But
Lavoisier continued his research and found that combining moist eminently
breathable air with sulphur, phosphorus and nitrogen yielded sulphuric,
phosphoric and nitric acid respectively. Eventually, he named this constituent
of air ‘oxygen’. By the mid-1770s, only a few prominent chemists – including
Priestley and Cavendish – remained unconvinced. The antiphlogiston camp
had triumphed.

But Lavoisier’s greatest contribution to modern chemistry was still
to come. In 1787, with his colleagues Louis Bernard Guyton de Morveau, Claude
Louis Berthollet and Antoine Fourcroy, he devised a radically new system
of chemical nomenclature. They published the Methode de Nomenclature Chimique
in which familiar chemical terms such as ‘oxide’ and ‘sulphate’ appeared
for the first time. In doing so they forged the international language
of modern chemistry.

In a series of tables they listed 55 elements or substances that had
not been decomposed. These included light and ‘caloric’, ‘azote’ (nitrogen)
and ‘inflammable air’ (hydrogen); carbon, sulphur and phosphorus; the 16
known metals, and a list of organic ‘radicals’. Compounds were named, as
they still are, after their constituents. Metal calxes were now called oxides,
and the salts (for instance, sulphates, nitrates and carbonates) were named
after the acid from which they were formed. The archaic symbolism and mystical
language of alchemy were ousted. At last there was a common language in
which to describe in a more detached way the smells and sights experienced
in the laboratory.

The Nomenclature had also included a rather unwieldy scheme for chemical
characters and symbols, devised by Jean-Henri Hassenfratz and Pierre August
Adet. Elements were represented by straight lines at various angles, metals
by circles and alkalis by triangles. Lavoisier, a fervent believer in the
power of algebra in chemistry, used the scheme in a paper on the way metals
dissolve in acids. ‘In order to show at a glance the results of what happens
in the solution of metals,’ he wrote, ‘ I have constituted formulae of a
kind that could at first be taken for algebraic formulae, but do not have
the same object . . . we are still very far from being able to obtain mathematical
precision in chemistry and therefore I beg you to consider the formulae
that I am going to give you only as simple annotations, the object of which
is to ease the workings of the mind.’ He did not go quite so far as to use
the equals sign, but he came very close to writing the sort of chemical
equation familiar to modern chemists. The Swedish chemist Jons Jacob Berzelius
improved on the Hassenfratz-Adet scheme a quarter of a century later and
by the 1830s chemists were using equations to represent chemical reactions.

Final work

In 1789, Lavoisier published his chemical swan song, the Elementary
Treatise on Chemistry. This aimed to present a more detailed and slightly
updated account of the new system of chemistry and included extensive descriptions
of experiments and apparatus. It became his most famous work; a much more
ambitious project that he began later was cut short by the guillotine.
Like the Nomenclature it was translated into several languages, and it became
a model for the teaching of chemistry for decades.

Lavoisier continued his research, but the French Revolution gathered
momentum after the storming of the Bastille on 14 July 1789, and Lavoisier,
along with other members of the hated Ferme Generale, the corrupt institution
responsible for collecting taxes, was arrested on 24 November 1793. On 8
May 1794, he followed his father-in-law to the Place de la Revolution and
the guillotine. Marie Anne was spared this fate and continued to host weekly
scientific gatherings.

In his last letter from prison, addressed to his cousin Augez de Villers,
Lavoisier had written: ‘I shall be remembered with some regrets and perhaps
leave some reputation behind me. What more could I ask?’ He left a reputation
for applying powers of persuasion and logic to scientific discoveries, and
bringing order to a chaos of chemical ideas. He transformed chemistry into
a modern science.

Paul Board is an environmental chemist at Simon Laboratories, Llandudno,
Gwynedd. Further reading Antoine Lavoisier: Science, Administration and
Revolution, by Arthur Donovan, Blackwell, 1993.