The history of combustion and conservation of mass
From a million-year-old campfire to the Keeling Curve: how people learned that burning destroys nothing.
People used fire for a million years before anyone knew what it was. For most of the 1700s chemists believed burning released a fire substance called phlogiston. Then scientists started weighing everything, in sealed vessels, and found the opposite: burning takes in part of the air, and not a speck of matter is lost.
Carl Wilhelm Scheele, Sweden (Priestley independently in 1774)
1756 and 1774
Mass conserved in sealed vessels
Mikhail Lomonosov, St Petersburg; Antoine Lavoisier, Paris
1789
Conservation of mass in a textbook
Antoine and Marie-Anne Lavoisier, Paris
1958
Continuous record of CO₂ in the air
Charles David Keeling, Mauna Loa
c. 1 million years agoFire before chemistry
1000000 BCE – 1600
Fire before chemistry
Early humans learn to keep and use fire long before anyone can say what a flame is.
1000000 BCE
c. 1 million years ago
Ash deep inside a cave
Early humans, probably Homo erectusWonderwerk Cave, Northern Cape
Deep inside Wonderwerk Cave, far from the entrance, scientists found ash from burnt plants and bits of burnt bone in layers about a million years old. Tiny cracks and colours in the bone show it was heated where it lay. It is among the oldest secure signs of people using fire, though some researchers argue for older ones.
Why it mattered. Fire gave early humans warmth, light, safety and cooked food, the first chemistry anyone ever controlled.
Homo erectus ("Peking Man"); dug up from the 1920sZhoukoudian, near Beijing
Burnt bones and thick dark layers in the Zhoukoudian caves were long taken as the hearths of Peking Man. A careful study in 1998 found burnt bone but no clear ash or hearths, so whether these people kept fires there is still argued about.
Why it mattered. It shows how hard it is to prove who first tamed fire, and why old claims need testing.
Chemists explain burning with phlogiston, a fire stuff that escapes in the flame, while careful weighing starts to turn up awkward facts.
1673
Metals that get heavier in the fire
Robert BoyleLondon
Boyle heated lead and tin in sealed glass vessels. When he opened them and weighed the metal's ash, called a calx, it was heavier than the metal. He guessed that particles of fire had passed through the glass and stuck to it.
Why it mattered. A real clue, misread: he opened the vessel before weighing, so air rushed in and the extra weight was really oxygen.
Building on Johann Becher's ideas, Stahl taught that everything that burns holds a fire substance, phlogiston, which escapes in the flame. Charcoal was almost pure phlogiston; a metal was its calx plus phlogiston. The idea explained a lot and ruled chemistry for most of the century.
Why it mattered. It was the first big theory of burning, and it was wrong in a way that pushed chemists to start weighing things.
Black heated magnesia alba and chalk and weighed them. They lost weight by giving off a gas he called fixed air, the gas we call carbon dioxide. He later found it in the breath and in the gases from burning charcoal.
Why it mattered. Careful weighing showed that a gas could be part of a solid, and gave chemists the first product of burning.
Lomonosov repeated Boyle's test but kept the flasks sealed. Heating metal inside them did not change the total weight; only letting air in did. He had already written to Leonhard Euler in 1748 that matter lost in one place is gained in another. His results were recorded in his notes and were little known in Western Europe.
Why it mattered. He reached the conservation of mass years before Lavoisier, though historians still debate how complete his proof was.
Oxygen is found three times over, and the Lavoisiers show with sealed vessels and balances that burning is joining with it and that mass is conserved.
1771
1771–1772 (published 1777)
Scheele makes "fire air"
Carl Wilhelm ScheeleUppsala
Scheele, a pharmacist, heated mercury oxide, saltpetre and other substances and collected a gas that made things burn fiercely. He called it fire air. His book describing it was held up at the printer until 1777, so others were credited first.
Why it mattered. He was probably the first to make oxygen, a reminder that discoveries often happen twice.
Priestley focused sunlight with a large lens onto red mercury calx. A gas came off in which a candle burned with a dazzling flame and a mouse lived longer than in ordinary air. He called it dephlogisticated air and told Lavoisier about it in Paris that October.
Why it mattered. Priestley found oxygen but kept the phlogiston theory; Lavoisier saw what it really meant.
Lavoisier heated tin in sealed glass vessels and weighed everything before and after. The total did not change. When he broke the seal, air rushed in, and the weight rose by exactly what the tin had gained.
Why it mattered. The metal was taking something out of the air, not losing phlogiston: the key experiment behind this box's first chapter.
In a paper on combustion in general, Lavoisier argued that burning, rusting and breathing are all a substance joining with part of the air. He named that part oxygène, "acid maker", because he wrongly thought all acids contained it.
Why it mattered. It replaced phlogiston with the idea we still use: burning is joining with oxygen.
Lavoisier and Laplace put a guinea pig in an ice calorimeter and measured the heat it gave out and the fixed air it breathed out. Burning charcoal to make the same amount of that gas gave nearly the same heat. Breathing, they concluded, is a slow combustion.
Why it mattered. It linked the flame and the body, the idea behind this box's chapter on fat and breath.
Marie-Anne Lavoisier translated Richard Kirwan's Essay on Phlogiston from English into French, adding notes that took its arguments apart. She kept the laboratory's records, took part in experiments and hosted scientists from across Europe.
Why it mattered. Her work spread the new chemistry, though she was rarely credited in her lifetime.
Antoine Lavoisier, with drawings by Marie-Anne LavoisierParis
Lavoisier's Traité élémentaire de chimie stated that nothing is created in any operation, of art or of nature: the same quantity of matter exists before and after. It listed 33 elements and used a new, logical naming system. Marie-Anne drew its 13 plates of apparatus. Five years later Lavoisier was guillotined in the Revolution.
Why it mattered. It is often called the first modern chemistry textbook, and it made conservation of mass the ground rule of chemistry.
Safety lamps, candle lectures and engines put combustion to work, and measurements show where all that carbon goes.
1815
A flame that can't escape
Humphry DavyRoyal Institution, London
Coal mines were full of firedamp, methane, which exploded when miners' candles lit it. Davy wrapped the flame in fine iron gauze. The gauze soaks up heat so fast that flame can't pass through it to the gas outside.
Why it mattered. It saved many lives, and showed that a flame needs heat as well as fuel and oxygen.
In Christmas lectures for young people, Faraday burned candles and caught what came off. He showed that the wax turns into water and carbon dioxide, using oxygen from the air, and that we do the same when we breathe. He gave the lectures again in 1860, and they became a famous book.
Why it mattered. It is still one of the best explanations of combustion ever written for young people.
Otto built an engine that sucked in fuel and air, squeezed it, burned it and pushed out the exhaust, in four strokes of the piston. Squeezing the mixture before lighting it made it far more efficient than earlier gas engines.
Why it mattered. Nearly every petrol car and motorbike engine still works this way.
In a short paper, Einstein showed that anything that gives out energy E loses mass E ÷ c². For burning, the loss is about one part in ten billion, far too small to weigh.
Why it mattered. It is the one tiny exception to Lavoisier's law, and it matters only in nuclear reactions.
Charles David KeelingMauna Loa Observatory, Hawaii
Keeling began measuring carbon dioxide in the air with great precision, high on a Hawaiian volcano. His record, the Keeling Curve, rises every year, with a small wobble as plants grow and die with the seasons.
Why it mattered. It shows the conservation of mass on a planetary scale: the carbon we burn does not vanish, it stays in the air.
Carmakers, under the US Clean Air ActUnited States
From the 1975 models, most new petrol cars in the US got catalytic converters to meet new limits on carbon monoxide and unburnt fuel. They needed unleaded petrol, because lead ruins the catalyst. Three-way converters that also clean NOx followed a few years later.
Why it mattered. They cut the poisons from incomplete burning, though every gram of fuel carbon still leaves as CO₂.
Ruben Meerman and Andrew BrownUniversity of New South Wales, Sydney
Meerman, a physicist, and Brown, a chemist, followed every atom in 10 kg of body fat as it is burned. They found that 8.4 kg leaves as carbon dioxide through the lungs and 1.6 kg as water. Many doctors and trainers they surveyed had guessed it turned into energy or heat.
Why it mattered. It is Lavoisier's law applied to your own body: mass lost has to go somewhere.