Revolutionary Instruments: Lavoisier's Tools as Objets d'Art

Jacques-Louis David, French, 1748-1825. Antoine-Laurent Lavoisier (1743-1794) and His Wife, Marie-Anne Pierette Paulze (1758-1836). 1788, Oil on canvas. Image courtesy of the Metropolitan Museum of Art.

Jacques-Louis David, French, 1748-1825. Antoine-Laurent Lavoisier (1743-1794) and His Wife, Marie-Anne Pierette Paulze (1758-1836). 1788, Oil on canvas. Image courtesy of the Metropolitan Museum of Art.

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.