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.

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.