Elements of Chemistry

Overview

The Traité élémentaire de chimie appeared in Paris in 1789, five hundred pages in three parts, and it is the first book that looks like a modern chemistry textbook. Lavoisier was a tax farmer, an administrator of the gunpowder works, a member of the Academy of Sciences and a wealthy man; he did chemistry in the mornings and on Saturdays, in a laboratory he could afford to equip better than anyone else in Europe. In 1794 the Revolution executed him. The judge is supposed to have said the republic had no need of scientists, which is probably invented; Lagrange’s remark that it took an instant to cut off that head and a hundred years might not produce another like it is better attested.

The structure tells you what he thought mattered. Part I is the theory: gases, heat, the composition of air and water, combustion, acids. Part II is a systematic naming of every known substance. Part III is a detailed description of apparatus and technique, with thirteen plates drawn and engraved by Marie-Anne Lavoisier, who also translated English chemists for him and took notes at his experiments.

That a third of a chemistry book should be about equipment is a statement. Lavoisier’s method is measurement, and the balance is the hero of the book.

The argument

What he was overturning. Phlogiston theory held that combustible bodies contain a substance released on burning. It was a real theory with real explanatory power: it made burning, rusting and the smelting of ores one kind of process, and accounted for why charcoal leaves almost nothing behind.

Its problem was weight. A metal heated in air becomes a calx, what we call an oxide, and the calx is heavier than the metal. If burning releases something, the residue should weigh less. Various rescues were tried, including the suggestion that phlogiston has negative weight.

The experiments. Lavoisier’s response was to weigh everything, including the gases, and to work in sealed vessels so that nothing escaped unmeasured.

He heated tin and lead in closed containers and showed that the total weight was unchanged, while the metal gained exactly what the air inside lost. He ran the famous twelve-day experiment with mercury, heating it in a measured volume of air until a red calx formed and the air was reduced by about a fifth; the remaining air suffocated animals and would not support a flame. Then he heated the red calx more strongly, recovered the mercury, and got back a gas of exactly the volume the air had lost, in which a candle burned fiercely. Air was not an element. It was a mixture, one part of which combines in burning.

He named that part oxygen, from the Greek for acid-former, on the mistaken belief that it was the principle of acidity in all acids. With Laplace he burned hydrogen and oxygen and produced water, and decomposed water by passing steam over hot iron, which disposed of the last of the four classical elements.

He then did the same for animals. Working with Laplace and an ice calorimeter, he measured the heat produced by a guinea pig and the carbon dioxide it exhaled, and compared them with burning charcoal. The conclusion was that respiration is a slow combustion.

The principle. Out of the sealed-vessel work comes the sentence the book is remembered for: nothing is created, in art or in nature, and in every operation an equal quantity of matter exists before and after. Lavoisier states it almost in passing, as something his practice assumes. It converts chemistry from a descriptive art into a science where you can write an equation and require it to balance.

The language. Part II is the fruit of the Méthode de nomenclature chimique, which Lavoisier had published two years earlier with Guyton de Morveau, Berthollet and Fourcroy. The old names recorded appearance, origin or discoverer: oil of vitriol, flowers of zinc, butter of antimony, Glauber’s salt. The new ones state composition. An element gives its name to its compounds; different proportions are distinguished by endings, sulphate and sulphite, sulphuric and sulphurous. Once you know the rule you can read a formula off a name.

The preface defends this at length, arguing that we cannot improve a science without improving its language, because reasoning is done in words, and that a badly named field cannot think clearly.

Part II also contains a table of thirty-three simple substances, defined not as ultimate constituents but as things nobody has yet managed to decompose. Most are still elements. Two, light and caloric, the supposed matter of heat, are not, and Lavoisier’s inclusion of them shows the definition being used honestly.

How to read it

Read the preface entire. It is the best short statement of his method and of the argument about language, and it is where he explains why he starts with the simple and moves to the complex.

Read the combustion chapters and chapter 13 of Part I for conservation of mass. That is the core.

Look at the plates even if you skip the text of Part III. They show the apparatus in working detail, and they are a reminder that this revolution was made with glassware, balances and enormous patience.

Notice what he gets wrong. Oxygen is not the principle of acidity, as Davy showed with hydrochloric acid. Caloric does not exist. Lavoisier’s confidence in both is a useful corrective to the idea that a scientific revolution consists of replacing error with truth.

Read it against Priestley, who isolated the gas first, called it dephlogisticated air, and defended phlogiston to the end of his life. The same experimental facts sat in front of both men. What Lavoisier added was a different question and a balance.

Questions it raises

• Priestley made the gas and Lavoisier explained it. Which of them discovered oxygen, and what does the disagreement reveal about what discovery means?
• Phlogiston explained a great deal and was wrong. What is the difference between a theory that is useful and one that is true?
• Conservation of mass is asserted as a principle his practice assumes. Could it have been proved, and how?
• An element is defined as what has not yet been decomposed. Is that definition a weakness, or the only honest one available?
• Lavoisier argues you cannot improve a science without improving its language. Is that true of fields outside chemistry?
• He was a tax farmer whose wealth paid for the laboratory and whose office got him executed. Does the way science is funded belong in an account of it?