Lavoisier: Elements of Chemistry

Antoine Lavoisier (say lah-vwah-zee-AY) lived in Paris more than 200 years ago. He had a secret weapon for doing science: a very, very good balance for weighing things.

In his time, people thought that when something burns, a stuff called "phlogiston" (flo-JISS-ton) rushes out of it. But Lavoisier weighed metals before and after he heated them in air, and found something odd: the metal got heavier. If something was leaving, how could it gain weight?

His answer: when things burn or rust, they are not losing something. They are joining with part of the air. He named that part of the air oxygen. And he showed that in a closed jar, the total weight never changes: nothing is made from nothing, and nothing disappears. Matter just rearranges.

His wife, Marie-Anne Lavoisier, was his partner in the lab. She wrote down the results, translated science books from English, and drew the careful pictures of his equipment that appear in his book.

Who, when and the problem

Antoine-Laurent Lavoisier (1743–1794) was born in Paris to a wealthy family, trained in law, and turned to science. He joined the Academy of Sciences in 1768 and, the same year, the Ferme générale, a private company that collected taxes for the king. From 1775 he ran the government's gunpowder works, where his laboratory at the Arsenal became one of the best in Europe. His wife, Marie-Anne Paulze Lavoisier, worked beside him. His Elementary Treatise of Chemistry appeared in 1789; Robert Kerr's English translation, Elements of Chemistry, followed in Edinburgh in 1790.

The problem: chemists of the 1700s explained burning, rusting and breathing with phlogiston, a substance thought to leave a body as it burns. A burned metal (a "calx") was metal minus phlogiston. But calxes weigh more than the metals they come from. New gases were being discovered fast, Joseph Black's "fixed air," Henry Cavendish's "inflammable air," Joseph Priestley's and Carl Wilhelm Scheele's very pure air, and chemistry had no clear way to name them or to say which substances are simple.

The main ideas

• Weigh everything. Lavoisier treated the balance as the chemist's chief instrument and kept track of the mass of every substance, including the gases, going into and out of a reaction.
• Mass is conserved. In a chemical change, the total mass after equals the total mass before.
• Burning is combining with oxygen. Combustion, the rusting of metals and even breathing take in part of the air. With Pierre-Simon Laplace he showed (1782–1783) that an animal's breathing gives off heat in the same proportion as burning charcoal.
• Water is a compound of hydrogen and oxygen, not an element (1783).
• An element is a substance we have not yet been able to break down. His table lists 33 "simple substances."
• A new language. With three colleagues he renamed chemistry in 1787 so that a compound's name tells what it is made of: oxide, sulfate, carbonic acid. We still use that system.

The weight of a burned metal

Robert Boyle had heated tin in sealed glass vessels in the 1670s and found it heavier afterward, but he opened the vessels before weighing them. In 1774 Lavoisier weighed the sealed vessel before opening it: no change. When he opened it, air rushed in, and the weight went up by just as much as the tin had gained. The tin had taken something out of the air inside.

Words and ideas

Setting the record straight

• Lavoisier did not discover oxygen. Carl Wilhelm Scheele made it around 1771–1772 (published 1777) and Joseph Priestley in 1774; Priestley told Lavoisier about it at dinner in Paris that October. Lavoisier's achievement was to understand what it is, show its role in burning and breathing, and name it.
• He was not the first to say matter is conserved. The idea that nothing comes from nothing is ancient (Lucretius, earlier on this ladder, argues it), and Mikhail Lomonosov in Russia stated it for chemistry in the 1750s. Lavoisier made it the working rule of every experiment, and checked it with the balance.
• Not every name was right. "Oxygen" means "acid-maker": Lavoisier thought every acid contains it. Humphry Davy showed in 1810 that muriatic (hydrochloric) acid does not.
• Light and heat. His list of elements included light and "caloric," a fluid of heat, as well as several "earths" like lime and silica, which were later broken down into metals and oxygen.
• A famous line. Lavoisier was guillotined in Paris on May 8, 1794, during the Reign of Terror, with other members of the tax company. The judge is said to have declared that "the Republic has no need of scientists," but the remark is reported only in later accounts, and historians doubt it was said in those words.

What later science kept, and what it changed

• Kept: conservation of mass in chemical reactions, combustion as combining with oxygen, the idea of a chemical element as a substance that can't be broken down chemically, and the system of names.
• Built on: John Dalton (1803–1808) explained fixed combining weights by atoms, each element with its own weight. Lavoisier's 33 simple substances became, by 1869, Mendeleev's periodic table of 63.
• Changed: caloric gave way to the idea of heat as energy (Rumford, Joule). Einstein's $E = mc^2$ (1905) shows that mass changes very slightly when energy is released, but in chemical reactions the change is far too small for any balance to detect.

A revolution? Lavoisier himself spoke of a revolution in chemistry, and the change from phlogiston to oxygen is a favorite example for historians of science. Thomas Kuhn, in The Structure of Scientific Revolutions (1962), used it to argue that scientists can see the same experiment differently: Priestley and Lavoisier watched the same gas, and one saw air without phlogiston while the other saw a new element. Priestley never accepted the new chemistry. Other historians argue the change was more gradual and depended as much on better instruments as on new ideas.

Language and thought. Lavoisier took his ideas about language from the philosopher Étienne Bonnot de Condillac, who held that a science is a well-made language. Is that true? Could a better word help you think more clearly about something you are studying?

Marie-Anne Lavoisier. She studied drawing with the painter Jacques-Louis David, drew the thirteen plates of the Treatise, translated Richard Kirwan's defense of phlogiston into French with critical notes (1788), and after Antoine's death had his last memoirs printed. How much of a shared lab's work belongs to each person is a question historians are still exploring.

Science and the state. Lavoisier spent much of his life in public service: tax collection, gunpowder, agriculture, and the committee that created the metric system. That service made him rich and, during the Revolution, made him a target. His life raises the old question of how a scientist should relate to the government of the day, which readers can consider from many sides.