In 1788, just as the stage was set for revolution, France’s most celebrated scientist met with France’s most celebrated artist. This sitting for a portrait of the illustrious scientist and his wife may not have been an entirely cordial meeting. The scientist, Antoine-Laurent Lavoisier (1743–1794), was one of the king’s men; the artist, Jacques-Louis David (1748–1825), would four years later vote for the king’s execution. The rencontre yielded an immense canvas still regarded as one of the greatest portraits of the 18th century.

The meeting in 1788 between Antoine-Laurent Lavoisier (1743–1794) and Jacques-Louis David (1748–1825) resulted in one of David’s finest portraits, an icon of the Enlightenment now hanging in the Metropolitan Museum of Art. The painting shows Lavoisier and his wife and partner in science, Marie Anne Pierrette Paulze (1758–1836). Behind Mme. Lavoisier is a folio, indicating that she is an artist. (As a younger woman, Mme. Lavoisier had studied painting under David’s tutelage, and she may have been the one who instigated the meeting between her husband and her teacher.) In the background of the painting are several pilasters, a signature of David’s neoclassical style. But the most important symbols of Lavoisier’s career are the pieces of chemical equipment. Never mind that they belong in the laboratory and look strangely out of place on a writing desk. They are shown prominently in Lavoisier’s studio so that the viewer knows that this elegant man was a chemist.

It would be interesting to know how the specific pieces of equipment depicted in the portrait were chosen. One can imagine the Lavoisiers showing David their nearly 200 pieces of scientific equipment, many of them beautifully crafted by Nicolas Fortin, Lavoisier’s instrument maker since 1783. Lavoisier and his wife might have chosen pieces for their scientific significance, but David was likely also looking for pieces that would contribute to the overall composition of the portrait. He might also have wanted to paint instruments that would showcase his own skill as a painter at representing reflective surfaces —and this they certainly accomplish. The viewer has no doubt that the glass is glass, the brass is brass, the water is water, and the mercury is mercury.

It may be that the work on Lavoisier’s desk is the manuscript of Traité élémentaire de chimie, which one year after the painting was created would introduce to the world the basic concepts and nomenclature of modern chemistry. The scientific community recognized its importance immediately. Published first in Paris in 1789, it was quickly translated into English as Elements of Chemistry, and Lavoisier became the acknowledged leader of the Chemical Revolution. The popularity of the Traité led to a second edition, published in Paris in 1793, less than a year before Lavoisier stepped up to the guillotine on 8 May 1794. His chemical revolution was well under way as his head and body were carted off to a mass grave.

In the first of the two volumes of the Traité, Lavoisier presents the conclusions and principles derived from his experiments. The second volume describes his experimental methods in detail. Appended to the second volume are 13 plates that show some 170 pieces of laboratory equipment finely drawn to scale by Mme. Lavoisier. Most of these flasks, bottles, jars, siphons, furnaces, tables, and basins do not grace David’s portrait, and those that do are probably the best-known pieces of laboratory glassware in the art world. They also were vital components in several of Lavoisier’s experiments —experiments in which he discovered scientific principles that lie at the very center of modern chemistry.

Mass of Reactants = Mass of Reaction Products

As one might strengthen a rectangular gate with a diagonal brace, David strengthened his rectangular portrait with strong diagonals from the upper left-hand corner to the lower right. Mme. Lavoisier’s right arm, Lavoisier’s quill, the bright fold in the table cover, Lavoisier’s unnaturally long leg, and a beam of light coming from the upper left window all point to a glass balloon on the floor at the lower right of the canvas. The gleaming balloon shows David’s skill to great effect, but it is also important for its use in the establishment of the law of the conservation of mass.

Lavoisier was a superb quantitative chemist, a master of the volumetric flask, the beam balance, the barometer, and the thermometer. Most of his quantitative experiments were performed in closed systems and involved either the consumption or production of gases, which were measured in volumes. In order to balance his equations, the volumes of gases had to be converted to masses. To determine the mass per volume of atmospheric air, nitrogen, oxygen, hydrogen, and carbon dioxide, he weighed the gases in glass balloons, like the one in David’s painting, with capacities of about 17 liters. Each balloon had a brass cap cemented to its neck, through which a metal tube with a stopcock was soldered. Lavoisier measured the balloon’s precise volume by weighing it first empty and again filled with water. He then dried the balloon and evacuated it as much as possible using a brass air pump, visible in the painting. He then closed the stopcock and screwed it to a reaction vessel that contained the gas to be weighed. As the stopcock was opened, the gas rushed into the balloon. Lavoisier then closed the stopcock and weighed the balloon again with, as he writes in the Traité, “the most scrupulous exactitude.” He subtracted the weight of the evacuated balloon and made corrections for temperature, pressure, and incomplete evacuation by the air pump. It is remarkable that the ratios of his measured weights of various gases are not very different from the ratios of their molecular weights, of which Lavoisier had no knowledge. Once established, his volume-to-mass conversion factors would allow him to compare masses of reactants and reaction products.

The law of conservation of mass, which French students call Lavoisier’s law, would soon have enormous repercussions not only for quantitative chemistry but also for understanding the very nature of matter. Lavoisier had shown that regardless of the physical state of the substances involved in a chemical reaction, the total mass of the system must remain unchanged. Such a concept required some number of indestructible particles of constant weight to be present in the reactants and in equal numbers in the reaction products. This led to the atomic hypothesis of the English chemist John Dalton and to the modern understanding of the physical structure of matter.

Water → Hydrogen + Oxygen

The middle instrument on the table is a glass tube about 2 inches in diameter and 24 inches in length, with a flared mouth. This plain and simple device adds to the verticality of the objects on the table, but it also had great meaning for Lavoisier and the Chemical Revolution. With it he was able to show that water was not elemental, but rather that it could be further broken down into hydrogen and oxygen.

Since ancient times water had been considered a basic element. But by 1781 the world was forever changed when water was shown to be, of all things, a combination of two gases. Joseph Priestley, Henry Cavendish, James Watt, and Lavoisier all contributed to that momentous discovery, with Priestley producing water by heating lead oxide in an atmosphere of hydrogen and Cavendish and Watt producing it by burning hydrogen in atmospheric air. All three were so preoccupied with trying to explain their findings in terms of phlogiston theory that it remained for Lavoisier, who in 1783 repeated Cavendish’s earlier experiments, to interpret the reaction correctly: water was being synthesized from hydrogen and oxygen.

But Lavoisier felt that proof of the composition of water was not complete. In the Traité he wrote: “Chemistry affords two general methods of determining the constituent principles of bodies, the method of analysis, and that of synthesis. It ought to be considered as a principle in chemical science, never to rest satisfied without both these species of proofs.” He set out to show the reverse of Cavendish’s synthetic experiment through his own analytic one: the breakdown of water into hydrogen and oxygen.

With the tube completely filled with mercury and inverted in a basin of mercury, as in the portrait, Lavoisier introduced under the lip of the tube small amounts of water and iron filings, both of which floated to the top. The filings gradually lost their metallic luster, and he knew from earlier oxidation experiments that the iron was becoming oxidized, thus removing oxygen from the water. As the iron oxide accumulated on the surface of the mercury, gas collected in the top of the tube. He sampled the gas and found that it burned quietly with a white flame. It was “inflammable air,” which he would later call hydrogen, because it had been “born of water.” Lavoisier considered this the final proof that water is composed of oxygen and hydrogen.

Ethyl Alcohol + Oxygen → Carbon Dioxide + Water

The vessel at Lavoisier’s left hand was suitable for storing oxygen and regulating its release by the stopcock at the top. Surprisingly, it is not included in Mme. Lavoisier’s illustrated inventory, although she did depict functionally similar pieces; it may have been acquired after her plates for the text had been completed. David has given this engaging piece a commanding position on the desk. With its long stem and brass cap, this masterpiece of Nicolas Fortin resembles a giant gold-lipped goblet. There are two stopcocks, one in the stem and one in the metal tube leading out of the airtight brass cap. A long glass tube passes through the cap and down to the bottom of the vessel.

In the Traité Lavoisier described the use of similar vessels. The glass foot of the stem is submerged in a basin of water, and the glass tube is plugged (as it is in the painting). With both stopcocks open, an air pump, screwed to the top stopcock, removes the air from the vessel, causing the water from the basin below to flow into it. Once the vessel is emptied of air and filled with water, the upper stopcock is closed. The glass tube is then attached to an oxygen generator, which produces oxygen by heating either mercuric oxide or red lead oxide. As the oxygen bubbles up through the water, the displaced water exits through the lower stopcock. When sufficient oxygen has accumulated, the glass tube is plugged, the lower stopcock is closed, and the vessel is ready for use as an oxygen storage tank. Certain experiments required a carefully controlled flow of oxygen from the storage tank, and for that the upper stopcock was critical. This is illustrated by an experiment whereby Lavoisier determined the composition of spirit of wine (ethyl alcohol) by combustion.

Lavoisier created a combustion chamber by inverting a bell jar in a basin of mercury and withdrawing part of the air so that the mercury level would rise. He slipped an alcohol lamp containing spirit of wine with “a small morsel” of phosphorus in the wick into the mercury under the lip of the bell jar. It floated to the surface of the mercury, where Lavoisier lit it by quickly pushing a red-hot wire up through the mercury to the lamp and igniting the phosphorus in the lamp’s wick. Thus spirit of wine burned in a closed system of atmospheric air. In pure oxygen the rate of burning would have been explosive, but with the nitrogen of the atmospheric air as a moderator the flame was manageable. As oxygen was consumed by the burning alcohol, water accumulated on the surface of the mercury but the flame grew weaker. To keep the flame going, Lavoisier allowed additional oxygen from the storage vessel, carefully regulated by the upper stopcock, to bubble up through the mercury and into the combustion chamber. In the storage vessel, as oxygen was released via the stopcock, the main chamber filled with the water from its underlying basin by way of the open stopcock in the stem. Too slow a flow of oxygen would extinguish the flame; too rapid a flow would risk overheating and cracking or exploding the bell jar. Lavoisier had learned the hard way that burning alcohol in oxygen in a closed system was hazardous. In his Traité he tells of an instance that “had very near proved fatal to myself, in the presence of some members of the Academy. A violent explosion took place, which threw the jar with great violence against the floor of the laboratory, and dashed it in a thousand pieces.”

When the flame was finally extinguished by the buildup of carbon dioxide in the combustion chamber, Lavoisier closed the stopcock on the oxygen storage tank. Lavoisier had measured the initial weight of the lamp and spirit of wine, as well as the volume of oxygen in the storage tank and the volume of atmospheric air in the combustion vessel. He weighed the water present on the surface of the mercury by withdrawing it with a curved pipette, and after the combustion chamber was dismantled he again weighed the lamp. Before dismantling, Lavoisier measured the volumes of the gases in the system before and after injecting a potassium hydroxide solution into the chamber through the mercury to absorb out the carbon dioxide and by measuring the remaining gas volume, correcting for standard temperature and pressure. The volume of the initial atmospheric air was subtracted, and the gas volumes were converted to weights. The weights of the reactants, alcohol and oxygen, could then be compared with the weights of the products, carbon dioxide and water, to balance the chemical equation.

This and similar procedures with other plant materials led Lavoisier to conclude that “the true constituent elements of vegetables are hydrogen, oxygen, and charcoal [carbon]: These are common to all vegetables, and no vegetable can exist without them.” The door to organic chemistry had been opened. Lavoisier’s experiments showed that the combustion of organic substances resembled animal respiration, consuming oxygen and producing water and carbon dioxide. This bolstered his long-held theory that animal respiration was a form of slow combustion. He would later show that a human being consumes oxygen at a rate proportional to the amount of physical work being done, opening the door to physiological chemistry.

From Portraiture to Politics

At the time the portrait was painted the Chemical Revolution had been firmly established. Now, thanks largely to Lavoisier, balanced chemical equations could be written; heat could be quantified; air and water (considered primordial elements since antiquity) could be broken down into their components; and, as chemical compounds were given compound names, chemistry could be discussed with a new and reasonable nomenclature. Respiration, that mysterious pneuma of the ancients, had become a chemical reaction akin to the burning of an alcohol lamp.

Lavoisier’s instruments are masterfully painted by David, and their realism is astonishing. But as the paint dried, the tools of the Chemical Revolution went back onto laboratory shelves. Lavoisier supervised the government’s gunpowder manufacture and collaborated in collecting taxes for Louis XVI. David, a radical revolutionary, continued his support of the National Convention. The two men went their very separate ways into the political revolution, David as its champion and Lavoisier as its victim.

It would remain for Mme. Lavoisier, the real centerpiece of David’s painting, to promote the contributions that she and her husband had made. After her husband’s death she retrieved the proofs of his unfinished Mémoires de chimie and managed to publish these classic papers, which contained his final interpretations of his work, in 1805. She continued to promote her husband’s discoveries and to be an important figure in the scientific and intellectual life of Paris. But there must have been moments when she longed for those days spent in the laboratory working with her husband, surrounded by the beautiful objets d’art of Nicolas Fortin.

David belonged to a political revolution—Lavoisier to a scientific one. For a brief historical moment these revolutionaries combined their genius to create a work that beautifully captures the brilliance of the social, political, and intellectual upheaval that whirled around them.