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

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.”