by Paul Braterman
If Isaac Newton is the father of modern physics, then Antoine Lavoisier is the father of modern chemistry. Newton was knighted, and died in his bed at age 84. Lavoisier died at age 50, on the guillotine.
Lavoisier originally trained as a lawyer, but studied science at the same time, and set about earning admission to the Academy of Sciences. This he achieved at the remarkably young age of 25, with a combination of pure science (composition of gypsum), and applications (problems of street lighting and water supply). He invested his inherited fortune in membership of a curious body called the Company of Tax Farmers. This was involved in the collection of indirect taxes throughout the whole of France, while its members individually lent money to the Crown, thus simultaneously taking on the roles of bankers, administrative civil servants, and investors in government securities.
Lavoisier's administrative responsibilities included supervising the Gunpowder Administration. Gunpowder is a mixture of sulphur, charcoal, and saltpetre (potassium nitrate). At the time, this last was obtained from fermenting organic matter with human and animal manure. French production had become haphazard, and lack of gunpowder was one reason for France's defeat in the Seven Years War/French and Indian War of 1754 – 1763. The Dutch had developed a system using beds of manure mixed with rotting vegetation, which Lavoisier copied, to such good effect that within a few years France was able to supply the material to its allies. French exports of saltpetre played an essential role in the American Revolution, and Lavoisier was able to write “One can truly say that North America owes its independence to French gunpowder”.
Reform, Revolution, and the Terror
In the years leading to the French revolution, Lavoisier campaigned for moderate reform. Reform, however, either constitutional or financial, was impossible in the face of entrenched privilege. When, in 1789, the Estates General finally met, the tensions within it set in motion the series of events that was to end in revolutionary Terror and, eventually, Napoleonic autocracy.
At its height, the revolution had no time for moderates, especially moderates who had been involved in the previous regime. Lavoisier, despite his brilliant success at the job, was removed from the Gunpowder Administration in 1792. The Academy of Sciences won a temporary respite by dominating the newly established Advisory Office on Practical Arts and Trades. Lavoisier himself worked within this Office, and was involved in developing the metric system, drafting a national educational curriculum, and advising on methods for printing banknotes; not a trivial problem before synthetic dyes became available. However, even before the revolution, allies of the revolutionary Marat had condemned the Academy as “elitist”, and it was abolished by the Revolutionary Convention in 1793.
Wealthy bankers are rarely popular, especially during periods of economic unrest; tax collectors, never. Moreover, the members of the Company of Tax Farmers had exercised their functions in the service of the King. Lavoisier and other prominent members of the Company were charged with fraud and arrested. When no evidence of this fraud could be found, the Revolutionary Convention simply declared them guilty of conspiracy against the people, and sent off Lavoisier with twentyseven of his colleagues to the guillotine. Meantime, the armies of the fledgling Republic were driving back invaders from half a dozen nations, using the products of Lavoisier's work at the Gunpowder Administration.
Three months later, the Convention itself had turned against and executed its extremist leadership, and the revolutionary Terror was over.
Guinea pig, mesmerism, and placebo
We start with two of Lavoisier's minor scientific accomplishments. “Minor” is a relative term, and either of these alone would ensure him a place in history.
Next, mesmerism. Lavoisier was part of a committee set up by the Academy of Sciences to examine the phenomenon of “animal magnetism” (a visiting American called Benjamin Franklin was also on the committee). This was a lucrative piece of quackery devised by one Dr Franz 
None of this extinguished belief in animal magnetism. Mary Baker Eddy, founder of Christian Science, wrote that malicious animal magnetism was being used in mental assassination, and magnetic healing devices of no clinical value are “a billion-dollar boondoggle“.
Chemistry, elements, and The Elements of Chemistry; the importance of the balance
Below, left: a selection of Lavoisier's instruments, as illustrated by his wife Marie-Anne Paulze for the textbook. Paulze was a scholar in her own right, translating and annotating scientific papers and keeping records of her husband's experimental work.
In 1774, Joseph Priestley in England found that while mercury is converted to its calx by heating in air, stronger heating of that calx would decompose it to the original metal, and to a gas which could support respiration longer than ordinary air. Lavoisier showed that formation of the calx involved uptake of this same “respirable air”.
With hindsight, all that remained was to join up the dots, and joining up the dots was inevitable. As to why it was Lavoisier who took the crucial step, it is worth recalling that he had been trained as a lawyer, a skill involving the precise use of unambiguous language. It is also worth noting the complaints he made about the way in which he himself had been taught chemistry. He contrasted the teaching of mathematics, where each step is shown to follow from the preceding, with that of chemistry where the standard presentations “make use of terms that have not been defined, and suppose the science to be understood by the very persons they are only beginning to teach.”[2]
For 
If you want to reform the language of a subject, the obvious place to start is with the definition of its key concepts, and the key concept of chemistry, then as now, was that of an element. For the ancients, as Lavoisier pointed out, there had been the four elements of earth, air, fire, and water. To these, the alchemists had added salt and sulphur (he could also have mentioned mercury), while others had spoken of different kinds of “earth”. Faced with this, Lavoisier concluded that instead of attempting to find the nature of “those simple and indivisible atoms of which matter is composed”, we should define an element as a substance that cannot be broken down into simpler components. This provides we would now call an “operational definition”; an element is defined in terms of experimental tests. It also, as Lavoisier explicitly recognised, took account of the possibility of changing knowledge. Thus air had already been shown to contain more than one component, so it was not an element, and water also was shortly to suffer a demotion.
Things would now fall into place very quickly. “Respirable air” could not be broken down any further, nor could it be made from simpler components, so it qualified as an element. This Lavoisier called oxygen, producer of acidity, since from his work with phosphorus and sulphur he believed it to be a component of all acids. Calxes were compounds between their metals, and oxygen. Mercury was an element. So was carbon, the purest form of charcoal. Carbon reacted with oxygen to give “fixed air”, the same gas that John Black had produced in Glasgow by heating limestone, which was therefore not an element but a compound. In the reaction between charcoal and the calx of mercury, carbon was again converted into “fixed air”, this time by removing oxygen from the calx, leaving the metal free. This was the same kind of reaction as had been practiced, although not understood, for over 6,000 years, whenever metals were extracted from their ores using charcoal. Calxes were compounds between the various metals and oxygen, while the metals themselves were elements. So on moderate heating, mercury reacted with oxygen from the air to form an oxide, but stronger heating would decompose this oxide to its elements. Once oxygen had been extracted from the air, what remained was an inert residue, which Lavoisier named “azote”, or lifeless. This is still what it is called in French, although the modern English name is nitrogen, in recognition of its presence in nitre.
What about water? Lavoisier knew about “inflammable air”, a gas of low density formed when metals react with acids, and was indeed the first to suggest its use in balloons. Believing that its reaction with oxygen would produce another acid, Lavoisier in combination with Laplace (now best remembered as a mathematician) allowed oxygen and inflammable air from pressurised tanks to react within a glass bulb, and collected the liquid formed. To their surprise, it was nothing more nor less than pure water. Hence the name hydrogen, producer of water, for inflammable air. They were not the first to carry out this experiment, and to verify that the weight of water was equal to the combined weight of the reacting gases, but they were the first to recognise the significance. If water could be produced from two simpler substances, then whatever Aristotle may have said on the subject, it was not itself an element.
Not content with determining the nature of water by synthesis (combining elements together), Lavoisier verified his result by analysis. Again, he was not the first to carry out the crucial experiments, but the first to understand them. He reacted iron with water, collected the gas produced, and verified that this was indeed hydrogen by burning it and recovering the original material.
We can summarise the “new chemistry” that Lavoisier developed throughout the 1770s and 1780s as follows:
1) Combustion or calcination involves the formation of a compound with oxygen.
2) Air is a mixture of oxygen and azote.
3) Water is a compound of hydrogen and oxygen.
4) Heat is an elemental substance that is released during combustion.
5) Combination with oxygen produces acids, and all acids contain oxygen.
6) Matter is composed of simple bodies (i.e. elements) free or in combination. Bodies are regarded as simple as long as all efforts to simplify them further or to produce them by combining other bodies have failed.
7) Matter is conserved during chemical changes, as is the amount of each of the “material principles” i.e. elements.
8) A new notation was needed to express these facts.
In the rearview mirror
It's worth a look at how well these conclusions have stood up over more than two centuries. (1) and (3) read as true today as they did when Lavoisier wrote his Elements. (2) is something like 99% true. In addition to nitrogen and oxygen, air contains about 1% argon and small but vitally important minor amounts of carbon dioxide, as well as traces of other gases. (4) was soon to be replaced by our modern view of heat as a form of energy
(5) seems a little bit strange, even from the perspective of the time, and we now know that many acids, such as hydrochloric, contain no oxygen at all. So we can regard (5) simply as an empirical generalisation, which seemed reasonable at the time, but which is now rejected in the face of fresh evidence. That's how science works.
(6) is essentially what we think today, with a “simple body” being the same thing as what we call an element. Lavoisier himself listed 33 of these, of which 23 are still recognised as such. (7) is the law of conservation of mass, and was to provide the vital link between experimental chemistry and the atomic theory of the ancient philosophers.
As we have seen, Thomas Jefferson regarded (8) as a premature exercise. He was wrong, and much of Lavoisier's nomenclature is still with us. Lavoisier spoke of metal oxides, and identified salts by reference to the metal and the acid that they contained. Thus what we call potassium nitrate is what he called nitrate of potash. Lavoisier's stated aim was to establish an unambiguous nomenclature in which the names of substances were systematically related to their composition, as an aid to clarity of thought, and in this he was wholly successful.
If he had survived the Terror, Lavoisier would quite probably have lived another 20 years or more, long enough to witness and contribute to the next round of developments. His clarity of thought and language would have helped make clear the underlying issues, such as the difference between an atom and a molecule, and might well have saved the world of science half a century of confusion.
1] These days, of course, we ideally use double-blind experiments, in which neither the subjects, nor the experimenters in contact with them, know which drugs or procedures are genuine and which are controls.
2] Elements of Chemistry, Dover edition (1965), p. xix. I fear much the same could be said about the present-day introductory curriculum in chemistry, where students are immediately exposed to concepts derived from quantum mechanics and thermodynamics, without any basis in experience for these concepts.
An earlier version of this material appeared in From Stars to Stalagmites (World Scientific Press, 2012). Portrait of Lavoisier and wife by Jacques Louis David, in Metropolitan Museum of Art. Guinea pig from Pixabay. Animal magnetism from an 1863 work by Du Potet, via Occultopedia. Lavoisier's instruments via Chemistry Explained website.