Chemistry 1789

Elements of Chemistry

Antoine-Laurent Lavoisier

In every reaction matter is conserved — so weigh it, and chemistry becomes an exact science.

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In depth · the introduction

Burn a candle, rust an iron nail — for centuries no one could say what was really happening, until a man decided to put it all on a scale.

The big idea

Lavoisier's idea was deceptively simple: weigh everything. Seal a reaction in a jar, and the total weight never changes — so whatever seems to vanish or appear has merely moved or combined. With that one rule he showed that burning is not losing something but gaining something from the air, that air and water are not simple substances, and that matter could be sorted into a small list of basic 'elements.'

It turned chemistry from a collection of recipes into a science of measurement. If the numbers don't balance, your account of the reaction is wrong.

How it came about

For most of the eighteenth century, chemists explained fire with 'phlogiston' — an invisible stuff that burning things supposedly released. Lavoisier distrusted it, because a metal heated in air gets heavier, not lighter, which is the wrong way round for something losing a substance. So he weighed with obsessive care, sealing metals and air in closed vessels and tracking every grain.

The weight the metal gained, he found, was exactly the weight the trapped air lost. Burning was combination, not loss. He named the active part of the air oxygen, recast respiration as a slow fire, and in 1789 gathered it all into this textbook — much of the laboratory work done with his wife and collaborator Marie-Anne, who also drew the instruments. Five years later the Revolution sent him to the guillotine; the mathematician Lagrange said it took an instant to cut off that head, and a hundred years might not produce another like it.

Why it mattered

This book gave chemistry both a method and a language. The method — settle every claim on the balance — is still how the science works. And the rational names, where a substance's name tells you what it is made of, let chemists across countries finally understand one another. Modern chemistry begins here.

A way to picture it

Think of a strict accountant's ledger: nothing ever leaves the books unrecorded. If a burning candle gets lighter, the missing weight didn't vanish — it floated off as invisible gas. Trap that gas and weigh it too, and the books balance to the gram. A chemical reaction is just matter being moved between columns, never created and never destroyed.

Where it sits

Before Lavoisier, chemistry still carried the habits of alchemy and the phlogiston theory. He made it quantitative — a sibling to the physics of his century, and a cousin to the conservation laws that run through it (his caloric would soon be replaced by the heat of Carnot's engines). After him the line runs straight on: Dalton's atoms in 1808 give the conserved matter its grains, and the elements he first tabulated are eventually arranged into the periodic table.

The original document

Original source text

Antoine-Laurent Lavoisier · Traité élémentaire de chimie, présenté dans un ordre nouveau et d'après les découvertes modernes · Paris: Cuchet, 1789

A textbook in three parts, written to teach chemistry as a measuring science under a reformed language. Its English title, in Robert Kerr's 1790 translation, is "Elements of Chemistry, in a New Systematic Order, Containing all the Modern Discoveries."

Preface of the Author

Lavoisier argues that a science cannot be improved without improving its language, and that we should reason only from facts established by experiment, never assuming what we set out to prove. He resolves to admit as elements only those bodies that no experiment has yet decomposed.

Part I — On the formation and decomposition of aëriform fluids, on the combustion of simple bodies, and the formation of acids

Heat is treated as a subtle fluid, "caloric," that swells bodies into gases. Combustion and the calcination of metals are recast: a burning or calcining body does not lose a "phlogiston" but combines with oxygen drawn from the air. Water, decomposed and recomposed, is shown to be a compound of oxygen and hydrogen, not an element.

In the chapter on the vinous fermentation, Lavoisier lays out a reaction as a balance sheet — the sugar of the grape must on one side, carbonic acid gas and alcohol on the other — and states the principle that governs the whole book:

We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements. Upon this principle, the whole art of performing chemical experiments depends.

[ … ]

Part II — On the combination of acids with bases, and the formation of neutral salts

A table of thirty-three "simple substances" — bodies not yet resolved into anything simpler — stands as the first modern list of the chemical elements. (It still counts light and caloric among them, and a handful of "earths" later shown to be metal oxides.) From it Lavoisier and his collaborators build a systematic nomenclature in which a compound's name announces its composition: sulphuric acid, sulphates, oxides.

[ … ]

Part III — Description of the instruments and operations of chemistry

A manual of apparatus and precise measurement — the gasometer, the calorimeter, the careful weighing on which the whole method rests — illustrated by thirteen folding plates drawn and engraved by Marie-Anne Paulze Lavoisier, his wife and laboratory collaborator.

Paris · 1789