Known as the “father of modern chemistry”, Antoine-Laurent Lavoisier (1743–94) was a French chemist. He named the gases oxygen and hydrogen, and worked out the role oxygen played in combustion (burning). He compiled an improved list of known elements and helped to reform the way chemicals were named. He also discovered that, although matter could change its form or its shape, its mass always remained the same. In so doing, Lavoisier completely changed the way chemistry was practised. Lavoisier also helped to develop the metric system in order to make weights and measures uniform throughout France.

Early life

Antoine Lavoisier was born into a wealthy family on 26th August 1743 in Paris. At the age of five, he inherited a large family fortune when his mother died. When he was just 11, Antoine attended the Collège Mazarin, also in Paris. In his last two years (1760–61) there he studied maths, botany, astronomy and chemistry. He was particularly inspired by Abbé Nicolas Louis de Lacaille, a mathematician and astronomer, who encouraged the young Lavoisier's interest in meteorology (the study of weather). Later, Lavoisier helped in the compilation of the first geological map of France. 

Academy and marriage

Lavoisier was elected to the French Royal Academy of Sciences as "assistant chemist" in 1768 when he was aged just 25. As a loyal member, he helped prepare reports on a wide range of subjects, including: the water supply of Paris, hypnotism, the Montgolfier balloon, the manufacture of gunpowder, the rusting of iron and breathing in insects.

Phlogiston

Before Lavoisier's time, many chemists had a very different understanding of the way chemical changes took place to how we know they do today—through the rearrangement of atoms of elements and compounds. The German chemist Georg Stahl (1660–1734) suggested that flammable materials contained a material called phlogiston. Substances burned not because they absorbed something from the air, but because they lost phlogiston. A flame ceased to burn, the theory went, when the air became full of phlogiston—not when the supply of oxygen (whose existence was then unknown) ran out.

Combustion

In 1772 Lavoisier began his investigations into combustion. During his experiments, he burned phosphorus and sulphur in the air and found that the compounds resulting from this combustion—then known as "calces", now called oxides—actually weighed more than the original amounts of phosphorus and sulphur. The weight gained he found was equal to that lost from the air.

Lavoisier became convinced it was ordinary air, not phlogiston, that was responsible for causing combustion. At the time he had no knowledge of the work of other chemists into what made up the air (gases, as we known them now to be). Lavoisier thus devoted himself to studying their findings. He was helped in this task by his wife Marie-Anne, who translated various published works from the original English.

The nature of oxygen

The Scottish chemist Joseph Black (1728–99) had discovered carbon dioxide in 1757; English chemist Henry Cavendish (1731–1810) had identified hydrogen in 1766; and, in 1772, Scottish scientist Daniel Rutherford (1749–1819) and English theologian and scientist Joseph Priestley (1733–1804) found that air contained nitrogen and oxygen (without naming them as such).

​​​​​​​By 1778, Lavoisier had come to realise that it was one part of the air that combined with metals and non-metals during combustion. Lavoisier called this part "eminently respirable air". He believed that it was also the source of acidity in substances (a belief we now know to be mistaken), so he named it oxygen, Greek for "acid-former".

To prove his theory, Lavoisier performed an experiment the following year. First, he heated mercury over a charcoal furnace. An oxide of mercury was formed. Lavoisier observed that about one fifth of the air had been used up and that the remaining gas, which he called "azote" (Greek for "without life", now known as nitrogen) did not allow burning. He then heated the mercury oxide, and obtained exactly the same volume of gas that had disappeared previously. This gas, oxygen, caused flames to burn brilliantly. On mixing the two types of gas—the gas left in the first experiment, and that obtained in the second experiment, a mixture similar to air was produced.

Respiration

From an experiment carried out in the winter of 1782–83, Lavoisier was also able to prove that respiration was a similar process to combustion. Taken up by blood in the lungs, oxygen reacts with the carbon in digested food to form carbon dioxide, the waste gas that is breathed out in breath. Heat released during this chemical reaction was the source of the warmth of an animal's body. So respiration was a slow form of combustion.

Listing the elements

Lavoisier, together with fellow scientists Guyton de Morveau, Claude-Louis Berthollet and Antoine François de Fourcroy, proposed a new list of 33 chemical elements to the Academy of Sciences in 1787. These were substances that could not be broken down into simpler substances by any chemical processes known at the time.

​​​​​​​Together with we now call oxygen, hydrogen, nitrogen, carbon, sulphur and phosphorus, the list also featured light, caloric (which does not exist but was then believed to be a form of heat), 17 metals, five so-called earths (mainly oxides of certain, then unknown, metals) and a range of other compounds.

Last days

The French Revolution, which broke out in 1789, was a dangerous time for any French person with an aristocratic (noble) background. Lavoisier was arrested during the Revolution and accused of selling watered-down tobacco. His real crime, however, was that he was a member of the private tax collection company, the Ferme Générale. The company made its profits from the collection of taxes, and was deeply unpopular in France at the time. Despite his scientific achievements, Lavoisier was put on trial as an "enemy of the people"